Heat Pumps and Electric Heating - Set Temp for propane back up to HP

I apologize if this has been dealt with elsewhere, but we're on dail-up and it takes forever to search.

In any event, we have an electric heat pump with a propane back-up. Our electric bills in the winter are crazy high and several people have suggested that that's because the heat pump is too inefficient at temps below the mid- to upper-30s.

Right now, the back-up only kicks in when its very cold outside - less than 20.

Are these folks correct - is my heat pump working too hard at those temps? If so, how can I get the back-up to kick in at a higher temp? Some have suggested a kind of thermostat that can control this.

Help!!!

Doc

you need to understand your equipment, its efficiency, and your fuel rates.

Here is a fuel comparison calculator that gives you a good idea between HP heating vs propane gas heating.

Keep in mind a HP loses efficiency as the outside temp drops.

IMO
:)

Warmair.com - Fuel Cost Comparisons (http://www.warmair.com/html/fuel_cost_comparisons.htm)

Yes there are thermostat that will give you control over when to bring on your backup Propane furnace. Your HP may even have a jumper or other feature on its board for low temperature lock out.

As mentioned I would try to understand everything about your system in case you already have the ability to do this. Also, your cost per gallon of propane, furnace AFU, KWH costs and HP's Coefficient of Performance (COP) are all factors in determining what is the most economical temperature at which to bring on your auxiliary.

Thanks guys for the info. This is more complex than I had anticipated, but it's good to know all this.

Turns out, propane is MUCH cheaper than electricity here.

I suspect I should contact the contractor that installed the system and ask them these questions.

again, thanks!

Turns out, propane is MUCH cheaper than electricity here.


I find that very hard to believe!

What is your LP cost per gallon, and your electric rates.


What do you have for t-stat/control for your system now?

Make and model of both furnace and HP.

Jay is absolutely correct.

I gave the OP a link to a fuel comparison calculator.

as the old adage goes, you can lead a horse to water but you can't make them drink it.

OP needs to review his propane costs and electric rates.

IMO
:wall:

OP needs to review his propane costs and electric rates.

... and the furnace AFUE and heat pump's COP numbers.

Thanks for the input and suggestions.

I did some calculations - per 100,000 useable BTUs of heat, I'm paying $2.59 on the electric HP and $1.99 on the LP backup furnace.

I also called the outfit that installed our HVAC - the guy I talked to said the way the system was set up was the most efficient it could be but if I wanted to change the temp at which the backup kicked in I should simply use the t-stat to manually go to backup. We don't have a programmable t-stat.

I found that response odd. Is he right - am I making a big deal about nothing?

I still find it's hard to see that LP is cheaper than the heat pump itself.

Again, can you share us your LP cost per Gallon, and your Electric rate per KwH, also make and model numbers on both furnace and outdoor unit.

I did some calculations - per 100,000 useable BTUs of heat, I'm paying $2.59 on the electric HP and $1.99 on the LP backup furnace.

I also called the outfit that installed our HVAC - the guy I talked to said the way the system was set up was the most efficient it could be but if I wanted to change the temp at which the backup kicked in I should simply use the t-stat to manually go to backup. We don't have a programmable t-stat.

I found that response odd. Is he right - am I making a big deal about nothing?

How are you calculating your electric cost?

No you are not crazy for wanting to get the most out of your system. Getting more control over when your aux kicks in may be desirable from both an economic and comfort standpoint. But first lets figure out what your true costs are. Please provide what jay is asking for.

I'm paying $1.49 for a gallon of LP and $.15 per KWH for electricity.

Will get the make and model numbers of the HP and gas furnace tomorrow.

Thanks!

Okay. The furnace and HP are both Goodman. The Model # for the furnace is GKS9. The outside unit model # looks like: GSH130421AC. I can't find the manual for the outside unit but best as I can read the back of it that's the model #.

The t-stat is a basic Honeywell, nonprogrammable. The model number isn't on the outside and I can't seem to get the cover off. Hope this isn't important.

I calculated the KWH cost by using our last year's bill: dividing the total KWH into the total cost. Is that correct?

