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Old 01-04-12, 02:52 PM   #1
abogart
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Default Warm-air furnace, reducing excess capacity

Here is my first useful post (not including the intro forum) on EcoRenovator! I'll start with the project that I have been dealing with today.

For a while now I had noticed that my gas warm-air furnace tended to be running short cycles, starting right back up after the cool-down sequence. The furnace is a Ruud Silhouette II 100,000 BTU natural gas unit. A while ago I had taped an indoor/outdoor digital thermometer to the return duct with aluminum tape and the outdoor probe taped inside the outlet plenum to measure (approximate) temperature rise. I watched the outlet plenum heat up to about 155 deg. F during a cycle, at which time the gas shut off and the furnace started the cool-down sequence, which involves running the Inducer/Draft blower for about 90 seconds, after which the main blower continues to run for another 90 seconds. The label inside the access panel states that max. outlet temperature is 170 deg. F. So I'm thinking that the temp at the heat exchanger probe was hitting 170 and tripping the over-temp switch.

I shortly after checked the filter and noticed that it was completely plugged . It was one of the high-filtration, disposable types which apparently clog up easily and don't allow much air to pass through. So I bought one of the basic blue fiberglass types today and put it in. After that the outlet plenum topped out at around 145 deg. F, eliminating the over-temp issue .

Anyway, with the return temp at about 65 and the outlet at 145, I have a rise of 80 degrees. The label in the access compartment states that acceptable rise is 50-80, so with a brand new, clean filter I'm already at the max. temperature rise. I don't like how the thing runs such short cycles. The inducer fan runs for a good 45 seconds before the igniter lights up, then another 30 seconds or so before the gas kicks in. With the 90 seconds that it runs after the gas shuts off, the thing is running for 2 minutes and 45 seconds during the total cycle that isn't even producing heat. When the flame is only on for a couple minutes, this makes for a lot of wasted energy and excess wear on the equipment.

I have already set the heating blower speed to the max speed on the main blower. The house is about 3500 sq. ft. but I have it sealed up pretty tight. So apparently this thing is either oversized, or it's putting out more than 100,000 BTU's. Now I know everyone says not to, but I went ahead and lowered the pressure on the gas valve regulator enough to bring the outlet temp to a steady 125. That gives me a rise of about 60, still in range of the 50-80 recommended by the factory. The flames look good, solid light-blue, and all four burners light just fine. I don't have a manometer, but I'm thinking that maybe this thing was just adjusted wrong during the install .

So does anybody know what happens when the gas pressure is reduced without changing the orifices? I'm thinking this might lead to a rich flame, but I can't really tell due to the inducer blower sucking it all right into the tubes. It's a low-efficiency model (rated 80% AFUE ) but I'd like to tweak it a little to make it more efficient if possible. I notice that the exhaust vent runs pretty hot. I just think that if more of that heat were removed by the heat exchanger, it wouldn't be going out the chimney. Reducing the amount of flame would cause more heat to be removed from the exhaust gasses before they leave the furnace, meaning higher efficiency.

Anyway, I'm just rambling here. Anybody have any input?

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Old 01-04-12, 03:13 PM   #2
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First things first I'd figure out why your furnace is short cycling. How long is the average on and off time?

If you truely are heating up the house THAT fast (and I find that hard to believe), you should be able to adjust the temperature swing that your thermostat allows if you have a programmable thermostat. If you don't, stop reading and go buy one now, it'll pay for itself by the end of winter and you'll be more comfortable. The default temp swing for almost all of them is +/-1F. That means if you have the heat set to 68F it'll turn on when the temp hits 67F and turn off when it hits 69F. I have mine set to a +/- 1.5F temp swing. This makes the furnace run for longer periods of time and reduces cycling.

