Use of Heat Pump in Cold Weather

[Tom]

Does anyone know how to determine how cold it should get before you stop
the heat pump(HP) and switch over to gas. I have a new Trane HP and gas
furnace. In this location (Atlanta), electricity costs 3.02 cents per KWH
and natural gas is 80 cents/ therm. Knowing this, it should be possible to
calculate the crossover point:

Any ideas?

Thanks in advance.

Tom D.

 

[Steve Spence]

Sounds like you have an air coupled heat pump with electric resistance
strips. Easy way to tell is see when the electric resistance strips kick in,
or turn them off, and see when you no longer get enough heat. Should be
around 40-50 F. I have yet to see an air coupled heat pump work well in any
climate. It's either too hot or too cold for them to work efficiently. Go
ground coupled instead.


--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[Harry Chickpea]

Sounds like you have an air coupled heat pump with electric resistance
strips. Easy way to tell is see when the electric resistance strips kick in,
or turn them off, and see when you no longer get enough heat. Should be
around 40-50 F. I have yet to see an air coupled heat pump work well in any
climate. It's either too hot or too cold for them to work efficiently. Go
ground coupled instead.

One works fine for me in So FL. Summertime temperatures max out at about 95F
and wintertime lows are about 50F. I don't even have the heatstrips connected
in mine. The OP might consider connecting a relay to the heat strip power that
would kick in the gas for a fixed period of time and then reset to the HP.

 

[Bob Adkins]

Does anyone know how to determine how cold it should get before you stop
the heat pump(HP) and switch over to gas. I have a new Trane HP and gas
furnace. In this location (Atlanta), electricity costs 3.02 cents per KWH
and natural gas is 80 cents/ therm. Knowing this, it should be possible to
calculate the crossover point:

Mine works down to ~35F, but heats slowly unless I kick on the heat strips.
It works great at 40F and above. Gas is super expensive here(south-central
Louisiana), so I'm way better off using a heat pump, even using heat strips
10-15 days per winter.

Bob

 

[Steve Spence]

He wants to turn off the strips, and not let them kick in, ostensibly
because his gas is cheaper than his electric, which I doubt.



--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[daestrom]

He wants to turn off the strips, and not let them kick in, ostensibly
because his gas is cheaper than his electric, which I doubt.

Why would you doubt this? Here in CNY, I pay <$0.75/therm for NG and

$0.13/kWhr. If you go strictly by energy content, NG is *much* cheaper (so

is propane around here). Below 35F or so, the heat-pump's COP drops (unless
you have ground source) and makes NG the cheaper way to heat your home.

Electric only looks better than NG when running a heat pump, and only when
the COP is relatively high (i.e. mild weather outside or ground sourced).

I've been thinking about buying A/C for my place, but it's only *really* hot
a couple or three weeks a year. Somewhat hot for July through August.
Never could justify the expense for just a couple of weeks. But then I got
to wondering if the cost of an air-source heat pump isn't much higher, I
could use one of those for about 7 months of the year.

Use NG furnace (already have it) for the cold winter months (about 3
months), heat-pump for spring and fall when it's only in the 40's or 50's
(when its COP is really good) and have the A/C for the heat of summer. And
the air-source heat-pump for spring/fall is *way* cheaper than a ground
source installation, right??

Not the *most* efficient, but would get enough use out of the heat-pump to
justify the capital outlay and I'd get my A/C for those summer nights when I
can't sleep.

But what's the price comparison between a conventional whole house A/C unit
and a comparable sized air-source heat-pump? Most people want to quote me a
heat-pump that will work in the dead of winter in CNY (way too big), or
ground-source.

I mean, if I'm going to get an A/C unit *anyway*, how much more would the
'heat-pump' version of the same size be? I know it wouldn't heat all
winter, but part of spring and fall?

daestrom

 

[News]

Why would you doubt this? Here in CNY, I pay <$0.75/therm for NG and (so
is propane around here). Below 35F or so, the heat-pump's COP drops (unless
you have ground source) and makes NG the cheaper way to heat your home.

Electric only looks better than NG when running a heat pump, and only when
the COP is relatively high (i.e. mild weather outside or ground sourced).

I've been thinking about buying A/C for my place, but it's only *really* hot
a couple or three weeks a year. Somewhat hot for July through August.
Never could justify the expense for just a couple of weeks. But then I got
to wondering if the cost of an air-source heat pump isn't much higher, I
could use one of those for about 7 months of the year.

Use NG furnace (already have it) for the cold winter months (about 3
months), heat-pump for spring and fall when it's only in the 40's or 50's
(when its COP is really good) and have the A/C for the heat of summer. And
the air-source heat-pump for spring/fall is *way* cheaper than a ground
source installation, right??

