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-   -   Using Gas heat during Defrost (http://www.diychatroom.com/f17/using-gas-heat-during-defrost-89747/)

mgroves 12-16-2010 10:54 AM

Using Gas heat during Defrost
 
Recently, and on another DIY thread, I got some help regarding wiring my defrost signal back to the gas furnace so that the gas furnace will run during heat pump defrost. We finally got some cooler weather and both units are working fine....but I now realize the give and take, I believe of running backup heat during the defrost.

That is - now that the gas furnace is running full blast during defrost (I only have one speed - high), the outdoor units take twice or more as long to defrost given that the inner cool doesn't cool off as much, and thus the outdoor unit doesn't warm as much as quickly. So, there is a trade off, and that's fine, but my larger concern is, with the gas fire blowing on that indoor coil that the outdoor compressor is attempting to cool is this going to burn up my compressor much faster by doing the defrost this way? I know that on our carrier infinity in our other home, that the gas fire is limited to low setting, which barely warms the air coming out of the vents, but obviously allows for a faster defrost. Must be a reason for this, right? Is it to protect the compressor?

I do like the comfort as the system had been blowing 52 degree air up for about 3-4 minutes without the gas fire during the defrost. Now, with the gas fire the air coming out is actually warmer than the heat pump air (when the heat pump is heating the house). The thermostat doesn't get confused and want to go to a higher stage during defrost so that's another positive.

I guess though that if this is actually wearing my compressor unnaturally then I might just remove the wiring fix that I accomplished with the forum's help.

However, if this isn't all that harmful to the compressor, then I have another wiring question. My concern is that the thermostat decides during the middle of a now longer defrost that it should call on the backup heat (thus turning off the signal to the heat pump). I'd rather have the defrost cycle finish. So I wondered if I should wire the defrost signal coming from the outdoor unit back to the heat pump to make sure it stays on until the defrost signal goes back to zero. Has anyone done that?

It would seem to me that no matter what the thermostat is attempting to do (ie it turns on W or E and turns off Y in the middle of a defrost), the only time both the heat pump and furnace would be running together is during defrost (if I wire W from heat pump to Y of heat pump this will lock the heat pump on for the duration of the defrost cycle then return to whatever control the thermostat is giving), so why would this not work? To me it would seem this would already be programmed into the heat pump so defrost cycles would complete but I have had defrost cycles interupted by calls for gas heat in the past.

As always thanks for your comments,
Mike

Marty S. 12-16-2010 05:02 PM

Mike
With the w wire from the heat pump hooked to the w on the furnace the defrost will happen faster, not take longer. It's not going to hurt your compressor since that's the way they are designed to work.
Thermostats that change from HP to gas based on run time can switch in the middle of a defrost and I'm not sure if there's a way to eliminate that. One's that operate on temperature differentail will not, unless the room drops another half degree waiting for the furnace to come on.

beenthere 12-16-2010 06:06 PM

The furnace running during defrost shortens the defrost time, since it is putting heat into the coil.

If the stat switches to furnace for heat in the middle of a defrost, so be it. It does no harm. And when it warms up. The defrost board will decide if it needs to defrost the coil or not.

Don't try to force it to finish the defrost cycle.

mgroves 12-17-2010 03:34 PM

Well, this sounds very good. It just seemed to me that having a full on gas fire with temperatures of the air at about 135 degrees might be outside the limits of the "air conditioner" side of the heat pump which is what is happening during a defrost. I mean one certainly couldn't expect to cool that air in the summer time with a heat pump or air conditioner so I thought that it would tax the compressor due to the whole hot/cold cycle engineering.

I thought that there were limits on the air source temperatures and perhaps that's why the Carrier Infinity System at my other house only goes low fire on defrost. According to what you are saying, if there was a way for me to do it, I should set the Carrier Infinity to HIGH FIRE so that the defrost would get done faster and the temperature in the house would not drop as is currently the case. Why didn't the engineers simply do this rather than set it on low? Stupid I guess.

Thanks to all for their responses.
Mike :thumbup:

beenthere 12-17-2010 05:25 PM

If the outdoor coil temp rises too fast. The defrost sensor would terminate defrost before the coil was completely thawed. Like many things. too far in either direction doesn't work as good as it does in the middle.

The compressor isn't taxed with the furnace running. Because the liquid refrigerant is very cool entering the indoor coil.

