Warm-air furnace, reducing excess capacity

[Indyplumber]

Originally Posted by abogart

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?

Check to make sure the blades on your inducer aren't clogged too. There is a high tempurature limit switch on the heat exchanger on most furnaces that will shut down the burner if the temp gets too high.

[abogart]

I have noticed that the inducer has some vibration to it, but haven't touched it because I wasn't sure how involved the process is. It looks like it just bolts on, but I didn't know if there was a gasket or anything in there that might get messed up if I remove it. (Relating inducer-blower to cylinder heads here )

[Indyplumber]

It may have a rubber gasket on it. You might be able to get some hi-temp gasket material and cut a new one if the old one breaks. While you are at it, check the fan blades on the blower motor too. It should be fairly easy to pull out and check. With your filter being clogged the way it was, the inducer and blower fan blades being clogged is the most likely culprit.

[MN Renovator]

The inducer doesn't share the same air as the blower cabinet so good filter or even none going past the air from the house, the inducer blower won't care. Inducers don't have filters.

[Indyplumber]

The inducer and blower motor both share the same house air. If the filter could become clogged enough to not pass air the same could happen to the blades on the inducer and blower motor. Nevertheless, it doesn't hurt anything to check both things. Its a simple enough thing to check and eliminate those things as a source of the problem.

[abogart]

I have also been wondering which blower speed I should use to get the most out of the gas that I'm putting in. The main blower has 4 speeds: Lo, M-Lo, M-Hi, Hi. When I bought the house the heating speed was set to M-Hi. Later, I had the idea that blowing more air over the heat exchanger would pull more heat out of the exhaust gases, so I cranked it up to Hi. Then I read something that said that faster air in the ductwork promotes more heat loss through the ducts. Also, I'm thinking that there might be a point of diminishing return in regards to lowering the outlet temp. For example, if the blower were pushing so much air that the temperature rise was only 10°F, would it have any effect at all on the air in the house.

On the same subject, I can adjust the blower-off timing for 90 sec., 120 sec., 150 sec., or 180 sec. I used to think that getting the most heat out of the HE and ductwork and into the house would make the most sense. But I read something that said that the ducts shouldn't be allowed to cool completely down.

I just KNOW somebody is going to tell me to read the manual.

[MN Renovator]

180 seconds won't cool your ductwork down, it'll still be warm coming out. The delay is usually a setting for comfort to where the goal is to get the heat from the heat exchanger into the house but to stop it before it gets below a temperature where it will be creating a breeze cool enough to make you feel cold if you are in or near a vent that is blowing air. My own furnace is 75 seconds without an adjustment but when it shuts down I'm still getting air over 40 degrees warmer than the return air temperature which is room temperature. If 'cooling' the ductwork was an issue we would see there being problems with the low power constant fan applications in variable speed furnace systems.

Blower speeds have 2 main issues Temperature rise over the heat exchanger and electrical usage. You don't want the temp rise to be too low or you can risk condensing in the heat exchanger which can lead it to a short life. If your fan speed is set at a higher speed than the ductwork will allow, as in temp rise barely changing with the adjustment, you might be wasting energy by turning it up.

With my furnace at 75k BTU, its set to the med-lo blower tap by default and temp rise is 55f. The spec is 55-85f. If I raised it to the highest speed tap the temp rise drops to 50f and if I drop it to the lowest fan speed tap, 1/10HP the temperature hits over spec in about 5 minutes when I tested it so I immediately shut off the burner. The 1/8HP tap does the job with a standard lower end pleated filter. If I use one of the 1" 3M Ultimate filters that look like they have hundreds of tiny pleats the temp rise is 80f on the same tap which puts me at the high end of the range so I have to be careful about if that filter gets dirty. Instead of spending money on those filters or raising the blower tap, I just use a less restrictive filter which works out great and even when it looks a little gray the temp rise is still in check. At the moment since my furnace is running about 30 hours in December and so far only about an hour a day so far this month too I've used the 3M Ultimate filter with the higher blower tap because 3.4 amps(408) is all my new PSC motor uses(versus about twice that with the old one) and I get the filtering that my house really needs as there is quite a bit of dust floating around the bedroom from changing my clothes and ruffling of the bedsheets. You do get better natural gas fuel efficiency with a lower temp rise but you'll never get down to 10f temp rise unless you are pushing TONS of air which your furnace will never be capable of even if the ductwork is as big as the furnace can support at the high speed blower tap.

Reading the manual will tell you to use the blower setting that puts you within the temp rise range. If both M-Hi and Hi do the trick, its up to you if you want to try to get more out of your fuel or more out of your electricity. I usually go for more out of my fuel but at the moment my priority is to get more filtering which means more air flow and another 120 watts while running which will be an extra dollar if my furnace runs for 75 hours this January(9 kwh at 11c/kwh). Your blower will use more power since your unit is a 100k unit.

[S-F]

Originally Posted by MN Renovator

Back to your temperature rise, this sounds like you have an inadequate ductwork issue, if there is enough restriction in your ductwork

Or leaks! Let's not forget leaks! What domestic HVAC contractor uses enough duct mastic, even in these enlightened days? Certainly not the boobs I deal with day to day.

Also I think you're spot on about air sealing! Even if you have no insulation at all (and I'm being quite serious here) you will do well with kick *** air sealing. Something on the order of 1 ACH @ 50 or so.

[MN Renovator]

I think getting to 1ACH at 50 would be extremely hard for anything that isn't new construction. I was surprised to see mine at 5ACH when I got my blower door after I had sealed a huge chunk of stuff and I'm not sure how its possible that my house still was leaking about 1500CFM. I'm thinking its largely the flue backdraft and possibly some inaccuracy from the blower door too because we didn't find that much stuff to seal. Air sealing is key though and everything I've done has definitely helped a ton but getting to 1ACH could be an extreme chore and might not really be practical.

Regarding the temp rise, if the temp rise is too high and the ductwork is restrictive, sealing the ductwork doesn't solve the ductwork being restricted, but of course it does help with causing the heat that is paid for to go to the right place.

[S-F]

Originally Posted by MN Renovator

I think getting to 1ACH at 50 would be extremely hard for anything that isn't new construction.

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.