New Heat Pump - Setup question

[Some Homeowner]

I have a new dual fuel heating system in my house (replaced my old heatpump) and I need some help programming the thermostat. The system is an air heat pump with and oil furnace backup.

The (Honeywell VisionPro) thermostat has an external temperature sensor. I can set the thermostat to turn off the compressor and use just oil at some cutover temperature (when it is too cold for the air heat pump to compete with oil).

What temperature would you recomend?

I asked the installers and (as if in a comedy show) they gave their answers simultaneously and they were 20 degrees apart. (seems like a wide window)

The heat pumps is a brand new 4-ton Bryant Preferred Series air heat pump (up to 15 SEER / 8.5 HSPF) . The slightly older hot air furnace is a ThermoPride oil furnace in good shape. (81.6 AFUE).

Thanks in advance for your advice.

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[Bubba]

Oil is expensive. Set it to its lowest...15. Bubba

[schumann.comfortcentral]

Here is the main problem: I do not have enough information to give you an answer. Here is why:

The balance point is always determined by: Heat/Load Calculation of House Outdoor Design Temp. BTU output of equipment.

These should be determined prior to install by contractor (ask for the Manual J, mandatory in some states), and unfortunately, your installers are not usually given this information (nor is it my experience that they are trained in this area).

RULE OF THUMB: Set it as low as possible, if not comfortable, then move it up. Try setting at 35, if you can go lower..go lower. If you can go higher...go higher. Your Heat Loss Calculation in conjunction with those mentioned above determine everything but bottom line, it is about your families comfort. But keep in mind that a heat pump is less efficient at lower temps. Oil in my area cost $3.30/gal, electricity is $.25/kw which makes the electricity a bargin. This is also part of your calculation I cannot determine for you.

[Richard]

The Installer's guide contains the worksheet that you'll need. They were supposed to leave that book near the equipment, according to its own instructions. If you post the model # I can give you a link to it.

[Mark]

I'd set it for as low as you can and not have the outside unit going into a defrost cycle. When it has to go into a deforst cycle, then its is doing a lot of thrashing around for not much heat. That turns out to be around 32 for my system but yours may be different. The cost of oil vs electricity is one factor. The cost of wear and tear on the compressor is another.

Mark

[Richard]

I don't know what the actual statistics are regarding straight cool vs heat pump compressor lifetimes. If you could somehow know when the compressor was going to take a dump, then you could factor that in.

In an electric heat pump system it's always going to be cheaper to run it in heat pump mode. With a dual fuel system that isn't necessarily going to be true. The economic balance point can be arbitrarily high, so high in some cases that it'll be cheaper to use gas or oil all of the time. In other cases the economic balance point may be lower than the thermal balance point, in which case it becomes a non-factor. That's what the worksheet is for. My best guess is to ignore the possibility of compressor failure when doing those calcs. It's just as likely that your heat exchanger will crack with overuse.

[schumann.comfortcentral]

Just an added thought: Compressor warranties are around 10yrs, Heat Exchangers are for 20yrs (even lifetime heat exchangers are only covered for 20y for subsequent owners--and those who bought into the lifetime). Given the statistics, as sad as it sounds, these have no bearing on what constitutes comfort and economy today for a current home owner.

[Mark]

The heat content of oil is 138,500 btu/gal. Assuming an 85% efficiency nets 117,725 btu/gal.

The heat content of electricity is 3412 btu/kWh. Assuming a COP of 3.5 nets 11,942 btu/kWh.

The ratio is 117,725/11,942 = 9.85.

You may have different efficiencies.

If your cost per gallon of oil is more then 9.85 times your cost for a kWh of electricity, then the heat pump is cheaper to operate.

This does not include the loss of efficiency due to the defrost cycle and does not include the wear and tear costs on the heat pump or the maintenance cost for an oil burner which can be significant.

Mark

[Some Homeowner]

Richard wrote in news: [e-mail]:

Richard,

its a 4 ton Bryant 265ANA048

Thanks!

[Some Homeowner]

Mark wrote in news:a555febd-1345-4be3-8882- [e-mail]:

Thanks Mark,

Now that I know these things (like BTUs per gallon or KWH) the math gets much easier. (I am an engineer, I just don't know HVAC).

For the COP of 3.5 can you relate that to HSPF? Or, is 3.5 a good enough estimate since it is a brand new unit in good working order?

AndyB

[Mark]

see

looks like average COP *3.4 = HSPF or HSPF / 3.4 = average COP

have fun Mark

[Richard]

innews: [e-mail]:

I don't know why, but I was assuming a package unit. Nevertheless there is a balance point worksheet and there are performance charts in the document below.

This should be all the information about the unit that you should need.

Here's the electrical diagram

That's all I could find. If you're an engineer then you should be able to figure out the economic balance point from this and what the other posters provided.

If you have the Evolution dual fuel control then the lit for that might have some additional info. I've never installed an Evolution/ Infinity dual fuel user interface, so I'm not completely familiar with them.

[Some Homeowner]

Richard wrote in news: [e-mail]:

Thanks a bunch Richard.

I got some readin' to do.

[Some Homeowner]

By the way, how does "outdoor temperature" figure in to the calculations?

[Richard]

You have to find the capacity of the system at different ambient temps in order to zero in on the economic balance point. Use the extended performance charts in the manual.

Now look through this till you find the economic balance point section. It should explain what to do with that info. HTH.

If it looks like the economic balance point is going to be lower than the thermal balance point, then you're going to switch to backup at the thermal balance point, in which case it would be a waste of time to continue with the economic balance point calcs. IOW, calculate the thermal balance point first. Usually when its winter you can bypass the calcs on this and just take note of the outdoor temp when the heat pump runs continuously.

[Sum Yung Guy]

Richard wrote in news: [e-mail]:

Thanks again for the great info!

[Richard]

innews: [e-mail]:

Sorry, that was the wrong link. Try this one instead.

You won't have that interface, but the directions for finding the economic balance point are independent of the control system.

[Gary]

Please explain sometimes my hp run continuously but keeps the indoor setpoint Great link

[Richard]

m:

Sounds like it's working.