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Heat pump question


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I tested a heat pump today on a job where outside temperatures were in the mid-50's. When I set the thermostat to a desired temperature only 2 degrees higher than the interior temp shown on the t'stat, I was surprised to find that the heat strips kicked in also. In other words, I couldn't get the unit to kick on without the heat strips coming on also. Clearly there's a problem. Is this a wiring issue? Heat pumps only comprise about 5% of what I see so I am not thoroughly versed in the nuances of their operation. I realize that telling the client why this is happening isn't something I need to do but I want to know. What do you guys say about this?

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Bill, why would that be the case on a "warm" day? It certainly isn't what I normally see here. Normally I can bump up the temperature two or 3 degrees without the heat strips coming on. More than that, and they do. Less than a two degree bump and the system won't come on at all. With a relatively warm outside temperature and a scant 2? increase in interior temperature desired why would the auxiliary heat kick in?

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It doesn't matter what the temp is outside - it's how most heat pumps operate. When the heat pump system "sees" that the room temp is 2 degrees below the t-stat setting, it "thinks" that it is not keeping up and kicks on the back up heating to catch up.

Pull the cover off one of the older style mercury heat pump t-stats and watch the 2 bi-metal and mercury bulbs. As you raise the temp 1 degree, the first bulb engages the heat pump. Usually, at the 2nd degree rise, the mercury in the second bulb flows down to the contacts engaging the back-up heating mode.

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With older thermostats (i.e. mercury switches) it typically takes about a 2 degree difference to bring on the backup heat. To test in the heat pump mode I turn off the thermostat, remove the cover, set the temp so only the top switch is on, then turn the thermostat to the heap pump mode. Then to keep it only running on heat pump I sometimes have to tweak the setting a bit.

On digital thermostats there is about a 1 degree spread. Raise the temp more than one degree above the room temp and the backup will come on (unless there is an outside thermostat to override this).

That is the common setup around here, which I think is about what Bill said.

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This was new construction with a digital stat. Similar installations I've found didn't behave this way where a two degree bump triggered aux heat. In this case, a 1 degree bump on the stat didn't trigger the unit to come on at all. So how do you tell if the heat pump works solo w/o heat strips?

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This was new construction with a digital stat. Similar installations I've found didn't behave this way where a two degree bump triggered aux heat. In this case, a 1 degree bump on the stat didn't trigger the unit to come on at all. So how do you tell if the heat pump works solo w/o heat strips?

You just have to be more gentle and patient.

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I don't know if this is common elsewhere, but many newer heat pump installs here have an outdoor sensor and a "heat strip lockout" feature that prevents the heat strips from coming on if the outdoor temp is above 35 degrees, unless you specifically select emergency heat on the stat (which requires going to a secondary screen). Virtually all older systems run the heat strips if you raise the set point. I've checked out many high bill complaints from folks who goose the stat whenever they feel cold, then turn it back down when they're warm... all day long.

If I want to run a heat pump, I usually just leave the front door open for a short period until the stat gets cold.

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I doubt that the temp sensor on any resid thermostat can reliably detect that 2 degree change. On the digital devices with their digital readouts, the signal from the sensor is interpreted by circuitry that determines what the call will be from the thermostat as well as what temp will be displayed on the readout.

Except for this minor detail, Bill K's response is right on the money.

Just my own opinion.

Marc

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