Cycling Losses for Gas Furnaces
 

How much does cycling losses of a gas furnace reduce AFUE vs. Thermal Efficiency? Loss of efficiency for oversized AC units is well documented, but I haven't been able to find anything conclusive on gas furnaces.

I think it depends on many factors, but you can reduce them by wiring in a circuit to start the fan really slow as soon as the burners are lit. And, of course, any good thermostat will let you slow down the response time.

The losses seem to be mostly from the time it takes for the heat exchanger to warm up and the losses from heat lost into the ductwork in places that its located where you don't want the heat to go.

For my house, I have all rigid metal ductwork and it takes 10 minutes of burning time before the delta T settles on its way out of a supply register across the house from the furnace. I don't occupy the basement in my house, it is storage space and laundry for me so I don't intentionally need it any warmer than it needs to be. If the ductwork was in an attic or in an unconditioned crawlspace, things get worse since the heat escaping that ductwork is lost to the outside. Of course if it takes 10 minutes to settle on the delta T, I should probably be a little upset that my thermostat kicks the show off at 10 mins of burner time even with the max temp span setting. With the expensive Honeywell units, I'd expect the duct losses to multiply, they have setting in those to cycle multiple times per hour. Mine never cycles twice per hour(as in every 30 minutes or less), no matter what load, even with design load yet the Honeywell's are set to something like 5 or 6 cycles for a condensing furnace. Not sure how that makes sense.

Manual J says that in my situation with supply air above 120f, winter design below 15f, with an enclosed unvented basement that duct loss is 25%.
This feels a little exaggerated to me but every time the system fires up it needs to warm up the ductwork before the full BTU output that is possible(subtract losses with what is still getting lost with hot ducts) gets to the living space, and once the furnace is off any extra heat in those ducts will go to where those ducts are. I think there is more cycling loss through the ductwork than inside a condensing furnace in most cases unless the cycle times are long and the ductwork is insulated.

One quick test is to tape a shoebox over the thermostat. If the short cycling stops, the cause is some of the supply air hitting the sensor and causing unwanted feedback. Then you either adjust the vents or install a thermostat cover as a permanent fix.

One way you could approach this would be to compare modulating gas furnace efficiency to non-modulating gas furnace efficiency, since the modulating feature is designed to overcome the cycling losses.

You'd need to be careful in selecting similar units for your comparison. For instance, a lot of the modulating units are also condensing units, which also adds to the efficiency.

I know that with regards to mini-splits, the modulating units (AKA: "inverter technology") made a significant jump in efficiency.

You might also look into modulating gas water heaters and also modulating gas hydronic boilers.

I'd be very interested to see what you find out.

-AC

I'm not sure that modulating furnaces solve the problem. At lower burn rates, the heat exchanger is colder and will exchange slightly less heat, a condensing furnace will still be over 90% efficient so the loss isn't that great though. Another point is that any losses to the ductwork have now become constant ductwork losses instead of intermittent ones. If going with a modulating furnace, I'd make it a point to use a variable speed or X13 motor and see if you can set it to have a very low temperature rise, if possible, to minimize ductwork losses. The trouble is that you'd need to be sure that you downsize your furnace as much as possible to do this and make sure you have enough ductwork to make sure that you'll get the airflow without raising power use or increasing static pressure too much. I personally wouldn't do a two-stage or modulating furnace unless I could be sure that my temp rise is something like 40-50 degrees or so. That takes a pile of air though 578 CFM for a 25000 BTUhr output (low stage of a two stage Bryant 40k furnace) for a 40 degree temp rise. The lowest air flow setting on that particular furnace is in the 550ish CFM so it seems they've got it right. I'd imagine the X13 type electrically efficient ECM motor like that uses about 200 watts with that kind of airflow but that's just a guess based on my PSC blower that gives me about 620cfm from 288 watts, which isn't so good for me because my 57000BTUhr output puts that temp rise at 85 degrees, so I up the furnace speed so its consuming 324 watts and a slightly more respectable temp rise.

