Heat Pump and power consumption

[KTurner]

We just got an electric bill and the usage is the highest it's been in the 9 years we've been in this house. I need to figure out if something is malfunctioning or if this is a combination of the cold weather and my wife fiddling with the thermostat. Our heat pump is about 7 years old and replaced one that was about 25 years old. The heating degree days isn't very different from the same time last year, about 10% higher. This bill has us using 103kwh/day while the corresponding bill from a year ago was 70kwh/day, an increase of almost 50%. Dec 2017 bill is roughly the same as Dec 2016 bill, with 61kwh/day and 58kwh/day, so this appears to be a sudden change. I don't doubt these numbers as they are actual measurements and not estimates. I realize that defrost cycles and big gaps (>2F) in current indoor temp versus thermostat setting will both cause aux heat strips to come on. I think we're pretty good at avoiding big gaps between current and desired temp. Defrost cycles... not sure how to track or estimate this. When the outdoor temp drops, obviously the heat pump has to use more power to keep the house at the same temp. Would this be the same amount of power for a longer time or does the amount of current go up also (ignoring the aux heat and defrost cycles)?

I took a couple readings from the meter. For a 13 hour period overnight we used 82kwh (>6kw/h ave). Looking at how long it took the meter wheel to make a revolution, it was about 1kw/h when the heat pump was off (and everyone awake, lights, tv, coffee pot, etc on) and 3kw/h when it was running - implying heat pump was using ~2kw/h. If the heat pump uses the same power but runs longer, then something else is contributing >3kw/h on average that entire 13 hour period - but wasn't when I took the measurements.

Keith

 

[eng1886trk]

A lot will depend on how cold it was and for how long. I just recently experienced my highest heating bill ever, and our new heat pump, with electric back up is only 4 years old, but we just also had the coldest start to January in the last 100 years and did not break 32 degrees in 15 days. When you look at the temperature they put on electric bills, they are pretty deceiving as they are an average for a months time. The thing to remember is a heat pump is only good at reclaiming heat to about 20ish degrees if I remember, after that it runs constantly and requires the auxiliary heat strips to make up the difference. I kept an eye on my thermostat and my aux heat was coming on fairly regularly with a constant unchanged temperature setting. Now that temps have evened out again and we are out of the cold snap, the heat pump has begun to turn off and cycle normally again.

Reason #24235 why I can not wait to get my chimney relined and wood stove fired back up.

 

[frugalangler]

I have used a heat pump for the last 18 years, backed by a 3-stage gas furnace. What I found is that keeping the house constant temp and not using set-back except for very long duration periods (weekends away, etc.) and not short 4 to 6 hour periods is the most economical. We have found that the heat pump 'maintains' temps but isn't great at recovery from setback. Also, refrain from bumping the temp up because you feel 'chilled', put on a sweater or in our case a small space heater when we're watching TV, it's more economical to run it than keeping an unused large space at comfort temp. We also push our system to run down to about 25* before switch-over to gas occurs, as running the heat pump to maintain temp is less costly than on gas.

If it's extremely cold you may be running on backup electric heat more than you realized, like overnight when the temps plummet and costing much more than you realize, and if that's the case and the outside temps never achieve heat pump operation level you may benefit from using setback at night to conserve the use of electric heat backup power. We've actually found that when constant sub-25* temps prevail, we can lower the thermostat a couple degrees because the 'feel' of the gas backup heat is warmer than the 'feel' of the heat pump heat, haven't found an automatic way to do that though.

Managing energy we've found consistency helps, avoid the temptation to increase thermostat when not active (like watching TV) and using alternatives such as snuggle blankets and quilts, or small personal radiant space heaters.

Just some of our experiences, hope it helps.

 

[kcflhrc]

I have a 4 year old Heil Heat Pump with propane back up, no heat strips. Not sure how efficient a heat pump is below 30 degrees F. I have the lock out set on mine at 30 degrees so it automatically switches to propane under 30 degrees. A lot cheaper to burn $2.00 a gallon propane than have that outside unit struggling. We have had some long cold stretches here in Eastern Kansas this year but hopefully we are on a warming trend. We also use a space heater in the living room at night and one to take the chill off the bedroom before bed. Typically the thermostat is on 68 day and night in the winter.

 

[coldsteelva]

Below 30 degrees and you're essentially heating with a toaster element. The actual heat pump is useless; that's why some have the lockout to switch. If you are having a truly long cold spell, switch the system to 'Emergency' and that should lockout the heat pump compressor from running (reducing your overall electric use) and only running the resistance elements and the fan.

 

[LittleBill21]

these comments need to be watched, some heat pumps now are efficient down to -5F, they most certainly are still putting out heat at 30F

 

[kcflhrc]

they most certainly are still putting out heat at 30F

Probably so, but at what cost? The Heat Pump may run for 20 minutes vs the furnace may run 5 minutes on propane for the same result.

 

[Spindifferent]

Has the cost per kWh increased from previous years?

 

[Sigarms]

Below 30 degrees and you're essentially heating with a toaster element. The actual heat pump is useless; that's why some have the lockout to switch. If you are having a truly long cold spell, switch the system to 'Emergency' and that should lockout the heat pump compressor from running (reducing your overall electric use) and only running the resistance elements and the fan.

Depending on the heat pump, you're comment is far from spot on.

Currently I'm watching a church up in the mountains of Tennessee use 4 heat pumps with 4 gas furnace for back up heat.

At 10 degrees outside air temp, the furnaces won't even come on, and the church is maintaining 67 degrees inside unoccupied. The system doesn't have strip heaters.

KTuner, if you haven't changed anything with the system per wiring (new t-stat), my money is on the wife playing with the t-stat. As Frugalanger mentioned, try to keep the temps even without a huge lag between temps if you're using a programable t-stat. Programable t-stats are "supposed" to save you money, but if you program it to heat and the system has to make up more than 3 degrees, you're generally going to burn your heat strips. This is one advantage where a duel fuel system helps. Use the gas furnace to make up the lag difference then have the heat pump maintain. The last thing you want to do is have the t-stat on 69, then to try and save money, set it to 64 while you're away, only to turn it back to 69 when you return.

Years ago I had a spike in my electric usage by a good bit. Finally narrowed it down to the well pump, which was a pretty penny to replace.

 

[kenmac]

The reason for your high elec. usage is your aux heat strips are being energized /used. The colder it gets the more your hp will go into defrost.

Sounds like a good reason to switch to a VRF unit, mini split.
I have one mini split @10 deg. my mini split was putting out 114 deg air. No elect. heat strips needed !
I'm going to replace my other older hp when the weather permits