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
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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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.
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
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.
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.
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
On Dec 15, 1:01 am, firstname.lastname@example.org wrote:
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.
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?
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
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
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
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.
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.
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