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Old 10-21-15, 04:52 AM   #1
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Default ASHP mod question

Warning... Newbie question...

Currently on 11th year of a Trane XL14i ASHP. Only mod so far is the addition of a HyperEngineering SoftStart Sure-Start compressor starter. I was having problems with compressor startups on very hot days (thermal overload shutdown, was trying to draw over 70 Amps on hot-startup), this seems to have solved that issue.

My Winter performance is pretty fair, I've had it run in heat-pump mode down to under 20F, and still keep up... though it was running at 100% duty cycle. (and I'm not sure I'm getting any more than a COP of 1 when it's that cold)

Just some random thoughts... would there be any benefit to wrapping "pipe heater tape" around the cool liquid return line from the house (after heat is removed), prior to the outside condenser coils... to help prevent coil freeze-up?... or even to extend the lowest running temperature...? (applying AC power to it by a simple OAT thermostat at <30F or so, and only when unit is operating)

I know there is still some heat "out there" that can be pumped into the house, but it's insufficient to do the whole job... so it shuts off and runs aux heat (natural gas furnace).

If I add 300 Watts to the refrigerant line before it goes through the outside condenser coils, I know that heat will be pumped indoors along with the heat gathered from the outside ambient air, but will it be on a 1:1 ratio?

Even at that, it seems it may still increase its ability to run at lower temps... prevent coil freezeup and defrost cycles, and therefore still utilize the 'free heat' from outside air longer, and defray running the gas furnace.

I have other 'mod' questions... but this one popped up first...

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Old 10-21-15, 07:24 AM   #2
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All the countermeasures you have mentioned will not really save you any energy during the cold season. Wrapping the line set might save you a hundred watts or less, the shorter the lines, the less potential gain. Actively heating your outdoor unit is probably already being done by a crankcase heater in the compressor.

Trying to force longer operation or less defrosting by adding heat is the wrong approach. Besides the fact that the supplemental heat transfer operation is inefficient, the heat pump is at its worst performance. Your gas backup furnace is most likely less expensive and much more effective to run. It's up to you: run your heat pump to death at half its rated heat output or let the furnace put out 500 percent more heat on a duty cycle. As temperature plummets, the furnace may become 10 times more effective compared to the heat pump. Depends on lots of factors.

A better approach to get the most out of the heat pump at low-temp operating conditions is to check out, and improve if necessary (or possible), the defrost cycle. Many of the earlier units and some of the nit so old units just defrost for a set period whenever the outdoor coil is below a certain temperature. How much frost and how long it takes to melt is irrelevant, the unit has a set defrost time. Newer defrost controls use ambient sensors, defrost termination sensors, and algorithms that save lots of energy. Not only does the unit spend less time sucking heat out of your house to defrost, some defrosting operations are skipped entirely by lack of demand.
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Old 10-21-15, 10:36 AM   #3
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Originally Posted by ronbo View Post
Warning... Newbie question...
All of your questions center on your heat source.

How much work have you done on reducing infiltration and increasing insulation?

Reducing heat loss can be much more cost effective than increasing heating.

I'm not an HVAC technician. In fact, I'm barely even a hacker...
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