Warm-air furnace, reducing excess capacity

[MN Renovator]

Originally Posted by S-F

You can get to 1 ACH 50. It just takes some work. With enough spray foam you could do it pretty easily. You'd have to have access to the inside of you walls and all of you band joists and top plates though. Also doors and windows would need a great deal of attention. And like I said, if you get that tight you'll be ahead of everyone else even if you have no insulation at all.

Every gap that was identified with the blower door will be sealed but access to areas that are covered by drywall is something I'm not looking to touch because I'm not experienced with drywall. If I have to take walls down, I assume I'm buying new drywall and I think the difference between the best I can do without tearing the walls down and the best I can do with them wouldn't make enough difference to be worth it. I wasn't planning on paying another $100 to get the results of my sealing but I might take this furnace blower and seal it around a window and try a depressurization with one of my neighbors FLIR cameras to see if I missed anything. Then walk around with it pressurized and check it. If only I had two magnahelics to measure the pressure drop over the fan and pascals, then I could roughly know the CFM. Beyond that, I think its just a number. Right? Does it really matter beyond bragging rights once I cleaned up the best I can.

If my natural ACH is about 87CFM and its -20f out and 70 inside then my infiltration BTU loss assuming I'm exchange a 90 degree difference is 8456 BTU/hr heat loss. My heat loss from watching thermostat cycles is 20k at 0 and 70 and 25k at -20 and 70. 33% is infiltration and 66% is insulation? So with the same insulation and half the infiltration I'm at 20k heat loss and I save 17% on the energy bill on the coldest days by cutting infiltration to half? Of course then again the blower door test that showed me at about 1500CFM was also having air pouring through the 5" flue back into the house, it was a ton of air. I should have blocked that and gotten a better reading. How much CFM does a natural draft furnace pull up the flue while its running? My energy auditor thought 200CFM. If that's the case then 4800 of the BTU/hr out of 20k heat loss is from the furnace's induced infiltration through fresh air intake. Would a sealed combustion furnace save me 5k and another 17% of what is left if I cut infiltration in half? Would I really use 63% of the energy and 51% of the energy if I go from 76% efficient to 95%? I'm thinking I'm accidentally exaggerating something somewhere. Central air furnaces don't exist at 12750BTU/hr I've already had a hard enough time with the door to door furnace shops and friends who are in the HVAC trade convincing them that I should have something smaller than my current 57k output. They want me to put in a 75k 80% or 60k 95% and none seem to know what a load test is or believe that I had any attention put to the thermostat during a design temperature period. I ask for a 40k 95%+ and they tell me only if I had a 1200 sq ft condo and never for a 2200 sq ft house.

[S-F]

Originally Posted by MN Renovator

Beyond that, I think its just a number. Right? Does it really matter beyond bragging rights once I cleaned up the best I can.

Ding, ding, ding!

Score one for common sense! Knowing your ACH 50 # is only relevant if you are going for some kind of certification like LEED or passivehaus. The only time I figure that stuff out is when an agency needs it. I was just saying that to impart a sense of the level of attention to detail you'd need to have for really getting all you can out of your house. IR cameras are great and I wish I had one but there are a lot of places you should air seal even if you don't see a big grey or black blob coming from it with the IR camera. All band joists, all top plates. Ruthless attention with the foam gun (even as I dislike spray foam it's cheaper, faster and cover more area than caulk) is what's needed. Where the rafters meet the top plates in particular is a tricky spot. You also really need to get to it from the outside also.

But we've gone way too far afield.

I don't know much about HVAC so I can't comment on your questions there aside from saying that it's probably not cost effective to replace what you've got already. Unless you think pretty long term.

[MN Renovator]

"I don't know much about HVAC so I can't comment on your questions there aside from saying that it's probably not cost effective to replace what you've got already. Unless you think pretty long term."

I know, the reason I asked around was mostly because the furnace is 29 years old and the A/C is only a little younger. I was mostly scouting for people who would work with me instead of fight me for a properly sized unit. I think I found a place that will listen as I found a review saying they did a load test and the price wasn't bad for higher end equipment. I don't need a load test but they will need to do one to believe me because my real world numbers are hard to believe. At this equipment age I'm creeping towards a cracked heat exchanger or an A/C refrigerant leak or failure. I know that with low usage the difference between efficiencies isn't going to make sense for me to pay over $5k to replace equipment, it would never pay off. It would take 17 years of gas bills before I pay that much in heating costs at my usage prior to really digging into insulating or air sealing just to pay the same as the furnace costs and that isn't savings, its cost. The new equipment wouldn't pay off the 'savings period' before it fails and costs a ton to replace again so I'll let this thing croak on its own. Insulation and air sealing is the way to go for energy and bang for the buck.

[abogart]

Originally Posted by MN Renovator

I know that with low usage the difference between efficiencies isn't going to make sense for me to pay over $5k to replace equipment, it would never pay off. It would take 17 years of gas bills before I pay that much in heating costs at my usage prior to really digging into insulating or air sealing just to pay the same as the furnace costs and that isn't savings, its cost. The new equipment wouldn't pay off the 'savings period' before it fails and costs a ton to replace again so I'll let this thing croak on its own. Insulation and air sealing is the way to go for energy and bang for the buck.

Very true. Unfortunately, some of us are obsessed with increasing the efficiency of our equipment anyway.

