Is there a HP Simple Rule of Thumb?

[Moman19]

I'm a homeowner with a new Carrier Infinity 3.5 ton Elite HP with Natural Gas for Aux heat. I live in St. Louis. Winter has been quite mild here so far and whether it's 50 or 15 outside, the HP seems to do just fine providing warm air out of the vents with little/no aux heat. The house is a 2000 foot, well insulated ranch with a finished basement.

While I understand there is an actual balance point as well as a financial balance point, (here's where I need the "rule of thumb") does it make FINANCIAL sense to stop the HP at some point and just allow the Aux natural gas to heat the home?

I ask because if the HP must chug away 24 x 7 to heat the home (say the lock out point is 10 degrees) without gas, I would assume the on-off nature of gas might actually be more cost effective, plus there would be less wear & tear on the compressor.

The installer gave little guidance in this area and told me to just let the Infinity stat figure it all out "automatically"........ Not sure I buy that!

[codywatkins]

For me, it makes financial sense to keep the heat pump running until it's too cold outside for it to keep up.

That's my balance point. When it can no longer keep up, that's when it can switch to gas. As long as it can keep up while chuging away, it should be fine.

Unless electricity is more expensive than gas in your area per therm.

[crmont]

I'm a homeowner with a new Carrier Infinity 3.5 ton Elite HP with Natural Gas for Aux heat. I live in St. Louis. Winter has been quite mild here so far and whether it's 50 or 15 outside, the HP seems to do just fine providing warm air out of the vents with little/no aux heat. The house is a 2000 foot, well insulated ranch with a finished basement.

While I understand there is an actual balance point as well as a financial balance point, (here's where I need the "rule of thumb") does it make FINANCIAL sense to stop the HP at some point and just allow the Aux natural gas to heat the home?

I ask because if the HP must chug away 24 x 7 to heat the home (say the lock out point is 10 degrees) without gas, I would assume the on-off nature of gas might actually be more cost effective, plus there would be less wear & tear on the compressor.

The installer gave little guidance in this area and told me to just let the Infinity stat figure it all out "automatically"........ Not sure I buy that!

A little light reading....
http://www.contractortalk.com/showthread.php?t=441

[mchild]

You will need to calculate it based on your energy cost.

Most heat pumps can operate at a lower cost as long as it can produce the heat the home needs. With heat pumps there is a leverage factor in that it produces more than it consumes. Even at out door temps of 10* it will likely be producing about twice what it consumes. The gas furnace, depending on whether it is 80%, 90% or 95% efficient is a;ready at less than one to one and less than the heat pump. But, if you have relatively high electric rates or low gas rates then that will be the deciding factor.

Get your energy cost and go to: http://www.warmair.com/html/fuel_cost_comparisons.htm to calculate what your cost is for 100k BTUs of heat using the different energy sources. For the heat pump COP number use 2.0 as that is about what it will be at 10* (higher at higher out door temps).

Also, letting the heat pump run is better for it than cycling on and off so I wouldn't worry about the long run times when it gets cold.

Good luck.

[BaldLoonie]

A 3.5 ton pump for 2000 sq ft well insulated in the midwest is BIG. That's why you can get by with just the pump in colder weather. But that's good for your bill as I'd bet at 15 the pump is still cheaper.

[dan sw fl]

A 3.5 ton pump for 2000 sq ft well insulated in the midwest is BIG. That's why you can get by with just the pump in colder weather. But that's good for your bill as I'd bet at 15 the pump is still cheaper.

3.5 ton for 2,000 Sq.Ft sounds JUST RIGHT

[Moman19]

Gentlemen,

Thanks for the advice and links. Much interesting stuff to absorb. So If I understand all this great info correctly, (assuming my electric rates and Natural gas rates are in the "normal" range) the economic balance point is going to be someplace close to the point where the heat pump can no longer keep up without Aux assistance.

Lets' say Gas comes on fairly often around 5 degrees outside. Would it be wise to set the HP cutoff at say, 7 degrees? Or, does it make more sense to set the HP cutoff at ay say, zero degrees and just let the Infimity Thermostat drive the ship?

[cem-bsee]

very difficult for us to decide from here --
why not do this yourself?
track KWH per DegreeDay for each billing period for YOUR house --
while U 'play'

you do get DD as part of your bill?

I can give 7y for my house --

[BaldLoonie]

3.5 ton for 2,000 Sq.Ft sounds JUST RIGHT

He ain't in Florida, Dan :D

In our climate, a Manual J would come out around 24,000 BTU for a "typical" well built 2000 sq ft house. They'd have a 2.5 ton cooling unit in most cases.

Friend had a 1700 sq ft 2 story cooled great with a 2 ton. But she had foam insulation so was tight as a drum. A family member has a new 1700 sq ft ranch with a 2 ton pump and 10kw. Cools fine but the 10kw loses it when it gets down into the single digits!

[dan sw fl]

A family member has a new 1700 sq ft ranch with a 2 ton pump and 10kw.
Cools fine but the 10kw loses it when it gets down into the single digits!

So, 2.5 or 3 heat pump is much better based on sizing it for heating.

[BaldLoonie]

In cold country, we size HPs to the cooling load. Some oversize pumps, that's works with a unit that has a good SHR, could backfire with a high SHR.

If it were my house, it would get a 2.5 ton with 15kw and a larger duct system. But you know RNC :mad:

Of course with Trane, especially in dual fuel, you have to go a 1/2 size larger on cooling to get capacity in heating. :rolleyes:

[Moman19]

He ain't in Florida, Dan :D........

Yes, but the summers in St. Louis do get as hot as Florida in July/August. Also, the 3-Ton unit we replaced had a tough time keeping the place cool on a scorcher with guests present.

[dan sw fl]

Yes, but the summers in St. Louis do get as hot as Florida in July/August. Also, the 3-Ton unit we replaced had a tough time keeping the place cool on a scorcher with guests present.

When equiipment is outside the Design Conditions, as mentioned, you're lucky to have < 5'F rise above normal.

[Rookie2dafield]

I might be missing somthing here correct me if I am wrong.


I live in south Texas I have never seen a heat pump with natural gas AUX heat.

Here Natural gas and LP cost less than a heat pump to heat a house. If this is true were you live I would use Emer. heat and use the gas.

This makes no sense if you have your house set up for gas why not just have gas heating.