Does anyone have a good idea on how much power an X13 or variable speed ECM blower uses? Does anyone have one of these furnaces where they have measured this or has a link to how much power they use at a specific CFM? I have yet to come across this data yet. Reducing cycling losses by raising CFM but staying within the manufacturer temp rise spec will help reduce losses through ductwork and you'd get more use out of your fuel at the expense of extra electricity. I don't think it would make sense to do this with a PSC motor and definitely not with a shaded pole furnace motor like the one I used to have. I'm actually thinking of upping its speed to the max in January (408 watts) which is about 800 CFM to bring the temp rise to about 65 degrees which puts towards the lower end of the 55-85 temp rise range for my older furnace. I think I'd get pretty close to 55 if I swapped my pleated filter out for a cheap fiberglass one for the airflow but I'd rather do what I can to not clog my A-coil.

Thank you for this useful post and detailed post.

It had made me more certain than ever that I am on the right track, and that I am truly doing the work of god, in disassembling and removing my gas furnace, which I haven't used in four years.

I'll no longer bang my head against the sheet metal elbow duct that stuck out from the wall at the bottom of the basement steps.

I'll no longer have floor ducts that have eaten bits of corn chips, gallons of dust, and escaping screws.

I'll no longer need to engage in territorial war against the squirrels over who actually owns the chimney that has gone cold... they can have it now.

I'll finally be able to rip out all of those horrible ducts and be able to properly insulate under the living room and kitchen floors.

The removal of the furnace will enlarge the space I need to stage work on installing radiant floors in my house.

Thank you.

-AC

What AC_hacker said.

Radiant floors do resolve ductwork loss problems, remove higher wattage blower fans from the equation, replacing them with what can be(although not always) super low wattage pumps, and you can pull cycles long if the water heating source isn't massively oversized.

The issue is usually that radiant floor systems involve tons of labor to retrofit and for people who aren't doing the retrofit as a DIY project, it would make sense to just spend that money on super-insulating the house. I've been doing the math on this now that my local PUC might be approving my gas company to raise the monthly service fee another $7/month and I'm looking at a positive cost benefit and reduced energy usage by going with mini-split heating. It'll take roughly R40 walls(XPS or polyiso outsulation), R60+ ceiling, and my rotting wood windows(replaced with u-0.2 or better) and siding(also rotting) would be part of the work to get my -13f design load under 10k BTUhr.

Ok, so I have a question considering a different, more common situation. I Know that many veteran ecorenovators have been there and done this, but lots of others have not. I see this a lot more than I should when working on peoples' homes.

Consider a younger, less mechanically knowledgeable citizen who has just purchased a "pre-owned" suburban or urban home. The layout is what they were looking for, within their budget, and passed inspection. It is a "postage stamp" property of less than 1/3 acre in an established neighborhood. The home is in a decent location relative to the owner's lifestyle. It's "where they want to be" for this phase of life.

After moving in and receiving some utility bills, new homeowner realizes the house is not as energy efficient as it could be. They call the local utility company and schedule an energy audit to find out where their home is on the energy-efficiency scale. What they find is confusing as heck to them.

It is found during the audit that the home is "not too shabby for the neighborhood". The envelope has been sealed up to "acceptable" levels and has been insulated to "above average" value. Not quite energy star, but better than most "affordable" site-built homes are being constructed today in the region. In these sections, the home gets a "B" from the utility.

The heating and cooling system doesn't fare so well in the audit. The 80+ year old ductwork was not replaced when the HVAC system was upgraded from a fuel oil or coal burning furnace/boiler in the early 80's. As usual, the upgraded unit was upsized to push enough airflow for the ancient ductwork. From the whistling registers and vents, it is obvious that the air is just flying through the system. The ducts have been gobbed up with mastic to seal up the leaks, but not insulated. Between the high pressure drop and the exposed ductwork, the auditor estimates that duct losses are 40% or more. The HVAC gets a "D".

During the conclusion of the audit, the homeowner sits down at the kitchen table and is given what seems to be a really expensive sales pitch. The auditor recommends to just gut the whole HVAC system and put in something new. A quick tour of the new systems on the market is given, with "good, better, best" systems of increasing estimated cost, naming major components but not much detail. A "top ten" list of local contractors is left with the report, and the auditor makes a hasty exit.

Now that the homeowner has been given all this information, they figure out they can't afford to do it all at once without experiencing financial hardship. The existing system still works properly, but there is much to be gained as far as energy efficiency. Assume a 10 SEER, 2 1/2 ton AC unit and a 50kbtu, 70% natural gas furnace.

What would you guys recommend as a starting point, assuming the homeowner can swing a couple thousand dollars or so today if it will reduce their utility bill?