The other thing I'd look at is where your thermostat is placed. I had an issue where the warm air was blowing right on the thermostat! A bit of adjustment to the hot air register/vent and it no longer blows right on it. This noticably decreased my gas usage.
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Old 01-04-12, 04:09 PM   #3
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I have been frustrated with this thermostat for a while now. It is an older Honeywell Magicstat digital programmable unit. The only cycle settings I get are on the back. There are two screws labeled A and B. There are three options that are set by turning one or both of the screws all the way in, or out 360 degrees. The settings are as follows:

Warm air: A = in, B = in
Hot water: A = out, B = in
Electric: A = in, B = out

The manual doesn't say what exactly each of these settings does. There is also a note that states "For high efficiency units (90%+ AFUE) set A = out and B = in." Quoting the manual, "In the unlikely event that you would like longer cycle times, for warm air set to the hot water setting." There is also a GAS/ELECTRIC switch that apparently turns the blower on as soon as it calls for heat.

I have had it set to the hot water setting for the whole heating season so far, but it seems like I get SHORT or SHORTER cycle times with this thermostat. The thermostat has been moved from a drafty back room of the house to a central wall in the living room closer to the front of the house, away from any windows. There is a register around the corner of the wall, but I have it closed completely. I have never actually seen the temperature change on the thermostat once it has reached its set temperature, so I'm thinking that it must be maintaining less than +/- 1 deg.

I have been debating about replacing it, but I wasn't sure if a newer thermostat would be any better, since they all seem to advertise that they "maintain precise comfort levels in the home within +/- 1 deg." So I wasn't sure if any other thermostats had an adjustment for cycle length. Is there any specific model that you recommend?
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Old 01-04-12, 05:04 PM   #4
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I personally use a LUX 5-1-1 model (I forget the exact model number). I believe all the programmable LUX models have the adjustable temperature swing option. You can check on their website, they have all the manuals there. A new one should run about $40 I think.
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Old 01-04-12, 05:35 PM   #5
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Quote:
Originally Posted by abogart View Post
For a while now I had noticed that my gas warm-air furnace tended to be running short cycles, starting right back up after the cool-down sequence....
I was having exactly the same issue with our furnace. The problem was an old and malfunctioning thermostat.

Quote:
Originally Posted by abogart View Post
I shortly after checked the filter and noticed that it was completely plugged . It was one of the high-filtration, disposable types which apparently clog up easily and don't allow much air to pass through. So I bought one of the basic blue fiberglass types today and put it in. After that the outlet plenum topped out at around 145 deg. F, eliminating the over-temp issue .
Unfortunately those blue fiberglass "filters" are nearly worthless for filtering air. I use mid-range, pleated paper filters and replace it every 2 months.

Quote:
Originally Posted by abogart View Post
... I don't like how the thing runs such short cycles. The inducer fan runs for a good 45 seconds before the igniter lights up, then another 30 seconds or so before the gas kicks in. With the 90 seconds that it runs after the gas shuts off, the thing is running for 2 minutes and 45 seconds during the total cycle that isn't even producing heat. When the flame is only on for a couple minutes, this makes for a lot of wasted energy and excess wear on the equipment.
I agree with Daox here - I think that your current thermostat has too small a temperature swing or is malfunctioning. By allowing a larger temp. swing (+/- 1 to 1.5 deg. F) the furnace will run longer and more efficiently.

If you can't adjust the temperature swing (range) on your current thermostat, it's probably time to buy a new one. I picked up a programmable thermostat for $25 (on sale) and will be comparing the next heating bill to the same period from last year.

My experience - $25 Thermostat vs. $250 Thermostat | EcoDaddyo.com

FWIW,
Tim
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Old 01-04-12, 08:48 PM   #6
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I did some research and Honeywell still doesn't have a swing setting on any of their thermostats, just warm air, hot water, and electric. I am really leaning toward the Lux thermostats. The Lux Smart Temp TX1500E seems quite well equipped and adjustable for only $34 on Amazon. I'm thinking I'll be buying one of those soon.

Quote:
Originally Posted by TimJFowler
Unfortunately those blue fiberglass "filters" are nearly worthless for filtering air. I use mid-range, pleated paper filters and replace it every 2 months.
I totally agree. This furnace actually has a General Air AC-1 pleated media filter unit on it. The problem is that when I bought the house (with the furnace) there was no filter media in it, only a cheapo blue filter wedged inside the oversized compartment with a dryer sheet box. The rack that is supposed to hold that media in place is gone and I haven't found out where to buy a new one yet. My recent experience with the pleated paper filters gives me the impression that this blower just sucks too much air for them to be effective for more than a couple weeks before they get dirty and start impeding airflow. Looks like I'm stuck with cheapo filters for now .
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Old 01-04-12, 09:25 PM   #7
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Reducing the output of a furnace(derating) is dangerous in that if not a high enough temperature inside the heat exchanger is reached then the there will be condensation in the heat exchanger and the flu. The condensation will rust out the heat exchanger and your flu. If your heat exchanger cracks you'll be buying a smaller furnace.

