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Old 09-07-15, 04:51 PM   #6
AC_Hacker
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Default Getting Back On Track...

If I recall correctly, you posted the idea of using an ASHP for most of the water heating, and then supplementing the final heating with a demand gas boost.

In effect you would be using the ASHP as a water pre-heater for the gas demand water heater.

I've been turning the idea over in my head for several days now to consider the merits of such a system.

First of all, a properly set-up ASHP water heater is less expensive to run than electric, because the same power would be used at a 150% to 300% advantage, no matter what the price of electricity.

It would also be likely to beat gas, because much of our electricity is generated not just by coal, but also by gas. There would be some comparison calculating to do, depending on the relative costs of each fuel.

Build It Solar, has some very useful calculators available, and one of them is for comparing the various fuels for heating a house. The calculator stays pretty up to date on fuel costs, but they can be changed by inputting your own value. One very interesting aspect is that the calculator also calculates the CO2 resulting from the various fuels, as well as the cost.

So 'heat a house' is essentially the same as 'heat domestic water'... you can see which one will be the most advantageous, but the calculated costs will not apply.

Back to your pre-heat idea, I usually see the pre-heat idea applied to solar preheating, with the final lift being done with electric or gas. In this case, the pre-heat is essentially free, so the advantage is very easy to calculate.

But your idea was to combine ASHP which already is quite efficient, with a demand heater, which is also efficient.

Tankless efficiency is predicated on avoiding standing heat loss. if your hot water is being used constantly, it may not be much more efficient. If it it is used infrequently, with long intervals in between, then avoiding the losses that would occur during that non-use period would be a big advantage. For instance, I am the only one using my hot water, so the tankless is really great. So, you'd need to factor this in to your consideration.

Also to consider it that the lower the lift that a heat pump needs to make, the higher it's efficiency. So by reducing the cut-off temp of the ASHP water heater, you could improve efficiency.

There are so many variables here that it would be pretty difficult to predict mathematically where the 'sweet spot' would be, or if there would even be one. But you could build the beast and try it out by varying the cut-off point of the ASHP water heater and measuring the fuel used. For that, you would need a device to measure the electricity used by the ASHP water heater alone. And you would also need a device to measure the gas used by the demand heater, which is a project by itself.

Yous should also be aware that for this setup, your gas demand water heater would need to be aware of incoming water temperature. I'm not convinced that the small inexpensive demand heaters have this feature.

Good luck!

Best,

-AC
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