# Heat pump is not running on Heat mode



## Plumber101 (Feb 25, 2009)

Hi, we bought a house with a heat pump and an electrical furnace as auxiliary heat. Unfortunately this winter we noticed that only the furnace was working to heat the house. The temperature was a little bit warmer today so I decided to run a couple of tests on the heat pump (Carrier 38YXA) and the furnace. I made sure that every wires (R, O, Y, G, C, W)Are these the only connections at the thermostat? were connect correctly at the thermostat, at the furnace and at the heat pump. I tested the load at the heat pump which was showing 240 V entering the contactor for an output of 120 V.

Since you have a heat pump you need a W2 or AUX. With the stat you have it appears it is for 1Heat and 1Cool and you need 2Heat and 1Cool 

I even disconnected the W1 cable at the thermostat and notice that the furnace was not working except for the fan (G wire). I had an older non-programmable thermostat so I even swap it but I ended up with the same result.

Then I remembered that the heat pump was working fine at the end of the summer, so I figured I could test it in COOL mode. So I did and it was working perfectly! The air coming out of the vent was cool and the heat pump appeared to be working. 

After all these tests the heat pump is still not working in HEAT or EM HEAT mode and I'm out of any idea. It looks like whatever thermostat I use, it always send a signal to the emergency heat. Anyone have an idea of what could be the problem ?


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## frank06 (Mar 11, 2012)

Hi Plumber101
1. Yes they are the only connection to the thermostat
2. Wire identified W2 on heat pump and furnace was connected to AUX on thermostat. 

I noticed a sensor in the heat pump with a mechanical dial from -30 to 100. I figured it's probably a Fahrenheit degree switch. It was set to 40 and now weather is around -15 C (5 F). What would actually be its purpose ? Can I set it down to 0 or -20 ?


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## Plumber101 (Feb 25, 2009)

Sounds like you have an Outside Temp Sensor (OTS). 

I usually set an OTS at 35Deg. Anything below 35 and yes the heap pump still works but, at a very low efficiency. Meaning you will have a better return on the dollar for heat using another form of heat than a heat pump.

But, I may be talking about something totally wrong. If you could post a pic of the sensor I could be sure.


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## hvactech126 (Nov 11, 2010)

it does sound like an ODT, however, on an ALL electric system you are more efficient to run the HP with the backup heat and it may be wired improperly and is shutting off the HP and turning on the aux heat instead of continuing to run the HP along with turning on aux heat below the setpoint of the ODT.


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## frank06 (Mar 11, 2012)

I'll go back outside, open the heat pump and try to take a couple of pictures. 

Ok so am I right to assume that the sensor will set the lowest temperature for the heat pump to work with the auxiliary heat (as if both should run at the same time) ? Below that point only the auxiliary heat will run. Is the system suppose to run that way, both the heat and the furnace ?

Should I leave the dial set to 35-40F considering that during the months of December-January-February the highest temperature is rarely higher than -20C (-5F). I had an electricity bill of nearly 700$ for two months so I'm trying to see if I should start the heat pump during those months.

I initially thought the dial was to set the lowest outdoor temperature for the Cooling mode to start.


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## Plumber101 (Feb 25, 2009)

frank06 said:


> I'll go back outside, open the heat pump and try to take a couple of pictures.
> 
> Ok so am I right to assume that the sensor will set the lowest temperature for the heat pump to work with the auxiliary heat (as if both should run at the same time) ? Below that point only the auxiliary heat will run. Is the system suppose to run that way, both the heat and the furnace ?
> 
> ...


The Aux heat shouldn't run until it is absolute needed. Like I said before I set a condenser to kick out about 35 deg. The efficiency of the condenser is very low and continues to decline as it get colder outside. You may be throwing money away running the condenser at very low outside temps. A heat pump thermostat should have temp set points as to when each come on. Such as when the indoor temp drops 2 deg below desired temp the heat pump will kick in and 3 or 4 deg below desired temp the aux heat will kick in. But it depends on the thermostat you have and how it is set up.

To me running both the aux heat and condenser at the same time below 35 deg is a wast of money.

