Viewing a single comment thread. View all comments

MarvelAtTheSky t1_j81w0lw wrote

I appreciate your well done calculations. The only missing link is the heat energy heat pump water heaters collect is costing money from another source in colder climates, such as ours. The BTU’s taken in are collected from that generated by our heating systems in all but a few months of the year. While these do lower the electricity used by the unit themselves, unless the place they are located has excess heat such as, for example, that from a greenhouse that experiences solar heat gain, they are making another system in our homes run harder or longer to generate the heat they are collecting. Heat Pump HVAC systems benefit from being able to move very large amounts of air in the outdoors so they represent huge savings now since newer versions can overcome their balance points at very low temperatures, but in northern areas the balance points of tanked heat pump water heaters can only be overcome by being in our heated spaces indoors and those spaces need to have a high volume of air available for their fans to move enough air to collect heat from to not need the electric element.


thenewtbaron t1_j8206d9 wrote

Well, I am sure you know that 6-10 feet under the ground it stays a regular 50-60 which is above the use temps. If you throw it into a furnace or washer/dryer room, that is free heat. And the ground is a huge tank of that heat (it's why it is hard to heat basements sometimes.

Most people don't have heating in their basements. So it really is negligible.

But I have already included the full cost of running it purely as a resistance heater for 8 months and it costs less.

The thing is that it doesn't take a lot of heat to compress down in a heat pump, even if it decreases the resistance usage by half over the whole year, we are at my second number.

If a house heat pump can get warmth for a home out of 40 degrees without using resistance heating, then a heat pump water pump can get it out of a 50 degree basement


MarvelAtTheSky t1_j827tbz wrote

Yeah, I do Ground Source Heat Pump calculations too. All Air Source Heat Pumps such as those on these water heaters require a large volume of constantly heated air that above their ‘Balance Point’. Because of such a small fan on the water heaters the balance point is pretty high at 50-60, but you don’t want it to operate at the balance point, it’s least efficient there as at that point it’s COP 1 or at 100% parity to resistance heat. You would want a heat source that is hotter to get the rated efficiency otherwise the compressor runs progressively harder to extract enough heat.

1kw = 3412 BTU, it takes 1 BTU to raise 1lb of water 1° F, 1 gallon of water roughly weights 8.33lbs. So, 8.33x50Gallons = 416.5 lbs of water x 73° rise to get to 120°F tap temp = 30404.5 BTU / 3412 BTU = 8.9kW input total the interval depends on the wattage rating of the water heater and usually is presented as a recovery time. But 30404.5 BTU needed, that is a lot of heat. Ground source heat pumps require acres and acres of tubing to collect that much heat and their COP is very high at like a 5 or more, water heaters are only a 2.2ish. So the house’s basement would have to be hundreds of feet deep or have a large footprint to collect that much heat or loose 30404.5 BTU on a consistent basis to be totally free of the water heater using the resistance coil. Unless the heat comes from the atmosphere outdoors it’s costing you something, somewhere to move into your space. Newer, REScheck compliant houses loose around 21000 BTU per hour, so at 30404 to totally negate the need for the water heater to not use the ‘backup’ resistance coil. Your furnace would be chugging hard produce those BTU’s and that energy use issn’t required to be on the Energy Star label and is what is costing you money in whatever fuel source it uses.


thenewtbaron t1_j829geq wrote

Hundreds of feet deep.. sure bud.

You still haven't answered the fact that you get a benefit of savings in hot summer, you know the four months you mentioned that it would benefit that ac. If you use resistance heating, it will be the same as a normal heater heater

So, even if the water heater used full resistance heating for 8 months, and heat pump to actually cool the home during the summer for 4 months... It is cheaper than a normal water heater.

It is cheaper. No way around that... And it pays for itself I'm 5-7 years.