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Old 01-05-12, 04:23 PM   #11
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A duct booster fan added to a system under high pressure due to having too many dampers closed isn't going to help because the velocity in the open ductwork is going to be high enough to where it will be a challenge to increase the flow any more. Ideally, to balance the system you would want to add more return and supply ductwork but to have it done right you would need the help of a pro unless you just added a flexduct run of supply and return to the coldest room so you could open some of your other dampers but that is a half-assed solution.

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Old 01-05-12, 05:13 PM   #12
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Well I don't see the ductwork getting upgraded any time soon. The house is at least a century old and the 6" supply ducts all kind of spider out from the furnace in the basement, with a short plenum in either direction. The return ducts are made from metal sheeting between two large boards, just below the floor joists. Many of the ducts feed two registers, one downstairs and one upstairs. Some, however, run straight from the supply plenum to the main floor. Those are mostly the ones that I have reduced or closed the dampers on.

There are four main rooms on the east side of the center wall on the main level (2 living rooms, dining room, family room), each with at least one supply on the inside wall, and one large return on the outside wall, with the exception of the front room which has a non-functional (decommissioned) return. Each downstairs room except for the dining room has an adjoining room on the west side of the center wall (kitchen, bath, study) which each have a supply register but no return. Basically, all of the returns are on the east side of the house, and all of the supply registers are on the west center wall, or west outside wall. The upstairs has no returns, but there are supply registers in each of the 5 bedrooms and 1 bath.

In the basement there are 3 return ducts measuring roughly 8" x 24" each. I'm no HVAC contractor, but it seems like these should provide adequate return airflow for a 100k furnace. The fact that there are no returns upstairs, however, makes for some unbalanced airflow issues. I think you can understand why I would have to close the damper on a short duct that feeds a register on the main floor, and open the damper on a long duct that feeds a downstairs register and an upstairs register. So basically I have been doing this careful balancing act of dampers trying to cut airflow to rooms that get too much air, and boost airflow to rooms that don't get enough air. I guess I always assumed that the blower was going to put out the same amount of air regardless of how much came out of each register. The way I have it set now, 5 of the downstairs ducts that lead to main floor register vents are either fully or mostly damped off, or closed at the register. Each room seems to maintain approximately the set temperature, when all doors are open.

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Old 01-05-12, 09:18 PM   #13
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Update: I followed the instructions in the furnace manual for calculating the BTU/h input rating. This involves watching the gas meter and determining the amount of time that it takes to use 1 cubic foot of gas, then comparing that time to a chart in the manual. 100k BTU should take 36 seconds according to the chart. The time was taken with the water heater in pilot only.

The first reading after my little adjustment yesterday was 42 seconds. Extrapolated from the data in the chart, this works out to 87,500 BTU. So I adjusted the gas regulator on the furnace until I got exactly 36 seconds for 1 cubic foot. I can tell that it was overfired before I adjusted it yesterday because the regulator was maxed out, which gave me a time of 30 seconds today on the same setting.

So now, at least by my home DIY methods, the burner should be set to the 100k that it's rated for. I measured a consistent 68 on the return and 135 on the supply, giving me a rise of 67 degrees. That's 10 degrees lower than it was before I messed with it. I also tried setting the thermostat to the recommended warm air setting. I didn't record how many times it cycled in a period, but it was more than at the hot water setting. I changed it back to the hot water setting and it does seem to go a little longer between cycles, still not quite long enough for my liking though.

I'm wondering if I might get a few efficiency points by adding a combustion air supply pipe. The manual says that it can be done, but there is no cutout in cabinet for one. I'm thinking that I could cut a hole and run some PVC pipe straight in to the cabinet. Of course I would want to cover the existing vent slits in the cabinet and access panel so that it only draws fresh air from outside (I can already see the dirty looks from the HVAC guys out there ).

