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Old 10-07-15, 12:29 PM   #7
jeff5may
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Depends on your hot water usage habits. Like Daox said, both of these heaters are much more expensive than their conventional counterparts and are built to handle what they do quite sufficiently on their own.

In another thread, you were asking about an 80 gallon HPWH tank. Unless you are using that unit for hydronic heating of some kind, or some other heavy usage scenario (commercial or rental units, family of 15, laundromat, etc.), it would handle some pretty serious practical usage with ease. The unit cited already has a backup source: a resistive electric heating element. The HPWH tanks are known for a 3-to-1 energy advantage or better over straight resistive electric tanks.

The propane burner heater is the polar opposite of the above unit. Where the HP units have relatively slow and low energy usage, these things operate like a hot air baloon: lots of usage fast then stop. The advantage of these units is perpetual hot water with no storage tank losses. These units need to be sized for your maximum flow rate, so generally more flow is more expensive. If you exceed the max flow of the unit, you get lukewarm water instead of hot water. Modulating condensing burners are an option which add serious expense to the purchase price. That being said, if you are running a laundromat, it could keep up with that after-work rush every day of the year if you bought a really big one. If you were a germophobe, it could fill up your hot tub every time you used it, so you wouldn't have to re-use the old water from last time. The heat pump unit couldn't do it: you would run out of tank before the hot tub was full.

If you were looking for a doomsday prepper machine, you could buy both and install a solar electric PV panel to the built-in resistor element in the HPWH. When the sun was shining, you would get essentially free hot water from the PV panel. If the power grid went out, you would still get reserve hot water in the tank every sunny day, and you could run the gas burner when that tank got depleted. Neither would cool your basement at all.

FWIW, the HPWH unit is not going to freeze you out of your basement either, check xringer's thread on his overly complicated water heating project for realistic detailed measurements. I believe he only uses his fossil fuel backup to verify that it still operates. If ultimate energy savings is a concern, it would not be a major undertaking to direct the cool HP exhaust air towards your crawl space. Once the air is blown in that general direction, a cold-retaining knee wall would keep the air where it belongs next to the warm ground to gain calories.

I see you mentioned propane as a fuel source, not natural gas. In this light, the HPWH would be way less expensive to run. Bottled propane costs a lot more than piped in natural gas.
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