Heat pump set back?

390 Views 11 Replies 6 Participants Last post by bob22, Jan 25, 2020

bob22

Discussion Starter · Jan 24, 2020

My daughter's rented house is heated/cooled by a heat pump (so all electric).
The other day she said it was putting out cold air (they've been setting it back to 61*, normal is 66* after a $400 bill last year).
Outdoor temp was likely 19* at the time.
Repair guy (an electrician son-in-law said) came out today and he said the guy said "both pipes were cold" (outdoor temp today was a lucky 50*). Son thinks the guy did something but not sure what.
Rental agent said their system is fine, to set it to 69* and not set it back and it won't need Aux heat.

My questions:
1. what is a reasonable set back to save some money (if there is one)?
2. How warm would the tubing be at the air handler? Would that temp change as outdoor temp drops?

Other advice/thoughts welcomed.

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Missouri Bound

· #2 · Jan 24, 2020 (Edited)

No setback for heat pumps.
The more efficient the heat pump, the cooler the tubing.
A better measure is at the supply and return vent.
A 15° differential is very good....on a 50° day that would be normal.
But anywhere from 10 to 20° can be expected, depending on conditions.
It all depends on the unit.
Remember that the units raise the heat of indoor air. If they start at 61° the most they can be expected to put out is around 70° - 75°.
And that may feel cool compared to skin temp.
And the colder it is outside the less effective a heat pump is.
Plus when you set back the thermostat, it takes longer to bring up the heat and then electric elements will kick in to help out. Keeping the thermostat at a constant temperature will minimize that event.

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user_12345a

· #3 · Jan 24, 2020

In heating mode the large insulated pipe should be hot and the smaller pipe luke-warm.

The air coming out of the outdoor unit should be cooler than ambient.

With pipes cold, the aux heat is probably carrying the entire load.

Setting back heatpumps with electric backup wastes energy.

A 15° differential is very good....on a 50° day that would be normal.

A 15F differential at 50F is very poor if the blower speed is set correctly.

At 400 cfm per ton, it's a little over half full capacity.

At 47 to 50F outdoor, should be getting full output - drops off slowly as it gets colder outside.

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beenthere

· #4 · Jan 24, 2020

No set back for heat pumps with electric aux heat. It just increases the heating bill when it brings on the aux heat during recovery.

when the heat pump is running in heat mode, the large line should be very hot. specially when its above 40°F outside temp.

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mark sr

· #5 · Jan 24, 2020

.... or you could do like I did - turn off the auxiliary heat strip. No matter how often my wife fiddles with the thermostat it doesn't affect the electric bill much. If it gets too cold for the HP to keep up by itself - it's a good excuse to start up the wood stove.

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user_12345a

· #6 · Jan 24, 2020

that works when the luxury of having a separate cheaper source of heat is there. in a a rental, not likely.

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Missouri Bound

· #7 · Jan 24, 2020

Leaving the thermostat at a temperature they find comfortable is probably the best idea.
Heat pumps were originally designed for warmer climates where cooling was the more important function. As they started to improve they moved north along with resistance heating and many with gas backup. Modern construction can have just any type of heat because of the insulation and tightness of new construction.
Existing homes, especially rentals are just stuck with what they have.
Your daughter can try closing the doors in a few rooms or even get a few small heaters for the rooms they use the most. It's hard to say sight unseen but they may find a way to cut down the heating expenses with some creative ideas.
Or they can move.

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bob22

Discussion Starter · #8 · Jan 25, 2020

Thanks to all for sharing some great info!

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user_12345a

· #9 · Jan 25, 2020

Let us know what happens.

it's worth getting a cheap digital thermometer and checking the temperature of the supply air with the heatpump running without the strips.

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roughneck

· #10 · Jan 25, 2020

A lot of folks get tricked every winter when it appears nothing’s wrong with their heat pump, then they get an insanely high electric bill due to the electric heat doing most of the work.
And keep that setpoint constant, the above posts are absolutely right when they say that the electric bill is being run up by setting the thermostat back.
If you must set back the thermostat, select a model that can lock out the auxiliary heat.

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mark sr

· #11 · Jan 25, 2020

user_12345a said:
that works when the luxury of having a separate cheaper source of heat is there. in a a rental, not likely.

I missed the rental part :blush:

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bob22

Discussion Starter · #12 · Jan 25, 2020

I've shared the thread with her and we'll see what happens. Last winter, one month they went mostly on aux heat due to broken part; luckily, landlord helped pay the $400 some bill.

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