I would think that if their house is that tight and insulation is that good, a 70% efficient furnace isn't going to make bills so high that spending thousands would be worth while. I'd probably ride it out with the current furnace and try to fix the ducting. This can be done relatively inexpensively if you DIY.

What does this mean? "Acceptable" to who? Any numbers??

I think Daox's strategy of turning on all the exhaust fans and scouting for leaks would be a great start. A case of cans of foam, and one of those IR leak finders would come into play here.

Next, I'd recommend judicious use of bubble wrap on windows. If they're double or single hung... on the lower panes of most windows.

This is really interesting, because around here, the standards are such that any home built today is much better than most homes built 30+ years ago. And to see what really good homes are like, just go check out a local Habitat for Humanity project, because they really have it going on regarding insulation.

This part is really depressing because it reminds me of my place and the wretched old parched air heating system I formerly had. Just thinking about it is almost like post traumatic stress.

So, I'd tell the young couple to do the above loss reduction strategies, and then flush the whole idea of ever retaining and improving central air, and start buying and self-installing mini-splits (not going with a multi-head unit), one at a time, as their budget allows and use the wretched parched air system (set the thermostat lower than on the mini-splits) until they have enough mini-splits to carry the house, then rid themselves of parched air, forever.

Kentucky should be a great place to use mini-splits.

-AC

Vexation Removal...
 

Best,

-AC

AC,

I honestly didn't know I would hit so close to home with this question.

What you had is exactly the kind of duct work I was speaking of: dryer-exhaust or furnace-stack sized ducts with tight bends leading to the furthest registers in the house. Maybe 8" round ducts leading from the plenum to the first branch. No balance in airflow whatsoever; the vents closest to the air handler get the lion's share of the flow.

Kentucky is an awesome place to use mini-split heat pumps. I have seen some installed, and the owners have nothing but good things to say about them for the most part. My hacked window unit holds its own down to about 30 degF, and it will carry my heating load for the entire home 95% of the time. If I had a larger heat exchanger outdoors, it would do even better.

I honestly believe if I purchased 2 tons worth of capacity in a multi-split unit, or maybe separate units, it would carry me for all but a couple of nights a year. Even then, thermal mass would probably carry me until 3 or 4 in the morning.

Close to home is exactly the case... not only my home, but the homes of personal friends, who are making the same dreadful mistakes as your young couple example.

In spite of my advice, and their full knowledge of the economy and comfort of a more modern type of heating (radiant, mini-splits, GSHP, etc) they are blundering ahead same as in the past... and only perfunctory attention to sealing & insulation.

Momentum can be a terrible thing.

-AC

I can relate to jeff5may on post #5

"They call the local utility company and schedule an energy audit to find out where their home is on the energy-efficiency scale. What they find is confusing as heck to them."

Did this, didn't receive anything on a scale basis other than CFM.
My auditor tried to tell me that my furnace was 60% efficient which isn't true.
He argued that ACH is pointless and to ignore it.
He then took the CFM converted it to ACH and told me that I had enough natural ACH to where if I didn't seal my house that I could skip additional ventilation.
When I asked him about my "60% efficient furnace" he recommended to get it checked annually and wait until it failed a heat exchanger inspection.

He also said that my 2x4 16" on center construction wrapped in 3/4 of XPS is a "R19 wall" because "that's code for the 1980's. ...sure an R13 batt plus R3.75 of XPS makes an R19 wall.

He seemed to be all over the idea of getting a water heater blanket like it would save the world and 'pay for itself in under a year'. ..sure the 4 therms a month it used was $2.60 at the time and 12 months would be $31.20. Quite certain I'm not going to save that much in a year by buying a $20 water heater blanket, I use some of that hot water, its not all going to the basement. Water heater insulation is a good idea and is fairly easy but he made it sound like the single most important thing to do.

I asked him about sealing attic facing top plates, got no info there about attic insulation other than to seal plumbing and electrical penetrations. I'm glad I read about it here and saw S-F's pictures.

It is found during the audit that the home is "not too shabby for the neighborhood".
Mine actually thought the 2x4 design with 3/4" rigid wrap was a good idea and is 'plenty enough insulation for a house'.

"The envelope has been sealed up to "acceptable" levels and has been insulated to "above average" value."

Apparently 1500 CFM is 'plenty good', 'don't worry about it', and 'if you seal much more you'll need a ventilation system.' worthy discussion points.

"Not quite energy star, but better than most "affordable" site-built homes are being constructed today in the region."