Well I think I have everything operating properly now. Manifold pressure on the furnace is set to 100K BTU/h input via the gas meter. Maybe some day I'll pick up a cheap gas manometer for $25 and find out for sure. We had a few nights recently where the outside temp dropped to around 7°F; The furnace cycled roughly 4 times per hour. On cycles seemed to be a little longer than off cycles. Now, though, we're at about 35°F here in MI and it's on for about 5 minutes and off for probably 15. I tried lowering the blower speed to M-Hi for heating, but it seemed to cycle on longer and off shorter, and some rooms got colder. So it's back to Hi now and everything is staying warm.

It would be nice to have some way to record system on and off cycles without having to stand in front of the thermostat all day. I'm thinking something like a device hooked to the thermostat wires or leads that transmits on and off cycles to a computer. Possibly even an electronic temp probe. Now I'm thinking a little outside the box here, and a little outside my wallet as well.

[MN Renovator]

"I'm thinking something like a device hooked to the thermostat wires or leads that transmits on and off cycles to a computer. Possibly even an electronic temp probe. Now I'm thinking a little outside the box here, and a little outside my wallet as well."

I have a thermostat that tracks how long it is running a call for heat or cooling. I could tell you the usage today, yesterday, so far this week, last week, or since it was last reset. This winter I have taken down the 'yesteday' reading every day. I can tell you how long its run everyday, including a full week's time in December where it didn't run at all. I have the Hunter 44550. It has a dip switch on the back to adjust for a 1 degree or 2 degree difference. I got mine off of ebay for about the same price as this one. Hunter 44550 Auto Save 7-Day Programmable Thermostat | eBay

I found it extremely useful to find out my heating load when we hit a 'design day', the coldest night of the year 2011 was -20 and it was there for about 9 hours so I took the usage for the first 9 hours of the day and found out how oversized my furnace was.

[abogart]

It was a tough decision (almost had to flip a coin). I reviewed quite a few models, researched all the features, looked at some manuals. In the end it came down to two models:

• Lux TX1500E
• Hunter 44550

The Lux model has just about all of the features that I was looking for in the price range that I am willing to spend. I like the overall look of the unit. The display is quite large and shows a good deal of information on it, such as the graphical and numerical filter change interval and system operation (heat/cool/fan). It doesn't look like the temp setting is displayed without pushing a button, which is a feature that I would like. AND as an added bonus, it displays relative humidity. The key selling point for this one is the amount of "tweaking" that it lets me do. Most of the options, such as filter change interval, swing setting, etc. are fully customizeable, not just on-off, 1 or 2.

The Hunter model has all of the basic features of a programmable thermostat with more of a minimalistic display. The main thing that got me about this one is the energy use tracking (as mentioned above by MN-Renovator). That is one feature that I could really use. Being able to make changes to the system or house and see in real-time how it affects heating and cooling cycles is a major plus. The things that I didn't like about this one are the lack of customization (tweaking) that it allows. Things like swing setting and filter reminder are on-off dip switches. Also, this model had some bad reviews (defective circuit boards and such), so I was worried about durability.

After several hours of deliberating, switching tabs between user manuals, and reading reviews, I decided to go with... (drum roll)

The Lux TX1500E model!

Why, you ask? Because I am a tweaker! I just can't pass up all of the settings that it has. That, and all of the neat stuff on the display. Although I would love to have the energy use tracking, I figure that maybe sometime down the road when I have more money (or save more money ) I'll upgrade to one of the Lux models that has energy use tracking. Depending, of course, on how well this one performs.

So now, for the next two days, I wait impatiently at the steps for the big brown truck.

[Piwoslaw]

Good for you!

Originally Posted by abogart

So now, for the next two days, I wait impatiently at the steps for the big brown truck.

And we're all waiting for you to install and tell us how big a difference it made.

[abogart]

Originally Posted by Piwoslaw

And we're all waiting for you to install and tell us how big a difference it made.

If nothing else, at least I'll feel better about replacing the antiquated Honeywell.

[abogart]

My family room wall now looks as if I have a modern, high-tech heating system for this old house... and I bask in the indiglo backlight of the display digits.

The system is operating much more efficiently now. Even on the lowest swing setting of 1 on the Lux thermostat, furnace cycles are longer than with the first-gen Honeywell on the "longer cycle" setting. I now have the swing setting at 4 and can watch the temp drop 1° below the setpoint, then heat to 1° above the setpoint. On this setting it seems to drop about 1.5° below the setpoint before calling for heat. All of the rooms seem to be heating equally. I might make some fine adjustments to the duct dampers to compensate for longer run-times, as some rooms lose heat faster than others. I actually had more trouble balancing the heating load when the furnace was short cycling. There aren't any rooms right now that are too hot or too cold, so the heat must be going where it's supposed to. I have also set a program to drop the temp 4° at night. I figure I'll keep playing with the swing setting and heating program until the family starts complaining.

We woke up to an outside temp of 3°F here in Michigan this morning. I didn't do any actual recording of cycles, but my best estimate is that the furnace is on for about 20 minutes and off for about 15. It's still only running about 3 cycles per hour in these frigid temps. This also tells me how oversized the unit really is, and how many leaks I probably have. Might look into doing a heat loss calculation when I get the time.

[Daox]

Glad to hear its working out for you.