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?