Not the *most* efficient, but would get enough use out of the heat-pump to
justify the capital outlay and I'd get my A/C for those summer nights when I
can't sleep.

But what's the price comparison between a conventional whole house A/C unit
and a comparable sized air-source heat-pump? Most people want to quote me a
heat-pump that will work in the dead of winter in CNY (way too big), or
ground-source.

I mean, if I'm going to get an A/C unit *anyway*, how much more would the
'heat-pump' version of the same size be? I know it wouldn't heat all
winter, but part of spring and fall?

daestrom,

If the air sourced heat pump is marginally more expensive to install than an
A/C then you don't save much at AFAICS. The heat pump at certain times of
the year maybe more economical to run than NG, but it must be only marginal.
Or is it? You would need some mechanism to tell the heat pump to switch to
heat the house. Outside thermostat? Differential controller?

Or are you inventing some justification to yourself to get the a/c anyway?

 

[Steve Spence]

I think he has a separate gas heater, so he wants to kick that on instead of
the resistance strips. easy to do. disconnect resistance strips, and use
their current to turn on gas furnace. We pay $.035 / kWh, and it's
completely hydroelectric. Massena NY

--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[Steve Spence]

yes, but he said he was paying $.03 / kWh, not $.13, so that puts his
electric almost the same as your gas.

1 therm of natural gas = 29 kWh of electric.
you pay .75 / therm or $.02 per kWh equivalent, he pays the equivalent of
$.87 / therm, or a difference of a penny per kWh or $.10 / therm. this does
not take into effect the efficiency of a gas furnace vs. an electric
resistance strip, so it appears he's ahead of the game by using the
resistance strips.


--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[daestrom]

If the air sourced heat pump is marginally more expensive to install than an
A/C then you don't save much at AFAICS. The heat pump at certain times of
the year maybe more economical to run than NG, but it must be only

marginal.

Well, I was thinking I could get more use out of a heat-pump than if it were
just A/C. Couldn't use it *all* year round, but a large percentage.

Of course, my electric is almost 6 times my NG rate ('News' figure, I
haven't verified it), so the heat-pump COP would have to at least run close
to this. But on the other hand, IMHO, NG won't stay this cheap much longer,
and if NG prices rise then the heat-pump idea is even more interesting.

Or is it? You would need some mechanism to tell the heat pump to switch to
heat the house. Outside thermostat? Differential controller?

As I understand it, heat-pumps come with a heating/cooling thermostat
already. House warms above ~78, cool the house, if it goes below ~67, warm
the house. The only extra control would be once the *real* heating season
starts, set the NG furnace thermostat and shut off the heat-pump till spring
and the crocus start blooming through the snow.

Or are you inventing some justification to yourself to get the a/c anyway?

Well..... yes and no ;-)

If I'm going to make the capital outlay for A/C *anyway*, then it just seems
like the heat-pump idea is better IF it isn't too much more money. But I
haven't been able to find any price comparisons between an A/C unit and a
comparable BTU/hr heat-pump. I know it must be more expensive, but are we
talking 10%, 50% or (shudder) 90%??? It's that difference between the two
that could make/break this whole idea. At 10%, I can probably get enough
high-COP operation in the spring and fall to cover investment (especially if
NG prices rise). But at higher numbers, would need to 'sharpen the ole
pencil' and pin it down more precisely.

I've got a few years of NG usage and daily heating-degree-day information,
so with a heat-pump's COP vs. outdoor temperature, I *should* be able to
figure the 'ideal' operating range for each. (once I know the added cost
involved) I want to factor in the extra capital cost of heat-pump to find
true ROI.

The extra running would mean more service/maintenance, but if NG prices
rise, it could end up saving me money in the long run to use A/C, heat-pump
and NG instead of just NG. And more comfortable too ;-)

But then again, there are a lot of folks around here with plain central A/C
and plain NG furnaces. So either nobody thought of it, or the HVAC folks
around here already thought of it and found a good reason why it doesn't
make sense and I'm just missing it. In other words, I'm either ahead of
them, or behind them

daestrom

 

[daestrom]

I think he has a separate gas heater, so he wants to kick that on instead of
the resistance strips. easy to do. disconnect resistance strips, and use
their current to turn on gas furnace. We pay $.035 / kWh, and it's
completely hydroelectric. Massena NY

$0.035 /kWh in NY???
You must be looking at just the line item on your bill. You *are* in
Niagara Mohawk service territory up there, aren't you??

