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RH77 10-25-2008 12:09 PM

Heat Pump Temp Lockout Settings to Gas Backup
As fall is settling in, there have been a few cold evenings where heat was requested. I tested the heat output of the Heat Pump and the Natural Gas Burner and both are ready for Winter.

Here's my Question...

(Background): I finally located a copy of the technical instructions for operation of the system through a programmable thermostat. Advanced settings allow multiple fan speeds, etc. There are 2 settings and a statement in the manual that have me wondering what is setting is best:
  • Heat Pump "Balance Point Setting" Temp
  • Heat Pump Lockout Temp
The Heat Pump will attempt to operate at the balance point setting, i.e. 35F. If the pump does not achieve results in 15-minutes, the gas burner is activated. The Lockout Temp is the lowest temperature permissible to operate the pump unit.

From the manual:

1. Capacity Balance Temperature:
This is the point where the heat pump cannot provide sufficient
capacity to keep up with the indoor temperature demand because
of declining outdoor temperature. At or below this point, the
furnace is needed to maintain proper indoor temperature.
2. Economic Balance Temperature:
Above this point, the heat pump is the most cost efficient to
operate, and below this point, the furnace is the most cost efficient
to operate. This can be somewhat complicated to determine and it
involves knowing the cost of gas and electricity, as well as the
efficiency of the furnace and heat pump. For the most economical
operation, the heat pump should operate above this temperature
(assuming it has sufficient capacity), and the furnace should
operate below this temperature.
As it states, it's complicated to determine. Has anyone had any experience with these settings? Cost is an issue, but so are emissions. The Grid is inherently more efficient/clean, but there's a point where the balance tips to favor Natural Gas -- in addition, the burner heats up the house very quickly compared to the Heat Pump.

So here I am :confused:

I have the Balance Point at 35F with the lockout at 30F. Perhaps I should conduct some tests at different temperatures and time the electricity usage compared to the gas and try to figure the amount consumed of each ???


bettered 02-14-2012 09:09 PM

Heating Efficiency vs. Cost
I ran into this comment: "The Grid is inherently more efficient/clean, but there's a point where the balance tips to favor Natural Gas-" The author is confusing efficiency with cost. The "grid" may be efficient, but the machinery that puts electricity into it grid is not. For making clean heat, there is no substitute for Natural Gas or Nuclear. A gas furnace can run in the 90%+ efficiency range. A power plant turns gas into electricity at 40% efficiency or less. "Electicity is more efficient" but it costs more because the gas to electricity conversion process is inefficient.

The "clean" observation is similarly flawed. The emissions are simply moved to the power plant. They don't go away. Hopefully, the power plant does a better job controlling emissions, but a fair comparison is that they have to burn twice as much gas to convert the energy from gas heat to electricity so that we can "efficiently" turn it back to heat.

Cost comparison is the only relevant basis, because it looks at overall conversion efficiency.

redpoint5 03-11-2012 12:01 AM

I'd be curious to learn more about this topic as well. My wild guess is that it's cheaper to switch to gas at a little less than 40 degrees.

bettered 03-11-2012 09:41 AM

HP Balance Pt
We recently replaced our A/C with a HP, keeping our gas furnace for "emergency heat" instead of "heat strips" which are frightfully expensive. We just got our 1st billls for less than a month on the new system. The electric bill was the same as the previous month at $110, but the gas bill was $80 vs last month's $120. This is remarkable because during the conversion we were running our gas logs - which have a higher BTU input than our furnace!

The em heat (furnace) is set to come on below 40 deg, and I have setback set for 64 until 10:30 am, by which time outside temp is generally above 40 even when the overnight low is in the 20's. The HP seems to run a lot, but that's not reflected in the bill. My opinion is that it's a struggle for a HP to extract warmth when the outdoor temp is near freezing. So far, I'm very satisfied with the change.

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