... Which raises the question: Would preheating the combustion air with heat from the exhaust vent (HVAC guys really glaring now) increase the overall efficiency of the system? My idea is to revamp one section of the galvanized exhaust vent with a double-walled heat exchanger can, similar to the heaters used on the mufflers of small aircraft (and old Volkswagens). Basically adding metal strips or spikes to the outside of the galvanized duct for heat transfer, then covering it with a larger diameter duct and sealing it with a cool fresh air inlet on one side, and a heated fresh air outlet on the other. I have worked with industrial furnaces that had combustion air recuperators on the exhaust vents and they seem to work quite well. I doubt that it would cool the exhaust enough to condense before exiting the house, and I don't really see it adding enough heat to the heat exchanger or cabinet controls to damage them. Just a thought...
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Old 01-05-12, 11:21 PM   #14
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Quote:
Originally Posted by abogart View Post
... Which raises the question: Would preheating the combustion air with heat from the exhaust vent (HVAC guys really glaring now) increase the overall efficiency of the system? My idea is to revamp one section of the galvanized exhaust vent with a double-walled heat exchanger can, similar to the heaters used on the mufflers of small aircraft (and old Volkswagens). Basically adding metal strips or spikes to the outside of the galvanized duct for heat transfer, then covering it with a larger diameter duct and sealing it with a cool fresh air inlet on one side, and a heated fresh air outlet on the other. I have worked with industrial furnaces that had combustion air recuperators on the exhaust vents and they seem to work quite well. I doubt that it would cool the exhaust enough to condense before exiting the house, and I don't really see it adding enough heat to the heat exchanger or cabinet controls to damage them. Just a thought...
Cooler air is denser which, then, has more oxygen.
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Old 01-06-12, 02:21 AM   #15
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Originally Posted by abogart View Post
I'm wondering if I might get a few efficiency points by adding a combustion air supply pipe. The manual says that it can be done, but there is no cutout in cabinet for one. I'm thinking that I could cut a hole and run some PVC pipe straight in to the cabinet. Of course I would want to cover the existing vent slits in the cabinet and access panel so that it only draws fresh air from outside.
I can see two potential problems:
  • Your furnace may be tuned for a more or less constant intake air temperature (whatever your basement temp is), while feeding it fresh air from the great outdoors means that it will see a whole range of temps and humidities. If your manual says it can be done, then it should be OK.
  • Having fresh air come in through the wall means that that cold air will be sucked into the house. This can be solved by putting a damper on the intake, so that it automatically closes when the burner no longer needs air.
    S-F started a thread on this:
    How to provide fresh air to my boiler?

Of course, the good side of adding a fresh air intake duct is that your furnace won't be sucking air from the rest of the house, which must be made up for somehow.
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Old 01-06-12, 06:23 PM   #16
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Originally Posted by Piwoslaw View Post
Your furnace may be tuned for a more or less constant intake air temperature (whatever your basement temp is), while feeding it fresh air from the great outdoors means that it will see a whole range of temps and humidities. If your manual says it can be done, then it should be OK.
An excellent point. Cold, dense air from outside would lean the flame, which is adjusted for 65 degree air. The heat scavenger should bring the intake air temperature up to about basement temp. I don't really want to heat it much more than that due to the various wires and controls in the cabinet, although they must be designed for some amount of heat because they receive a good deal of radiant heat from the flames.

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Having fresh air come in through the wall means that that cold air will be sucked into the house. This can be solved by putting a damper on the intake, so that it automatically closes when the burner no longer needs air.
Another excellent point. However, if I seal the cabinet, the only airflow in when the furnace is off will be due to stack effect from the warm heat exchanger and exhaust vent. Cold air would flow in through the intake (running over the warm heat scavenger in the process), in to the heat exchanger, and out the exhaust, which is being cooled by the same intake air. No outside air should enter the house at all if I seal it up well enough, whether the furnace is running or not.
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Old 01-09-12, 04:23 PM   #17
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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?
Hello. Im in the HVAC Trade and had my own business for 26 years.

There is a very good chance from running the unit with severely plugged air filters, that alot of dirt has ended up on the Cooling Coil which has made a restriction to airflow thru the system. If so, your Cooling Coil will need cleaning which is best left to a professional since it may need to be completely removed and taken outside for a pressure wash procedure , then reinstalled .

Other possibilities are :

1. The furnace wasnt properly setup when installed. The manifold gas pressure should be close to 3.5" w.c. using a pressure gauge hooked up to the leaving side of the gas valve.

2. The system should have been air balanced using duct dampers . It could be some or most of the duct dampers are closed which would also cause a restriction to airflow.

3. Your space thermostat's heat anticipator setting may not be set in accordance to the type of furnace you have , which will cause short cycling .
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Old 01-10-12, 07:48 AM   #18
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Thanks for the great information, it's always nice to get advice from people that work in the trade. As I said, I don't have a manometer, but I would imagine that setting the gas flow by the meter would have brought it pretty close to the optimal setting. As far as dust buildup on the evaporator coil goes, I never thought about that. I'll have to stick my head in the blower cabinet and see what's going on in there. It's due to be vacuumed out anyway.

Since I adjusted it its been cycling about 4-5 times per hour, about normal for the 30 degree nights we've been having lately. Still a little too sensitive for my taste but that will have to wait until I can get a new t-stat.
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Old 01-10-12, 09:38 AM   #19
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4-5 times per hour still seems like an aweful lot. Mine comes on roughly 1x per hour or less I'd say.
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Old 01-10-12, 11:10 AM   #20
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Originally Posted by abogart View Post
Thanks for the great information, it's always nice to get advice from people that work in the trade. As I said, I don't have a manometer, but I would imagine that setting the gas flow by the meter would have brought it pretty close to the optimal setting. As far as dust buildup on the evaporator coil goes, I never thought about that. I'll have to stick my head in the blower cabinet and see what's going on in there. It's due to be vacuumed out anyway.

Since I adjusted it its been cycling about 4-5 times per hour, about normal for the 30 degree nights we've been having lately. Still a little too sensitive for my taste but that will have to wait until I can get a new t-stat.
If youve got a thermostat that has an adjustable cycle rate , then adjust it so you get a 2 f. drop in indoor temp. before the next heating cycle starts. If 4-5 times per house at 30 f. is equivalent to a 2 f. drop in indoor temp, then youve nailed it.

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