Mine had huge thermal bypasses into the attic. I found roughly 500 sq inches or 3.5 sq ft of what were essentially holes in the attic that I've sealed up.

"During the conclusion of the audit, the homeowner sits down at the kitchen table and is given what seems to be a really expensive sales pitch. The auditor recommends to just gut the whole HVAC system and put in something new. A quick tour of the new systems on the market is given, with "good, better, best" systems of increasing estimated cost, naming major components but not much detail. A "top ten" list of local contractors is left with the report, and the auditor makes a hasty exit."

My auditor gave me the cheap fixes. Used estimates and payback periods and basically said to not do anything with only 7" of cellulose in the attic, not do anything about 1500 CFM at 50 pascals, don't consider adding additional insulation if you replace the siding, be sure to put a continuous(2 speed) bath fan in the house if you seal up the place.

I asked him about passive house insulation or at least super insulating my place and he talked about how it won't work out well for me without more thermal mass.

He left and I feel that I would have been better off with two box fans sealed in a window and renting a thermal imaging camera from Home Depot. Oddly enough, I'm looking to do that again after I have insulated, air sealed, and did everything else that I'm planning to do to this place since some of the images that I really wanted of my house during the audit, I did not get.

If you insulate well enough, I do agree with Daox though. You'll save tons more for the same amount of money by insulating and air sealing than you will on replacing the equipment. I'll save more than 26.6% of my energy with less than 1/4 the amount of a $8000 upgrade to a 96.2% efficient furnace and a 16 SEER AC. I'm thinking I'm already better off than 26.6% and I don't even have the $300 of cellulose plus blower cost in my attic yet. $140 of spray and rigid foam seems to have made a huge difference once applied to the right places.

What would you guys recommend as a starting point, assuming the homeowner can swing a couple thousand dollars or so today if it will reduce their utility bill? Bang for the buck would be to go into the attic and seal all of the wall top plates, electrical penetrations(use UL 1479 and UL 2079 rated caulking for junction boxes), and plumbing penetrations. Seal any second story or story+half knee wall holes. Pull the window trim and seal all of those gaps. Seal electrical and switch plates with the foam inserts you can get at your hardware store. Put on the thicker version of the water heater blanket. Seal the sill plates in the basement that face outside. Be sure the gaskets are good on all doors(top, bottom, and sides), check the windows too. Put on window shrink plastic, especially in the winter, but for windows you don't open in the summer, leave it on in the summer too.

After the easy bang for the buck stuff is done, return to the attic and get R60 in your attic if you are in a climate zone like mine or at the very least what the DOE recommends for ceiling/attic insulation. I was surprised that I could get just over 1000 pounds of cellulose for $250 after tax (add another $50 for blower) and will probably put me between R60-R75 from my original 7". I'm looking at about $500 with everything said and DIY done with my house and that feels super cheap to me. The next project is to add 4" of polyiso or XPS under the siding when I do DIY siding replacement on my house. That is more work and I'm less familiar, but I'll start with the easiest side first and then maybe do a side each summer until its done. A heating load calculation suggests I'm cutting my peak load by about 2.5 times and the heating bills should be reduced by even more than that. I'm planning to get solar shade screen in the summer to block out sunlight on the west side to cut the AC costs.

All of this will cost less than the $8000($6000 after government and utility rebates) for what amounts to minimal mechanical efficiency improvements in comparison to real envelope improvements. Oddly enough this makes my home even more of a candidate for ultra efficient mini-split equipment. I'm actually looking at heating and cooling my entire home to my satisfaction with one 12k mini-split heat pump and using the gas furnace on the coldest days as a backup to the mini-split or use its blower or maybe a little gas when I need a more even temperature when I have guests over. I'm particularly looking at benefits such as the super high SEER, dehumidification dry mode, local heating/cooling when I want to close the door and chill in one room on the computer or in a book all day, and long cycle benefits when cooling too.

Have you done a Heating Degree calc for your location?

If you post your zip code, I can do it for you.

-AC

AC,
My zip code is 42701.
I have more HDD's than CDD's on paper, but the summer climate has lots of latent energy in it. It tends to be muggy and swampy here a lot. Even during heating season.

That is really an excellent point. You have correctly identified a deficiency in the terms HDD and CDD. The idea of HDD and CDD is to have a statistical, localized value to use as an aid in designing heating and cooling systems, but both HDD and CDD ignore the part that humidity plays in the design. You are right, the humidity plays a very large part. I would assume that local designers would learn from experience, and develop a 'fudge factor' to make localized corrections. But it does seem inelegant.