The way to determine the output of your furnace is to clock your gas meter, there are sites on how to watch the dials and depending on the units that your dial displays you could the seconds for a few revolutions and there is a chart that will tell you the rate its burning at. You just need to make sure its the only gas appliance running when you are checking and the water heater is on pilot only and nobody is using the stove, oven, or clothes dryer.

With all that being said, I also have dealt with bad thermostats that seem to want to only less than 5 minutes. I replaced mine with a Hunter, the package it came with specifically said it can be changed between -1f/+1f and -1f/+2f. I like it because now it runs a 10 minute gas cycle every time which I think is long enough to be reasonably efficient but not too long to where I notice the temperature swing. The Honeywell thermostats with the screws on the back are GARBAGE! I was at someones house once and the burner turned on and the furnace turns the blower on at 75 seconds but while the blower was starting up, the Honeywell's anticipater assumed that the demand was met and so the blower kicked on without any gas running. We decided to see what happened if we raised the temperature 5 degrees on the thermostat with the furnace power switch off. The thermostat displayed raised 2 degrees over 10 minutes without the house actually getting warmer. Needless to say that Honeywell crap isn't there. I had a White Rodgers in my house when I moved in, junk too. Get something you can adjust the room temperature for when it cycles on and off.

I don't think you were hitting a limit switch, normally those act like a fuse or circuit breaker, if I pop my limit switch, I'll need to replace it and the only people I know who can buy one is an HVAC tech and they'll check my heat exchanger to be sure I didn't crack it in the process.

Back to your temperature rise, this sounds like you have an inadequate ductwork issue, if there is enough restriction in your ductwork due to bad design and it can't handle the airflow to keep temperature rise down, you either need to have a lower capacity furnace or your ductwork fixed. It could be in the return side or the supply side or a combination of both. An HVAC tech would measure your duct pressure to determine this, if its over 1" water, you need to fix your ducts or have a lower output furnace.

100k 80% furnace is an 80k output furnace. Sounds like an HVAC installer used a terrible rule of thumb for sizing the furnace when it was installed. Where I live, based on the rule of thumb used by tech's here you wouldn't have an 100k furnace unless your house was about 3800 square feet. The problem with those rules of thumb they are using is they figure your house is as leaky as the leakiest house and not insulated well like the houses of the 50's because they don't do a proper heat load calculation, they base it on bad data.

I provided my own personal load test data of thermostat run times in an 8 hour period when the weather was -20f outside, which is the design temp where I live. Even though I'm well under the needs of even the 40k 95% furnace I'd like to have installed, I was still recommended by the tech to put in a 60k 80% furnace to replace my 75k 76% furnace. If a house needs 25-30k of heat at -20f to stay warm and you put a 48k unit in, its still only going to run half the time on the worst day. Needless to say, I'll never talk to that guy again and you shouldn't talk to anyone who won't do a load test if your furnace ever needs to be replaced.

"So does anybody know what happens when the gas pressure is reduced without changing the orifices? I'm thinking this might lead to a rich flame, but I can't really tell due to the inducer blower sucking it all right into the tubes. It's a low-efficiency model (rated 80% AFUE ) but I'd like to tweak it a little to make it more efficient if possible. I notice that the exhaust vent runs pretty hot. I just think that if more of that heat were removed by the heat exchanger, it wouldn't be going out the chimney. Reducing the amount of flame would cause more heat to be removed from the exhaust gasses before they leave the furnace, meaning higher efficiency."