Now if you had an over sized oil furnace. Then it would be a slightly different story, and you would need a bonnet sensor to shut off the oil furnace before it got too hot.

serviceermanman 12-18-2010 10:32 AM

Quote:

Originally Posted by mgroves (Post 552061)
Recently, and on another DIY thread, I got some help regarding wiring my defrost signal back to the gas furnace so that the gas furnace will run during heat pump defrost. We finally got some cooler weather and both units are working fine....but I now realize the give and take, I believe of running backup heat during the defrost.

That is - now that the gas furnace is running full blast during defrost (I only have one speed - high), the outdoor units take twice or more as long to defrost given that the inner cool doesn't cool off as much, and thus the outdoor unit doesn't warm as much as quickly. So, there is a trade off, and that's fine, but my larger concern is, with the gas fire blowing on that indoor coil that the outdoor compressor is attempting to cool is this going to burn up my compressor much faster by doing the defrost this way? I know that on our carrier infinity in our other home, that the gas fire is limited to low setting, which barely warms the air coming out of the vents, but obviously allows for a faster defrost. Must be a reason for this, right? Is it to protect the compressor?

I do like the comfort as the system had been blowing 52 degree air up for about 3-4 minutes without the gas fire during the defrost. Now, with the gas fire the air coming out is actually warmer than the heat pump air (when the heat pump is heating the house). The thermostat doesn't get confused and want to go to a higher stage during defrost so that's another positive.

I guess though that if this is actually wearing my compressor unnaturally then I might just remove the wiring fix that I accomplished with the forum's help.

However, if this isn't all that harmful to the compressor, then I have another wiring question. My concern is that the thermostat decides during the middle of a now longer defrost that it should call on the backup heat (thus turning off the signal to the heat pump). I'd rather have the defrost cycle finish. So I wondered if I should wire the defrost signal coming from the outdoor unit back to the heat pump to make sure it stays on until the defrost signal goes back to zero. Has anyone done that?

It would seem to me that no matter what the thermostat is attempting to do (ie it turns on W or E and turns off Y in the middle of a defrost), the only time both the heat pump and furnace would be running together is during defrost (if I wire W from heat pump to Y of heat pump this will lock the heat pump on for the duration of the defrost cycle then return to whatever control the thermostat is giving), so why would this not work? To me it would seem this would already be programmed into the heat pump so defrost cycles would complete but I have had defrost cycles interupted by calls for gas heat in the past.

As always thanks for your comments,
Mike

YOU GUYS ARE NUTTS YOUR GOING TO BE SPENDING EXTRA CASH THIS SUMMER OR THE NEXT ON A LEAKY A COIL WHEN THE GAS FURNACE AND THE AIR ARE RUNNING AT THE SAME TIME THE PRESSURES ARE GOING TO CREAT A LEAK MARK MY WORDS DONT ALLOW THE UNIT TO DEFROST AT ALL TERMINATE HEAT PUMP AT 36 38 DEGRE F AND USE YOUR GAS:thumbsup:

Marty S. 12-18-2010 10:52 AM

Quote:

Originally Posted by serviceermanman (Post 553135)
YOU GUYS ARE NUTTS YOUR GOING TO BE SPENDING EXTRA CASH THIS SUMMER OR THE NEXT ON A LEAKY A COIL WHEN THE GAS FURNACE AND THE AIR ARE RUNNING AT THE SAME TIME THE PRESSURES ARE GOING TO CREAT A LEAK MARK MY WORDS DONT ALLOW THE UNIT TO DEFROST AT ALL TERMINATE HEAT PUMP AT 36 38 DEGRE F AND USE YOUR GAS:thumbsup:

Naa. Coils are rated at 450 psi and you don't get anywhere near that. Run one in defrost with the gas on and see what your suction pressure is. It's not going over 125 even with 410A.

mgroves 12-18-2010 11:18 AM

My heat pumps defrost even with temps as high as 43 degrees - I mean yes the outside coil will get cold enough to engage the defrost timer and 90 minutes later it will defrost. So, setting the lockout at 36-38 won't do much good.

I do see however that there is controversy regarding that much heat being sent over the coils in the indoor unit, which would explain why Carrier only allows low fire to operate during defrost (and that's basically worthless). I've searched online but there doesn't seem to be anything wrong with hooking up the defrost signal to the backup gas and there is no mention of making sure its a low stage gas furnace.

Maybe the answer to this question can help the debate - how many heat strips get turned on in defrost on a heat pump with strip heat backup? All of them? Why or why not all of them? Because that's essientially what I am doing - I would think this 75000 btu gas furnace at 80% is still hotter than full strip heating. I also suspect that full strip heating wouldn't be engaged and part of the reason might be to save energy. But I don't know so I'm asking.