A few years back, I was trying to zero in on exactly what my heating requirements actually were. I was doing a study in which I was heating my house with several electric resistance heaters, and I had a Kill-a-Watt attached to each. I was logging outdoor temperatures during this time, and plotting temperature against power, expecting to get a fairly smooth curve that I could then use to precisely model heating needs of my house. I was amazed at the scattering of data points I was getting, and what I thought would be some kind of a curve, was actually a cloud of points. What you have identified, explains the scattering, because I had not accounted for variations in humidity, and the power that would be required to deal with that humidity.

But back to the HDD & CDD issue, this is a very good reason to do a concept 'upgrade' of Heating Degree Days to Heating BTU Days, HBD (with the metric equivalent being Heating Kw-h Days, or HKD) and instead of Cooling Degree Days, they would become Cooling BTU Days, CBD (ditto metric equivalent, Cooling Kw-h Days, or CKD). This would incorporate average humidity for various months.

It's an important distinction.

jeff5may, I think we should prepare to storm the halls of science...

-AC

Wind speed has a LOT to do with load, especially on the heating side. 20F outdoor with no wind vs. a 15MPH wind makes a HUGE difference in heat loss. Did you account for wind speed when you were plotting your heat loss?

Thermal mass of the house is another thing that needs to be accounted for. If outdoor tempatures change quickly it can take up to 24hrs before the house feels the full effect of thermal load. This is why you don't need to size AC units for peak load with high mass homes.

Back to the original topic, it does appear that duct losses are significant in the cycling loss calculations. Not sure hot much the 45 second burn time to heat up the exchanger costs in efficiency, and how much is recovered during the 180 second blower off delay after the burner shuts off. 90%+, 80% and old school pilot light furnaces have different cycling losses.

Jeff (and others),

Interesting you bring up this case study (above, two posts back) as I have in front of me almost the exact same scenario.

In this home, the thermal mass of the metal duct is huge and it takes almost a minute after the furnace fan comes on for warm air to come out of the registers. The two main ducts are in the crawl space, poorly insulated (maybe R4) and have lots of losses (~120 sq inches cross sectional area each). I could hear them whistling when I scooted under there and can literally feel air blowing out of the duct joints (ducts outside of HVAC space).

I am debating on recommending running flex duct as it is inexpensive and I can get 100 foot lengths with no splices. On the other hand, what I would LIKE to do is to put in duct grade PVC and sleeve insulate it (R10). The problem is cost. The 12 inch diameter PVC is about 1/4 the cost of the flex duct and the installation is a snap with flex duct.

As you know, it is very difficult to clean flex duct of the dust, mildew and other junk that accumulates (even with good pleated filters). PVC and metal is breeze to clean. I really hate flex duct . . .

But cost is cost and I tend to look way down the road in terms of job quality. The HVAC installer is pushing me to suggest flex duct. Me thinks I will put both in the recommendation and allow homeowner to sort it out (and yeah, I have just contributed to the confusion you described above).

Has anyone sleeved flex duct - or seen a product to do this (extra insulation sleeve slid over flex duct to augment duct insulation)? The R value of flex duct is pretty poor, but at least it has very low exfiltration (if installed correctly).

Years ago, there was a product where you could spray foam the INSIDE of metal ducting both to seal up air leakage and put in some degree of insulation. The downside was it collected dust/dirt like MAD, decreased the cross sectional duct area and increased duct resistance to air flow. Haven't see that product in a while . . . .

Thoughts?


Steve

My furnace runs a blower off delay of 75 seconds and in that 1 minute 15 seconds at the recommended blower speed, it is still putting out 125 degrees of heat through the ductwork when it powers off and my thermometer will actually slowly climb after the airflow stops as the heat in the metal is hotter than the air passing through it and some of that hot slowly rises out of the ductwork but I'd imagine most of it ends up conducting off the surfaces of the ductwork into the basement where I do not need it. I ran the blower for 2 minutes and that seems appropriate but my furnace(and many others) are not adjustable in their blower off delay.