And I do believe you have an outside temp sensor.


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## frank06 (Mar 11, 2012)

Here are the pictures of the dial and the sensor. You can see I set the dial to zero and the heat pump did start. I'm happy with it, now I just need to determine what should be the best setting for it. 
Thermostat is a Honeywell RTH8500 and I verified the settings (every settings are set to what seems the most obvious (Heat pump with auxiliary, electric heat, heating cycle and emergency cycle are set to Electric furnace).


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## Plumber101 (Feb 25, 2009)

Looks like an outside temp sensor to me

It's set a 0deg which is 32 deg below freezing already. I wouldn't set it any lower if anything I would set it higher.


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## frank06 (Mar 11, 2012)

Thank you very much for your hints and help. I will probably try to set it to 15-20 F from now on and I will confirm with a company or two what setting do they actually use in the city.


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## beenthere (Oct 11, 2008)

You don't need that ODS. Its costing you money. Leave it set to 0, or even set it to -10.


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## hvactech126 (Nov 11, 2010)

in normal heating mode the HP will run, once it can't keep up the aux heat will kick in and run with the HP. Beenthere is correct, set it low! Plumber is wrong in setting the ODT on an all electric system at 35F. That will cost you more in energy. In Emergency heat mode only the electric furnace will run.


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## Plumber101 (Feb 25, 2009)

So, Hvactech what is the efficiency of a HP at 0deg, compared to cost of operation.


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## hvactech126 (Nov 11, 2010)

> Electric heat is cheap to install and very reliable, but the heat it produces is over 3 times more expensive than heat produced by the heat pump. For this reason, we want our heat pumps to continue to operate in the bitter cold and use only as much electric heat as necessary to “supplement” and keep the home warm. So even though the heat pump is running 24/7 and still can't keep up, the heat that it is producing is still much cheaper than electric resistance heat. The combination of electric heat at 100% efficient and heat pump heat at 300-400% efficient result a total heating system that is more than twice as efficient as electric heat alone. You could also say that adding a heat pump to an electric furnace will cut your heating bills by about 60%.
> 
> Trane heat pumps achieve efficiency ratings between 300-400%. How can any system exceed 100% efficiency? Rather than burning fuel to create heat like a gas furnace, a heat pump uses electricity to move/transfer/pump existing heat from outside to inside. Because it is only *transferring heat*, a heat pump can burn 100 watts of electricity to produce as much as 400 watts of heat. By comparison, a gas furnace would burn 100 watts of natural gas to produce only 95 watts of heat. Heat pumps are awesome!
> 
> You can think of this outdoor to indoor heat transfer like a car driving uphill. On a steeper hill, the car must work harder. Likewise, colder outdoor temperatures require more work from your heat pump. This relationship between outdoor temperature and heat pump efficiency means that efficiency is a curve rather than a set number. For this reason, the industry has established HSPF (Heating season performance factor) as an average measure of heat pump efficiency throughout the winter heating season.



Quoted from :
http://www.watkinsheating.com/blog/heat_pump_operation_101/
http://www.watkinsheating.com/blog/heat_pump_operation_102/

(this is not my blog) This was easier than putting it in my own words.


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## Houston204 (Oct 18, 2009)

Sounds like the wrong stat or an improperly configured stat.
What is the make and model of this stat?


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## beenthere (Oct 11, 2008)

At 0 degrees outdoor temp. Many heat pumps still use 1/3 less electric for every BTU they provide over electric strip heat.

Some older ones(8 SEERs) are only as efficient as electric strip heaters when its 0 outside.


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## beenthere (Oct 11, 2008)

At 0 degrees outdoor temp. Many heat pumps still use 1/3 less electric for every BTU they provide over electric strip heat.

Some older ones(8 SEERs) are only as efficient as electric strip heaters when its 0 outside.


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## hvactech126 (Nov 11, 2010)

here is the info on this heat pump it is not 8 seer. http://www.commercial.carrier.com/wcs/proddesc_display/0,,CLI1_DIV41_ETI4926_PRD710,00.html

The HP should still be more efficient that the aux at 0F


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