It is getting cold here. I switched my HP/Gas unit from:

FAN AUTO and HEAT

to:

FAN AUTO and EMERGENCY

to stop the HP from running and just run on gas. Unfortunately the gas heater started and the fan stopped (it had been running previously). After a couple minutes, the gas heater was still on, but the fan didn't turn back on, so I switched to:

FAN ON and EMERGENCY

The fan kicked on immediately. I plan to leave it this way and call an HVAC repair service. Does that sound good? Can you guess what the problem is?

The thermostat says Rheem T874R 1442 8805s, looks analog-type with Stage 1 and Stage 2 mercury switches inside.

Thanks,
Martin

 

How long did you let it run befrore you switched over to fan/on ? If it wasn't very long then the heat exchanger just may have not heated up enough.

If the burners were on for a while and the blower didn't come on it's either low speed on the blower is out or the fan relay isn't closing.

 

I let the unit sit for a couple of minutes. Do you think that I can let it sit for longer than that? I was worried that something would get too hot with the burners on and the fan off.

Martin

 

Keep the fan to fan on let it run 24/7 till the hvac guy gets out to take a look. it wont hurt anything to let it on The gas will come on and off with the tstat.

ED

 

Okay, now I don't know if I should call for service or not, because I tried running on FAN AUTO and EMERGENCY again before reading Ed's post, and this time I could tell that the fan was running very slowly. I could see airflow from the grilles by floating a kleenex over them. The fan isn't running fast like when the HP is running. Does that sound like proper operation?

The burner turned off eventually, but the "aux heat" light on the thermostat was still on. That sounds like maybe an overheat switch maybe shut the burner off, which doesn't sound good to me. Does it make sense that the burner should turn off even when the aux heat light is on?

Martin

 

Is the filter clean the coil clean??? Just turn the fan on at the tstat and let it on till a tech can look at it. the aux heat should turn on and off for what the tstat is set at.

ED

 

Ed,

The filters are clean, but there is a bypass around one of them.

How many filters does it have? There should be no air bypassing the filter, so for now just tape it if you can. If the air's been bypassing for a long time I would bet the coil is plugged. On the gas the blower should run on low speed but from the sound of it it's running too slow. Just run the fan in the on position till you can have this thing inspected.

Martin, I must appologize. I hit edit on your post instead of quote and I can't change it back.

 

No problem. I remember what I said... The filters are clean, but there is a bypass around one of them. Someone cut an access hatch in the side of the duct and didn't replace the insulation on the inside of the duct where the edge of the filter touches the side. I'm wondering if I can replace the insulation myself?

And, I looked at the HP coils and they look clean. Of course, I'm not sure what they would look like dirty.

Mattison, there are two filters upstream of the fan, burner and cooling coils. They are in a "V". I don't think that I can tape the gap where the insulation is missing (too big and tape probably wouldn't stick well). I'll wait for the repair man to fix it. Sounds like he should clean the coils, too.

Thanks,
Martin

 

Most of the time when you have the filters that go in like yours in a V. the have like a sheet metal W that fits in and over the point of the V the filters make.


ED

 

Well, the service man just left and I don't know what to think.

I do have a sheet metal W at the point of the V, but that isn't where the insulation is missing. The insulation is missing at the outer edge of the V where the edge of the filter rests against the side of the duct. Regardless, the service man was just here, and he said to ignore the bypass.

The service man also rewired the system so that the two speed fan only runs at high speed, even in emergency heat/fan auto mode. He said that the fan was running too slow on the slow speed, but now it runs at "heat pump speed" even when for gas heat.

He also changed the fan in this mode from time-delay to a temperature sensor. When the heater turns off, the fan runs for awhile to cool the sensor down, then the fan stops and the sensor heats up a bit and the fan turns back on, then the sensor cools down and the fan shuts off again. That is annoying.

Another side-effect of the temperature sensor turning the fan on and off is that there seems to be a burnt smell when the system runs now. He just left about an hour ago. Maybe that burnt smell will go away.

Any thoughts?

Thanks,
Martin

 

Well, it has been a couple hours since the service man modified the system. The burnt smell is mostly gone, but I'm still wondering if the changes he made were for the best (the fan being controlled thermally, and the fan running at heat pump speed when I'm in emergency heat mode).

Thanks,
Martin

 

I don't want to judge the service tech without seeing the job but that don't sound right. You should have zero air bipassing the filters and if that is a gas furnace it should have no problem running at low speed.

I also am not sure what he put in there to cycle the fan but if he changed the design he just voided the ul listing.

 

Well, the smell never did go away completely, and the fan operation was annoying, so I just switched the fan to "Fan On" until I figure this out.

I've got the wiring diagram for the gas furnace but not the HP, and I checked out some books at the library. However, learning to be my own HVAC repair man isn't easy. I'd appreciate any advice I can get, but I'm not calling that service man back, becuase it seems pointless now. I might call him back after I can explain exactly what needs to be done.

Martin (reading books about heat pumps and gas furnaces)