My original thermostat, which was installed from when the house was built in 1985 until 2010 when I swapped it out, used to run the furnace with a 5 minute gas burn time. The new thermostat that I installed runs the gas for about 10 minutes. I can only imagine that cutting the cycles in half has helped a ton but I have a feeling that I still have room for improvement if I could allow for 1 or 2 more degrees in temperature spread before it turns on. I think the key may be to get a thermostat that allows you to adjust the spread or anticipation settings to the maximum comfort/efficiency level compromise for the people living in the house. ..along with making sure that the minimum size furnace is installed in the space.

Steve, I like the idea of insulated ductwork but I think flex duct is a bad way to go about it. Flex duct is one of the most restrictive ductwork options and a quick search on sites like greenbuildingadvisor shows that it is generally a bad idea. It allows for bad shortcuts such as not installing it stretched to its full stretch, tight 90, 180, or even full U-turns in the flex, bad trunk setups. I've seen other recommendations where rigid ductwork should be used and flex only allowed in the final 10 feet of a run.

It's a frictional loss issue that is the problem even if it is installed properly. The best way to use flex duct but not encounter the issue is to respect maximum duct lengths and be sure that the ductwork is upsized to the appropriate level to be sure that the airflow is correct. The issue with that is unless your flex runs are relatively short, you'll need to make very large diameter runs and on top of that you'll need to factor in the thickness for the insulation and how you plan to fit that flex into the space you are putting it. Once you get to a certain size of flex duct, you might even have R8 flex but you've now got so much surface area of the flex that you've got new problems.

I like this duct grade PVC idea since it is rigid and won't be leaky or have high frictional losses but I don't know what to expect in terms of how to best insulate it. My head says to create a sealed box around it out of rigid foam(beadboard or XPS are cheap) or drywall since it is cheap and stuff it full of cellulose.

As a personal note, if my attic had flex duct in it when it was built(it doesn't), it would probably be R4 so I'd be inclined to replace it with the R8 and since my attic is loose-fill I'd probably baffle around it with drywall or wood and then just loose-fill it to R60. So basically the ductwork would run in a channel that is 2 feet wide and covered 2 feet above the ductwork with cellulose. That is the way you can easily insulate that ductwork if you have the room. In Minnesota, the idea of putting ductwork in an attic is ludicrously stupid because we have winters where we usually hit -20f once or twice and if the ductwork has 130 degree air going through it, we would have a 150 degree difference in the ducts versus the outside. Which is more than the 90 degree with the rest of the houses insulated components. R8 is a hysterical in such a circumstance.

Since I think flex duct is a bad idea and rigid metal doesn't seem appropriate in an attic. How about ductboard? Take the ductboard and then add additional insulation around that, which should be easier since the stuff is square. You could wrap it in EPS, XPS, polyiso, or do the cellulose box route that I suggested above. If you are cheap you could even bust out fiberglass rolls and wrap flex or ductboard in that but no promises on its performance since you can't install fiberglass in an ideal way to get the most R-value by wrapping it around something square or round.

125 degrees is HOT after 75 second delay, what is the peak tempature while running? Is time delay digital with a board or a manual limit switch? I've not run into any board controlled blowers that don't have a selectable time delay. Manual fan/limit switches are also adjustable in most cases. You could always add a sequencer/time delay relay/fan control center to add extra blower time if you think it would help.

My furnace only gets to 106F after 10 minutes of running. Once the burner cycles off the blower runs for 3 minutes then tempature at vents is about 80f. Furnace is a 44,000BTU 90% with the 1/3HP blower set on the lowest speed.

Timer based blower on and off time of 75 seconds, not based on temperature. I might be able to modify it electrically which might require swapping a resistor somewhere or something. Either that or maybe setup some kind of relay to hold it on for a certain additional time period after the power is normally cut off. ..I suppose a 90 degree fan switch might not be a bad idea, that would turn it on sooner too and shut off once it drops below 80 degrees to really get the heat out of the ductwork.

After 10 minutes of gas run, it will usually be at 140 degrees(70 degree temp rise). It varies a bit depending on filter age, if any dampers or registers are closed, and fan speed. I have a roommate right now who prefers a warmer temperature than I do so some of the dampers and registers are shut to position the heat where the cold-baby is.

70 degree rise with 140f discharge from the vent. Add 5-10 degrees for the tempature exiting the furnace, you are proably close to hitting the high limit. Is the furnace a standard 80% induced draft/electronic ignition? Increasing blower speed should get you more efficient operation.

