heat pump running nonstop vs. using aux mode

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  #1  
Old 01-18-05, 08:29 AM
calilou
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heat pump running nonstop vs. using aux mode

I know you're supposed to set a heat pump at whatever temp you want and then not touch it after that, but how can it be more efficient running nonstop during cold snaps than just turning it up for a few minutes and letting it go into auxiliary mode then it shuts off? It would seem logical that it would use as much if not more electricity to have the fan constantly running than just bringing it up to temp and letting it shut off. I'd appreciate any thoughts on this.

Also, I read in other posts that the temp coming out of a heat pump is whatever temp you set it at, but I can have the thermostat set at 70 degrees and when it's in the 30's outside, the cool air coming out of the registers actually drops the temp in the house until it goes into aux mode. Is this normal?

Thanks!
 
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Old 01-18-05, 08:44 AM
Carltine
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From what the service man just explained to me today. Running the fan constantly does not mean that the "heat pump" is running (aka compressor?) Running the fan just maintains the temp throughout the house keeping the temp more consistant. Allowing the aux temp to come on means (I think) that the furnace is using electicity to heat coils up within the furnace. Then the blower blows the "warmed air" through the ducts in the house. Hence, the further away from the furnace you are...the colder the "warmed air" because it cools as it passes through the ducts. He said the heat pump is very very efficent when the temp outside is between 40 and 60 degrees. Anything under or over that temp is basically baseboard ele. heat and very expensive. He also said to prevent aux heat from coming on but to turn up the temp in the home, turn up the thermostat in one degree incriments only (1 degree at a time until the temp is reached then up 1 more degree if needed) Now, I may be completely wrong and I am passing information 3rd hand but this is what I have been lead to believe and I take no offence in being corrected if somone out there knows more or knows better. I hope this basic information helps or leads you to what you are looking to know.
 
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Old 01-18-05, 10:33 AM
calilou
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Thanks. I have also been told to bring up the temp a degree at a time, it just seems to take forever to get up to temp that way and it seems like it will would use less electricity just letting it run in the aux. heat mode for 15 minutes than for the fan to run in normal mode for 3 hours to come to the same temp. Unfortunately I live in Virginia where heat pumps shouldn't be used. It gets too cold in the winter here. Today it's about 20 degrees and forget trying to get warm when it's below 40. We use space heaters to suppliment when it gets this cold. Fortunately, we live in a rental right now. When we build our house, it ain't havin' a heat pump!
 
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Old 01-18-05, 10:37 AM
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Carltine, No correction. You're pretty much on the mark. In winter time with a heat pump, the fan should be set to "auto". Some believe the fan should be set to on to maintain a more even temperature distribution, but it often causes draftiness and may encourage you to turn the temp up higher than it needs to be. calilou, I think, is referring to the fact that the whole system has to run constantly during colder weather to maintain the set temp. I've read opinion after opinion about this and the majority of "experts" say that leaving the heatpump on to help maintain the temp as the auxiliary heat cycles in and out saves a little bit of energy, but turning the temp up to get rid of the draft, negates all the savings. In my experience, once the temp. gets below freezing, the house is much more comfortable if you turn on the auxiliary heat.

The statement that a heatpump puts out air at whatever temperature the thermostat is set to is confusing: A heatpump will warm the air passing through it's coils as much as it can based on the amount of heat it can pull from the outside air. This often means that it provides very little temperature rise in the air passing through, but can provide just enough heat to maintain the temperature.

Doug M.
 
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Old 01-18-05, 02:23 PM
calilou
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Thanks Doug M,
That's exactly what I was wondering. Whether it really is more economical to leave the thermostat set at a constant temp and letting it run nonstop for long periods of time. I can see where that might be more efficient than cranking it up for a blast of warm air. Although, I'm with you when you said that when it drops below freezing that blast of warm air in aux. mode is welcome! I guess the bottom line is that when the temps drop, you don't have much choice but to let it run when you have a heat pump. It may be efficient when the temps are between 40 and 60, but truthfully, I'd take any other form of heating system over a heat pump.

Thanks,
Jeanne
 
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Old 01-18-05, 02:58 PM
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calilou, You should be able to switch the system to Auxiliary or Emergency heat via a switch on the thermostat. This bypasses the heat pump and heats with only the resistance heaters. That's what I used to do whenever temps dropped or were expected to drop below freezing (new house has gas furnaces...no more heat pumps). I found there to be little increase in the electric bill and great increase in comfort.

Doug M.
 