Thanks guys.

Okay. The furnace and HP are both Goodman. The Model # for the furnace is GKS9. The outside unit model # looks like: GSH130421AC. I can't find the manual for the outside unit but best as I can read the back of it that's the model #.

The t-stat is a basic Honeywell, nonprogrammable. The model number isn't on the outside and I can't seem to get the cover off. Hope this isn't important.

I calculated the KWH cost by using our last year's bill: dividing the total KWH into the total cost. Is that correct?

Thanks guys.

Correct - that will include taxes.

To compare 1 million BTUs:

For your GKS9 92% AFUE propane at $1.49/gal:
1,000,000 / 91,800 x $1.49 /.92 =
$17.64

For your GSH130421AC HP with electric $0.15/KWH with COP
2.97 at 35°F:
1,000,000 / 3413 x $0.15 / 2.97 =
$14.79
2.72 at 30°F:
1,000,000 / 3413 x $0.15 / 2.72 =
$16.15
2.57 at 25°F:
1,000,000 / 3413 x $0.15 / 2.57 =
$17.10
2.42 at 20°F:
1,000,000 / 3413 x $0.15 / 2.42 =
$18.16

So the statement that the heat pump is too inefficient at temps below the mid- to upper-30s is not quite accurate. In the mid to upper 30s you are saving with the heat pump. As you enter the 20s you are crossing your economic balance point since defrost eats into your efficiency (COP numbers don't include defrost events). In the low 20s it is costing your more to run the HP, but not substantially so.

If you want to improve the efficiency of your system you could get a programmable tstat with outdoor sensor that will lock out your HP below 25°F or 30°F outdoor ambient.

Dac did a good job on the numbers.

As he said a good t-stat with an outdoor sensor will cut off the heat pump for you. The ones I'd suggest is the Honeywell VisionPro, or the IAQ.

Jay and Dac - thanks for this - it's great!!

Really surprising that the HP is cheaper until we get to 20 degrees. Go figure (oh wait, you did....).

In any event, I will keep an eye out this winter to see when the system automatically locks out the HP. If it does so below 20 degrees, I'll look into the t-stat with the outdoor sensor.

Again - thanks you guys; this really helped!

Jay and Dac - thanks for this - it's great!!

Really surprising that the HP is cheaper until we get to 20 degrees. Go figure (oh wait, you did....).

In any event, I will keep an eye out this winter to see when the system automatically locks out the HP. If it does so below 20 degrees, I'll look into the t-stat with the outdoor sensor.

Again - thanks you guys; this really helped!

If your Honeywell is non-programmable it is likely the only thing it knows is to bring on your furnace if the HP cannot keep up. Depending on the heat loss of your dwelling (ie however tight it is) that could be into the 20s or teens. There is no true lockout without an outdoor sensor. Let us know what model Honeywell you have and we can say for sure.

The numbers don't lie, so I'd suggest switching manually as you cross 25-30°F in the evening and go back to HP as the outdoor temp rises above 25-30 in the morning.

Was this system put in by a dealer?

You may have a control already on the sytem that switches over.. Look for a control either on or near the unit outside that the t-stat wires up to, or a control on or near the furnace.

Was this system put in by a dealer?

You may have a control already on the sytem that switches over.. Look for a control either on or near the unit outside that the t-stat wires up to, or a control on or near the furnace.

Yes, forgot about that. There may be a jumper or something on the outdoor unit that will lock it out, or in your air handler perhaps. If it you consider a more advanced tstat you will want to know at what temp it is hard locking out, and if there is any way to override.

You may have something like this.

http://img35.imageshack.us/img35/7171/imgcachela01pressurecon.jpg

It may be found inside the unit outside.

I would second the suggestion for the Honeywell VisionPRO T8000 thermostats. You can program the point where the heat pump only runs, where both the heat pump and backup are allowed to run, and where the heat pump is disabled and only the backup heat runs. You will have to install the Honeywell C7089 outside temperature sensor for this function to work. It sounds like it would automate your procedure fully.