Total fan control can be done by installing one of these:
http://www.amazon.com/ICM251-Fan-Con...m_sbs_indust_3

MN Renovator

Agree with ALL your points, but need to mention that I am a hired consult on this (hired by the HVAC contractor). As I said, "I hate flex duct"!

That said, I did increase the x sectional area by 15% by using 12 inch diameter flex duct, but exactly as you say, now there is increased surface area for heat loss. We have to use the crawl space (maybe 20" of space) and it is nasty . . .

Now I am leaning towards the PVC ductwork . . . .

To all those that think contractors "rip off" consumers, here I am putting in FAR more time than I am paid, but trying to help out a homeowner. Not trying to be a saint, but I also know quite a few contractors that do the same.

Steve

No way I'd use flex in a crawl space around here. Wouldn't make it a year without a critter tearing it up. Hard pipe only for crawl installs.

Ducts vs Pipes
 

I'm surprised that you professionalistas are venting so much hot air on ducts when hydronics offers so many advantages.


THIS link explains this and other advantages, like the ease of retrofitting a parched air house.


-AC

Hydronics is practically non-existent in the south due to lack of AC capability.

I know that Germany isn't actually part of the south, but they have learned how to do hydronic (radiant) cooling, by circulating cooled water through ceiling panels. There was an issue with the possibility of condensation, but it was resolved by automatically monitoring the temperature and humidity of the room, calculating dew point, and adjusting water temp to keep it from crossing into the dew point.

Their energy costs are 2X what ours runs, so they have to be much more intelligent about how they use it.

-AC

So they use a mini split to do the dehumidification? Actually, that's a great idea. Install a very undersized unit to do the dehumidification and use hydronics to pick up the rest.

I don't know if it was exactly a mini-split, but that would work in a home... but I'm sure that there was some de-humidification involved.

randed tried to run water straight from his ground loop directly through a HX that was in a ceiling mounted blower unit to cool his shop. He reported that the setup was able to lower the air temperature, and that there was considerably more condensation on the fan HX that he had expected. But the overall experience that he reported was that his shop became more humid, and not much more comfortable.

So yeah, some de-humidification would be required, but not as much power would be required to make the whole thing work as a straight on air conditioner.

-AC

The real problem as it relates to this thread is that when using old-school, forced air "blast" furnaces, the duct losses are calculated separately in the design. The AFUE or thermal efficiency measurements are done for the heating unit only, under a tightly controlled set of conditions. They basically consider what heat is available to distribute, and tell nothing about the ductwork outside the box. Once it leaves the unit, there's no telling where the heat may go, only how much and how hot it is, at what pressure.

In the real world, the professionals in the industry are left to solve the problem of delivery. Any mechanical contractor or engineer will tell you that it's all about trade-offs. Finding the balance of airflow vs temperature vs raw btu's delivered here or there is an art in itself. It would be great to have a wide open, straight path to follow, but bottlenecks are unavoidable due to the nature of the installation. Working around or through the real world situations is what these pros are supposed to be devoted to. As always, your mileage may vary.

There are lots of ways to try to approximate the "transient" losses of ductwork, as well as "static" losses. Due to the uncertain nature of (especially residential) on-site distribution equipment actually installed, it is commonly much easier to just take measurements. The main idea is this: the more restrictive, leaky, and uninsulated the ductwork is, the less heat that will make it to its destination. As with anything, everything you do to improve the system adds up.

As with many other aspects of making a building more efficient, most of the remedy is a one-time expense. After it's done, it just keeps working for free. But it's the work that turns people off. Nobody wants to go crawling around in a dark, dusty, cramped space. Not even once. Don't even mention dragging materials in and out of there!

Residential water chillers with "mini split" style fan coils/air handlers could be the way of the future. Each room has it's own small air handler, therefore a way to deal with condensation. No ductwork losses and only one chiller/condenser unit. It's done in commercial buildings all the time on a larger scale.

I would guess spiral wrapping that PVC duct with whatever thickness fiberglass/bluejean/rockwool batting would give you the R values you want.

The PVC provides the air seal so butting it close as possible would suffice.


I like the mini-split idea as a stop gap energy efficiency improvement, and it basically is what I'm doing. It can provide for all your needs most of the year, and then you have the furnace as backup when needed.
I'm not sure it's a homeowner install, although the avg contractor can be just as bad as they generally move the traditional big units. Vacuum the system is not cheap or simple for a DIY'er, and support if problems arise may be non-existent.