My Niagara Mohawk 'supply charge' is $0.0568/kWh, but when you add on tariff
(1.37%), sales tax (3%) then delivery charge of 4 cents, another tariff of
3.17% and another sales tax of 3%, and some other 'adjustments', last months
electric of 725 kWh cost $91.63 ($0.126/kWh). (I average under 500kWh a
month, but hey, the holidays and all)

And looking back for two months, I see that a difference of 123kWh costs me
an additional $10.78 for a marginal rate of $0.087 for each additional kWh.

Looking at this months gas bill and figuring 95% efficiency of my furnace,
NG is currently running me about $0.0363/kWh. So my electric rate is only
about 2.4 times my NG rate last month.

daestrom

 

[News]

than
marginal.

Well, I was thinking I could get more use out of a heat-pump than if it were
just A/C. Couldn't use it *all* year round, but a large percentage.

Of course, my electric is almost 6 times my NG rate ('News' figure, I
haven't verified it), so the heat-pump COP would have to at least run close
to this. But on the other hand, IMHO, NG won't stay this cheap much longer,
and if NG prices rise then the heat-pump idea is even more interesting.


As I understand it, heat-pumps come with a heating/cooling thermostat
already. House warms above ~78, cool the house, if it goes below ~67, warm
the house. The only extra control would be once the *real* heating season
starts, set the NG furnace thermostat and shut off the heat-pump till spring
and the crocus start blooming through the snow.
anyway?

Well..... yes and no ;-)

If I'm going to make the capital outlay for A/C *anyway*, then it just seems
like the heat-pump idea is better IF it isn't too much more money. But I
haven't been able to find any price comparisons between an A/C unit and a
comparable BTU/hr heat-pump. I know it must be more expensive, but are we
talking 10%, 50% or (shudder) 90%??? It's that difference between the two
that could make/break this whole idea. At 10%, I can probably get enough
high-COP operation in the spring and fall to cover investment (especially if
NG prices rise). But at higher numbers, would need to 'sharpen the ole
pencil' and pin it down more precisely.

I've got a few years of NG usage and daily heating-degree-day information,
so with a heat-pump's COP vs. outdoor temperature, I *should* be able to
figure the 'ideal' operating range for each. (once I know the added cost
involved) I want to factor in the extra capital cost of heat-pump to find
true ROI.

The extra running would mean more service/maintenance, but if NG prices
rise, it could end up saving me money in the long run to use A/C, heat-pump
and NG instead of just NG. And more comfortable too ;-)

But then again, there are a lot of folks around here with plain central A/C
and plain NG furnaces. So either nobody thought of it, or the HVAC folks
around here already thought of it and found a good reason why it doesn't
make sense and I'm just missing it. In other words, I'm either ahead of
them, or behind them

Probably ahead of them. Users don't like new fangled things, and installers
anything whatsoever.

 

[Harry Chickpea]

If I'm going to make the capital outlay for A/C *anyway*, then it just seems
like the heat-pump idea is better IF it isn't too much more money. But I
haven't been able to find any price comparisons between an A/C unit and a
comparable BTU/hr heat-pump. I know it must be more expensive, but are we
talking 10%, 50% or (shudder) 90%??? It's that difference between the two
that could make/break this whole idea. At 10%, I can probably get enough
high-COP operation in the spring and fall to cover investment (especially if
NG prices rise). But at higher numbers, would need to 'sharpen the ole
pencil' and pin it down more precisely.

I've got a few years of NG usage and daily heating-degree-day information,
so with a heat-pump's COP vs. outdoor temperature, I *should* be able to
figure the 'ideal' operating range for each. (once I know the added cost
involved) I want to factor in the extra capital cost of heat-pump to find
true ROI.

FWIW, my air exchange scroll 3 ton 12 seer heat pump cost about $2400 in 1997.
The only problem I've had with it is a sticky contactor that had to be
replaced. The unit gets a lot of use in AC mode.

 

[Steve Spence]

No, I'm not in Niagara Mohawk territory, I'm on Massena Electric (Robert
Moses Power Dam).

From 11-19-03 to 12-20-03 I used 1478 kWh, and paid $66.42, so we are up to
$0.045 per kWh



--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[Steve Spence]

I've been down in the belly at the bottom of the dam, even more interesting
eye level with the generator coils.

--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[Steve Spence]

A good ground source unit will run you 12-15k installed.

--
Steve Spence
Renewable energy and sustainable living
http://www.green-trust.org
Donate $30 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

 

[George Elkins]

Does anyone know how to determine how cold it should get before you stop
the heat pump(HP) and switch over to gas. I have a new Trane HP and gas
furnace. In this location (Atlanta), electricity costs 3.02 cents per KWH
and natural gas is 80 cents/ therm. Knowing this, it should be possible to
calculate the crossover point:

Any ideas?

Thanks in advance.

Tom D.

This is a nice theoretical question. Try sci.engr.heat-vent-ac instead.