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Old 01-18-05, 02:59 PM
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calilou: Its all in the cost of what you want for heat. Go to http://www.warmair.net and you can compare fuel cost for where you are. About all we sell now inMissouri are heat pumps. They cost the lowes to heat a home here.And yes we let them run down as low as it gets. today hit 0o and yes they ran with the Aux kicking in now and then . We try to tell people that they can set the tstat to what they want and let it alone, Also stay away from in front of any registers. Thats like you dont want to use a program tstat on a heat pump .Because after the set back and it kicks back up to the temp the Aux come on to help so you burn up what you saved with the set back.
Also the COP on a pump at 30o is still where you get about 3 times more heat for your dollar as you would with Aux strips heaters and the same dollar.

my .02 cents
ED
 
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Old 01-18-05, 05:42 PM
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Here's a nice way to see what you might be saving..

Let the heat pump run, without the strips on, go look at your meter.
Now, let the strips come on, and go look at the meter again.

I didn't know that wheel could spin that fast! I do anything I can to keep those strips from coming on. If I knew how, I'd disable them entirely.

The hot water heater (currently a 50g electric) is the largest energy consumer in my home. Under that is the heat pump as long as the strips aren't on. If the strips are on, it's like having 4 hot water heaters running at the same time.
 
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Old 01-19-05, 01:03 PM
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Watching the meter spin can be a bit deceptive since electricity is billed by use over time (kilowatt hours).

If a heat pump uses 5 KW and the strip heaters use 20 KW (about 4 water heaters...) and on an average day the heat pump runs 20 hours or the strip heaters run 8. Every once in a while the heat pump defrosts and uses strip heaters (let's guess around 10 minutes out of every hour) so 3 hours of the 20 running hours are using 25 KW. Do all the math - Strip heaters - 8 hours X 20 KW = 160 KWH. Heat pump - 17 hours X 5 KW = 85 KWH + 3 hours X 25 KW = 75 = 160 KWH.

Reality may not be quite so close, but we're still probably only talking about 1 or 2 more KWH a day to run the strip heaters. 2 more KWH per day at my local rate of about 12 cents per KWH would only raise the electric bill 7 dollars for the month. If you're bumping the temp up in heat pump mode to get a blast of warm air or because of the draftiness, the heat pump is probably costing more than the strip heaters to run.

Energy use is a complicated issue. Don't be afraid to experiment. Different things work for different people. Maybe you could use heat pump mode while at work and switch to Em. Heat when at home or maybe setting the thermostat 5 degrees lower while at work, something you can't really do with a heat pump, would save more than the heat pump is saving. Try and see.

Enlightening story: I've been harping on the kids to turn off lights for years. Finally, one day my wife pointed out that our total utility bill varied very little from month to month no matter how much I yelled, but one trip to the doctor’s office for hyper tension would cost over $100. She was correct.

Doug M.
 
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Old 01-19-05, 02:46 PM
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>Watching the meter spin can be a bit deceptive since electricity is billed by use over time (kilowatt hours).

If you're using the most expensive method possible all the time, then it adds up..

> and on an average day the heat pump runs 20 hours or the strip heaters

I think the most mine has ran over a 24 hr period has totaled 9 hrs, but I'm assuming here that you're just throwing numbers out.

>thermostat 5 degrees lower while at work, something you can't really do with a heat pump
Why not?

> I've been harping on the kids to turn off lights for years.

I've been telling my dad for years that the TV and lights don't amount to squat on the power bill, he still doesn't believe me. I know the biggest variance for me is the water heater. Take a few whirlpool baths during the month and the bill goes up $50.
 
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Old 01-19-05, 04:08 PM
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If you're using the most expensive method possible all the time, then it adds up
Running something that uses 5 KW for 4 hours is the same as running something that uses 20 KW for 1 hour. If the heat pump takes 5 hours to do what the resistance heat can do in 1, the heat pump is the most expensive method of heating. As the temperature outside decreases, the heat pump provides less and less heat while continuing to use the same amount of electricity to do it. As that happens, the heat pump becomes more and more expensive to operate and, once you reach a certain outside temp, is using energy and providing nothing.

I think the most mine has ran over a 24 hr period has totaled 9 hrs, but I'm assuming here that you're just throwing numbers out.
Actually I was being generous. Mine would run constantly once the outside temp got down around 20 degrees. The heat strips would cycle on and off to maintain the temp. I was basically heating with the strips and running the heat pump for nothing. If yours has never run more than 9 in a 24 hr period, you probably have a better designed system, but if the heat strips could have done the job in only 1 of those 9 hours (just throwing out number...) you would have saved money using them instead.

Why not?
Because the resistance heat has to run to get the house back up to temp. after the setback period all the savings, and then some is negated.

Take a few whirlpool baths during the month and the bill goes up $50.
Ouch! What are your electric rates?

Like I said, everyone should do what works for them.

Doug M.
 
 

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