Mike

beenthere 12-18-2010 12:45 PM

Quote:

Originally Posted by serviceermanman (Post 553135)
YOU GUYS ARE NUTTS YOUR GOING TO BE SPENDING EXTRA CASH THIS SUMMER OR THE NEXT ON A LEAKY A COIL WHEN THE GAS FURNACE AND THE AIR ARE RUNNING AT THE SAME TIME THE PRESSURES ARE GOING TO CREAT A LEAK MARK MY WORDS DONT ALLOW THE UNIT TO DEFROST AT ALL TERMINATE HEAT PUMP AT 36 38 DEGRE F AND USE YOUR GAS:thumbsup:

I have lots of dual fuels installed that go into defrost and bring on the gas furnace(some are on oil and it runs during defrost). Non of them have leaked because of that.

beenthere 12-18-2010 12:47 PM

Quote:

Originally Posted by mgroves (Post 553162)
My heat pumps defrost even with temps as high as 43 degrees - I mean yes the outside coil will get cold enough to engage the defrost timer and 90 minutes later it will defrost. So, setting the lockout at 36-38 won't do much good.

I do see however that there is controversy regarding that much heat being sent over the coils in the indoor unit, which would explain why Carrier only allows low fire to operate during defrost (and that's basically worthless). I've searched online but there doesn't seem to be anything wrong with hooking up the defrost signal to the backup gas and there is no mention of making sure its a low stage gas furnace.

Maybe the answer to this question can help the debate - how many heat strips get turned on in defrost on a heat pump with strip heat backup? All of them? Why or why not all of them? Because that's essientially what I am doing - I would think this 75000 btu gas furnace at 80% is still hotter than full strip heating. I also suspect that full strip heating wouldn't be engaged and part of the reason might be to save energy. But I don't know so I'm asking.

Mike

The heat strips are after the coil. So it has no bearing on dual fuel systems.

mgroves 12-18-2010 01:56 PM

Good point. :)

I found this written in a user's guide for a dual fuel tstat -

During operation of HP in heating mode, HP itself determines
when a defrost is necessary. It initiates defrost by energizing its O
and W wires. The signal on the O wire switches HP from heating
to cooling mode and W signal starts furnace. Thermostat monitors
this action by sensing the signal (which it did not create) on the O
line. It responds by turning on its W outputs (both if 2-stage
furnace) to hold furnace on high heat. At completion of defrost,
indicated by removal of signal from HP on O wire, thermostat does
1 of 2 things. If it is satisfied, it turns off all Y, G, and W outputs
which results in all equipment off. If not satisfied, it will turn off
Y and G, leaving W on until it becomes satisfied. In this way it
assures that furnace will be used to satisfy a heat demand existing
after a defrost cycle is completed.

Note says HIGH HEAT and my carrier furnace doc clearly indicates what it does during defrost (it doesn't turn off the fan immediately while doing its other pre fire items and even shortens the purge period) so it is clearly allowed to be used.

The only other thing that comes to my mind is since both gas furnaces are the same size but the heat pumps are 2 ton and 3 ton, would the 2 ton be affected given that it isn't producing as much cold in the coil when being heated up by the same fire as the 3 ton? Is that going to create too high a pressure? I did read some place else that the gas fire isn't going to be able to heat the heat exchanges and get really hot before the defrost is over with, and that makes sense to. The air from the registers is only slightly warmer than the heat pump normally, and aren't heat pumps supposed to shutdown on high pressure conditions? Am I not protected from all of this high pressure therefore?

Thanks Mike


beenthere 12-18-2010 02:01 PM

That doc is full of BS. The thermostat doesn't monitor the defrost. And the heat pump doesn't energize the O terminal.

Who ever wrote it. Was out to lunch while writing it.

mgroves 12-18-2010 05:11 PM

lol...its part of the user's manual for a Carrier Duel Fuel Thermostat.

Mike

beenthere 12-18-2010 05:24 PM

Quote:

Originally Posted by mgroves (Post 553345)
lol...its part of the user's manual for a Carrier Duel Fuel Thermostat.

Mike

The tech writer is full of it.

The writers of those manuals. Read what an engineer writes down. And then they try to translate it to something non engineers can read.

mgroves 12-18-2010 05:40 PM

Well, I am just glad it says that the furnace is set on HIGH HEAT! That means I probably won't screw up my inside coil. I like having the heat on while defrosting. So I don't want to change it back to simply blowing out cold air while defrosting. I still can figure out why the installer hooked this W coming from the outdoor unit to the W2 on the thermostat. He obviously had no clue! Perhaps I should have an independent installer come out and check my Puron levels. Do you think?

Mike


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