Hot Air Furnance - Oil Fired - Blowing Cold Air

Steve_in_NH

Hello,

I went to a friends house for dinner tonight and they have been having an issue with their heating system. This is their first heating season with the house.

I offered to take a look for them, I'm no expert, but have a general understanding of heating systems and was curious.

The issue is that after the heating cycle has been on, and it does work and heats the house, once the temp is satisfied the blower remains on and blows cold air into the house.

The furnace is in the basement. The return vents on the first floor open directly to the basement, there is no duct work. I did not see any returns from the second floor. There is one duct that it looks like it was just cut off that is blowing heat into the basement. There are two plenums on top of the furnace. One has almost all of the duct work for the house on it. The other one goes to a large decorative cast iron vent near the front entrance door, and there is a large vent in the side of the plenum itself in the basement that was blowing cold air out at me. At first i thought it was the return vent, but then thought it was strange to be up so high so I put my hand in front of it and then i saw the other duct and traced that etc etc. I looked around the unit for a return and didn't see one right way. But there was a louvered area about half way up the unit, very small, about 1'-6"x5", that I'm thinking now must have been the return.

For some reason there are two thermostats located right next to each other on the wall upstairs. They are both older but different vintages. They were both set at 69 degrees. One of them a dial style was actually stuck from paint when I tried to adjust it (they did paint this room though so it could be from them). It was time to leave at this point so i didn't continue to play with the thermostats to see what they did.

I'm pretty sure the thermostats are the issue though, but I was also hoping to get a little lesson on how that systems works as some of the things I saw didn't make sense to me. Mainly the vent on the side of the plenum. And it needs a return right? so that little vent must be the return? What could the reason be to have two thermostats mounted right next to each other??

Thanks in advance. And its late at night so hopefully I didn't make to many grammar mistakes!!

STEVEusaPA

It probably is either the thermostat, but more likely the fan/limit control-either sticking or not set to the right parameters (unless it's board controlled).
You're going to need someone out there who knows how to troubleshoot with a meter.

EBEBRATT-Ed

It is most likely the fan/limit control. the thermostat should start the burner and the blower should remain off. When the plenum is warm say 120 deg the blower should start and the house will heat. When the thermostat is satisfied the burner shuts down and the blower continues to run until the the plenum temperature drops to about 90 deg and the blower will shut down until the next heating cycle.

No harm in running the blower all the time except for the annoying cold air blowing.

You didn't mention if the house had AC tied into the furnace. If so the thermostat wod have a fan "on-auto" setting.

Ductwork sounds like a hack job. The ductwork could cause a potential combustion air problem with the furnace if he supply and return ducts terminate in the basement

GW

Two stats for the same system is not normal, especially side by side. Sounds like a cobbled system

Was the vent blowing or sucking? Back in the old days heating contractors pulled air from the basement, not sure of their logic.

Unless we are the talking habitable and conditioned living space, the ducts should not be communicating with the basement.

Steve_in_NH

Yes the duct work seemed like it was a mess. I noticed at a couple location that where the round duct would reach the rectangle register they didn't seal the transition. The duct just pointed straight up at the register and stop about 2 inches short of it. None of the ducts were a return, they are all supply. The returns were just holes in the floor directly to the basement with registers to cover the openings. The basement is not living space and was not well insulated so it was cold even with all the extra heat escape. None of the duct work was uniform or made any sense. There was a large round duct 14-16" to the back corner of the dining room, two small rectangle ducts to the 2nd floor, the giant rectangle one to the entrance area. A smaller round 6-8" to the kitchen, etc...

Is the vent out the side of the plenum typical? The vent was square (18-24") and the entire width of the plenum. I could actually look right through it into the plenum and also look inside the large rectangle duct going to the entrance area.

No A/C. Both Thermostats are older simple type. One was round dial type the other is a rectangle with a metal slide at the bottom that you move to the desired temp.