No, this gets discussed on hvac-talk amongst the professionals regarding 2-stage units, the heat exchanger is sized for a certain amount of heat output from the burners, if there is less heat output then the efficient temperature exposure against the heat exchanger isn't met and if the inducer blower doesn't slow down to compensate for the now 'too large' heat exchanger for the burner output, you lose more of your efficiency. For 2-stage units, if the inducer blower doesn't support changing its speed, a lower burner flame will result in reduced efficiency. If you make an 80% efficient furnace into a condensing furnace(which is required to raise the efficiency beyond 80%) you'll need to have a different heat exchanger setup to produce the proper condensing properties and condensate drain lines and PVC piping wouldn't be needed. An 80% furnace isn't designed for it. Get a 90% and be sure to have a variable speed(ECM) motor if/when you replace it. Don't forget to get that heat load test, preferably after you air seal and insulate if you are going to air seal and insulate, which is something you should do if you haven't.
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Old 01-04-12, 09:42 PM   #8
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Quote:
Originally Posted by abogart View Post
I did some research and Honeywell still doesn't have a swing setting on any of their thermostats, just warm air, hot water, and electric. I am really leaning toward the Lux thermostats. The Lux Smart Temp TX1500E seems quite well equipped and adjustable for only $34 on Amazon. I'm thinking I'll be buying one of those soon.



I totally agree. This furnace actually has a General Air AC-1 pleated media filter unit on it. The problem is that when I bought the house (with the furnace) there was no filter media in it, only a cheapo blue filter wedged inside the oversized compartment with a dryer sheet box. The rack that is supposed to hold that media in place is gone and I haven't found out where to buy a new one yet. My recent experience with the pleated paper filters gives me the impression that this blower just sucks too much air for them to be effective for more than a couple weeks before they get dirty and start impeding airflow. Looks like I'm stuck with cheapo filters for now .
A quick search with Google Product Search for Generalaire AC1 found a link to this.
MERV 11 GeneralAire 12758 / 4001 Filter Media Alternative
http://www.amazon.com/AC-1-MERV-Gene.../dp/B000NMF45G

The first link has a good picture, seems the model number for the AC1 filter might be 12758 if the actual dimensions you see match this. Measure it out and see. A 5" filter is supposed to be a 6 month to a year filter depending on how dust it has to deal with but since there is so much media it should be able to hold a whole bunch more dust than a 1" thick filter.
Nominal Dimensions: 20" x 25" x 5"
Actual Dimensions: 19.6" X 24.1" X 4.75"


The Lux Smart Temp TX1500E, I just read a review on the amazon $34 listing, someone there says it has 9 settings from .25 degree difference up to 2.25 degree difference. Seems like this thermostat is a good pick unless you need the programmability to be 7 days of the week or if you have a two-stage a/c or something, otherwise this looks to be full-featured.
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Old 01-05-12, 06:46 AM   #9
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Thanks for all the great info, MN Renovator! I haven't been able to find those disposable ones that fit my unit before. But $134 per year in just furnace filters??? Ouch!

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Back to your temperature rise, this sounds like you have an inadequate ductwork issue, if there is enough restriction in your ductwork due to bad design and it can't handle the airflow to keep temperature rise down, you either need to have a lower capacity furnace or your ductwork fixed. It could be in the return side or the supply side or a combination of both. An HVAC tech would measure your duct pressure to determine this, if its over 1" water, you need to fix your ducts or have a lower output furnace.
I never thought about that part of the equation. Because it's a big, 2-story house, I have many of the dampers partially or fully closed to balance the heat. If I open them all completely, the room with the thermostat and several rooms downstairs heat up way too quickly and the upstairs bedrooms and several other rooms downstairs stay cold. I think the house originally had a wood or coal stove and, apparently, back in the day they didn't care if the rooms heated equally.

So there I go re-engineering something that isn't broken again . I suppose the way to fix that problem would be to install some of those duct booster fans for the far rooms, but they are a bit pricey. I might just open them all back up and see if I can balance it out without restricting them too much.
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Old 01-05-12, 10:45 AM   #10
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Excellent points so far.

Also to OP: when in doubt: it never hurts to replace any digital thermostat that have been in place for 5+years, for they will fail eventually (causing all sorts of misbehaviours).

I dumped those mercury switch-based (not in the landfill, of course!) Honeywell and replaced them with RiteTemp digital thermostats and been very happy so far.

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