Quote:
Originally Posted by wewantutopia
...To sum up. Instead of using a back up heat source when it is REALLY cold is it possible to not install a back up but rather boost the ground loop water temps and continue using the heat pump as the only source of heat...
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We are a motley lot here with a variety of opinions, so here is mine...
To achieve maximum efficiency, any heat pump system should be sized
somewhat smaller that the 98% maximum requirement. To meet the demand on days that exceed the 98%, you would use some axillary heat source like wood, gas or electric.
A heat pump, even a variable speed compressor system (AKA: 'Inverter Technology') will reach its maximum efficiency when running flat out (100%). At utilization levels below that, friction factors will reduce efficiency. (To make this easier to visualize, just imagine that you have your over-sized compressor chained to your foot and you have to drag it around all the time and it NEVER reaches maximum efficiency. It would be far better to have a smaller, lighter compressor chained to your foot and have it reach maximum efficiency more often.)
With a ground source system, by far the biggest part of the cost and/or effort is the earthwork and associated pipe installation. So if you over-size your system, you are paying extra in cost and effort for extra loop field. There are some very good reasons to do this, because during the heating season, there is some inevitable decline in loop temperature. The bigger the loop, the smaller the decline. The smaller the decline, the greater the ultimate efficiency due to the Carnot Efficiency Theorem.
But to get back to your idea of having an electrical demand heater augmenting the loop water temperature just before the point of use, I think that you have a wonderful idea here, and if you are a multi-talented kind of guy, it would make sense to use a Pulse Width Modulation scheme to apply your electrical energy to your water heater, as needed rather than on-at-setpoint, off-at-setpoint.
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Some other schemes to consider are that if you use your GSHP for cooling, you will be storing summer heat into your loop field, and your electric heater idea, as clever as it is, would probably not get used.
Also, if you are in an area of summer sun, you could use a hot water solar panel (or several panels) to pump solar heated water into your loop field during the sunny months, to be harvested during the winter.
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And as a final word, it is usual that in thinking of ways to stay warm, we tend to think of ways to make heat... I think it's an evolutionary thing.
But it is more modern and often more fruitful to think in terms of how to keep the heat we have. That would lead us to reduce infiltration losses as much as possible and to maximize insulation as much as possible.
Once these steps have taken place, the heating system size and cost will shrink to a more manageable size.
Best,
-AC