Heat Pump and power consumption

[KTurner]

And knowing its amp draw.....I can compute heating cost.

My main reason for starting this thread was that my HP was drawing more than I expected, due to defrost cycles (I'm pretty sure). As far as I can tell, the thermostat doesn't know a defrost cycle is happening. And during defrost, my unit is drawing about 8 times as much current than normal operation - due to the heat strips. Do you somehow factor this into your computed heating costs? How?

Keith

 

[LD1]

My main reason for starting this thread was that my HP was drawing more than I expected, due to defrost cycles (I'm pretty sure). As far as I can tell, the thermostat doesn't know a defrost cycle is happening. And during defrost, my unit is drawing about 8 times as much current than normal operation - due to the heat strips. Do you somehow factor this into your computed heating costs? How?

Keith

Dont have strips and dont defrost because I have geothermal.

But If you had an ecobee....I am sure the chart will tell you when strips are running. Wattage of the strips vs how long they are on gives you all you need to compute cost.

 

[WM75Guy]

I wish I could find a thermostat that would allow one setting for heat and AC but they all have a 3 to 5 degree spread. If you put it on auto and set the heat to 70 the AC won't come on till it's 73-75.

Our Trane autoseason thermostats will allow a 2 degree spread. We keep our thermostats set at a 68-72 range year round, except at night the bedroom goes to 66-72 from 10pm to 6am. The house is 3200 square foot all electric with a 3 1/2 ton HVAC unit for the living areas and a 2 1/2 ton unit HVAC unit for the bedrooms. The stables has a Goodman mini split unit for the tack room/bathroom. All totaled there are four refrigerators, 3 hot water heaters, and 4 1000W arena flood lights.

I am very pleased with our Trane units and our electric bill. The average bill last year was $283 with a range of $203 in April to $448 in September. The electric coop went with smart meters that allow you to look at usage by the hour. By looking at it I found my biggest power user is the clothes dryer. We have 2, one in the house and one in the stables.

When we built I used roof decking with a radiant barrier. The attic has R44 insulation. The windows are insulated double pane. The majority of the lights are fluorescent in the kitchen, pantry, laundry, closets, and all lamps. The rest is mostly can lights that are being switched to LED as they burn out.

 

[KTurner]

Dont have strips and dont defrost because I have geothermal.

But If you had an ecobee....I am sure the chart will tell you when strips are running. Wattage of the strips vs how long they are on gives you all you need to compute cost.

What I'm saying is I don't think it can do that, at least not completely. From what I can tell... There isn't a standard input on the thermostat that listens for a 'the heat strips are running' signal. If the thermostat is what tells the heat strips to come on, then yes, it could report on that (but it would just be assuming the heat strips are running). A defrost cycle doesn't involve the thermostat - it is triggered by the control board in the outside unit. For me, each defrost cycle can chew through about 2kw and can happen every 30 minutes or so.

 

[techman]

For me, each defrost cycle can chew through about 2kw and can happen every 30 minutes or so.

Average is closer to 1 KWH for a 10 K strip. You are correct that it could happen every 30 min or so, but only when very cold and they are likely already cycling on. The most common issue is either the heatpump is undersized and can't keep up, or charge is too low (making it ice up when it should not be) or too high (reducing overall efficiency). As I had commented earlier, a unit with "demand defrost" does a much better job and eliminates "frivolous" defrosts.

paul

 

[LD1]

guessing its an undersized system. As said....contractors like to size for cooling for whatever reason. And heating needs of an average house in my area are at least double the cooling needs. Which makes the average "recommended" system grossly undersized. Thus the reliance on strips and lots of defrost.

No idea where Kturner is located......so cannot say if this is the issue. Obviously somewhere like florida or texas probably has more cooling needs than heating....but here in the midwest.....that aint true. And if you let a contractor talk you into sizing for AC......then your system will never be cheap to operate in the winter

 

[kenmac]

Wonder if the unit is having a TXV issue with your hp. TXV on my son's unit was sticking causing the unit to switch back and forth between HP and strip heat. Replaced the TXV and now it's working great.
My son's chief complaint was... My power bill is high !

It's hard to tell you what may be wrong with your unit from behind a computer screen. All we can do is give suggestions

 

[goeduck]

As I had commented earlier, a unit with "demand defrost" does a much better job and eliminates "frivolous" defrosts.
paul

Paul, could you explain this feature? Is it only on modern heat pumps?

 

[techman]

Paul, could you explain this feature? Is it only on modern heat pumps?

Most heat pumps (90% or so) use Time-Temp Defrost. When the unit runs, if the outside coil is below about 35 deg, a timer starts. If the timer times out (typically 30, 60 or 90 minutes) a defrost will start. It will end when the coil thermostat reaches about 45 deg. If the unit cycles off before the timer is reached, everything resets and it will start again when it runs again. Likewise if the coil is above the 35 deg point, the timer never starts. So when it is very cold and the heat pump runs long periods, it will defrost every time the timer is reached.

Demand defrost has been a feature of high end heat pumps, usually only on the premium models. It is, unfortunately, not a modern vs old heat pump, but an upgrade feature. Demand defrost uses temperature measuring sensors on the coil and for outside temps. When the outside temp is below a set value (typically about 35 deg), the coil and outside air temp should be within 5 deg or so. As the coil ices, the coil will get colder than the air, since air can not flow through the coil to warm it up. When this coil-ambient difference hits a threshold value a defrost is initiated. It is ended when the difference is again below 5 deg or so. There is also a timer to end defrost if the temps are never reached. Normally a defrost will occur only when needed due to icing.

If you read my post #24 on this thread you will see that a replacement/retrofit defrost controller is now available for most heat pumps. You can have one installed by a service tech, but consider the cost if you do not see an excessive number of defrosts in your local climate area.

paul

 

[kcflhrc]

My Heil Heat Pump has a wire strap for the defrost cycle, you either put it on 30, 60 or 90 minutes. I have mine strapped to 60 minutes. Which basically means the unit will defrost every 60 minutes of run time. If the defrost cycle initiates the outside unit automatically shuts off, then the propane lights and runs on propane until the defrost cycle is over. Then the propane shuts off and the heat pump takes over again. Pretty simple stuff actually.