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MarvelAtTheSky t1_j817pxy wrote

I do home design including Manual J / efficiency calculations as one of my many jobs. My recommendation on a heat pump tanked water heater is really consider if where it needs to be placed has excess/unwanted heat and very good airflow to were the heat comes from.

A heat pump works by pulling heat energy out of the air, which requires another source, usually a furnace in norther climates, to provide more BTU’s to replace it. In the Summer this helps your A/C, but since there is lower heat available in the space it will use its standard ‘backup’ electric resistance heating element, making it into a just standard electric water heater. Also the majority of the heat pumps on tanked heat pump water heaters don’t last very long, only 10 years at most, which is how long many tanked water heaters last anyway, depending on water quality. So even with tax credits and rebates, is there going to really be a return on investment for the heat pump water heater. Heat pump tankless waters make a lot of sense in southern states because they can just go in a garage or outdoor closet and the more moderate climate affords then ample heat energy from the outdoors all year long without the threat of freezing or needing more ambient air heat, but in PA, they are rarely a good investment.


thenewtbaron t1_j81lk5a wrote

I am going to have to disagree with your assessment on the return of investment

a rheem 50 gallon regular electric 9 year tank costs 700$
A rheem 50 gallon heatpump 10 year tank costs 1700$

So, with a heatpump, you are playing 1000$ more for the difference. that is 100$/year.

Will the system save 100$/year? odds are, yes.

if we take the estimated annual usage of the regular tank and the heat pump and divide it per month we'll get.

regular tank 3400kwh/12 = 283kwh/month
heat pump 837 kwh/12 = 70kwh/month.

If the heat pump runs like a regular water heater for the months that have below 70-80 temperatures but like a heat pump at 70-80 temperature averages then 8 months at regular and 4 months at heat pump, that comes to 2544kwh. The rate average on the energyguide is 14 cents/kwh.

so, you'd save 856kwh or about 120$/year. that means you'd get 20$ worth of savings in a year, 200$ for ten years.

Now, the Rheem HP water heater says it can work down to about 40 degrees, my basement floats around 60 in the winter and if I throw it near the washer/dryer it will get a bit more heat from those being run. Add to the fact that not everyone in my household takes showers at the same time, so it will allow the heatpump to be able to regen the heat over a longer period of time.

So if we up the usage to half the year rather than a quarter of the year, we are at 2118 kwh, so saving 1280 kwh per year, we are at 180$ per year total savings. or a savings of 80$/year or 800$/10 years including the 100$/year purchase.

or another way to think about it, is that is will pay itself off in about 5 years, and you should get another 5 out of it. If you have the money, why not, it will save you money and at the worst of times, it is just a regular water heater.

the tanksless ones are dumb though for northern climes


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.


shanafme t1_j81od0d wrote

Regardless of what this guy says, I installed a HP hot water heater in my basement this past summer and I'm saving big bucks. My previous electric water heater was probably using 30-40% of my monthly electric bill. With the HP, my electric bills have probably halved (even considering recent price hikes). Sure, it will be a few years before I break even, but I think it is worth it, especially with the price of electricity increasing as it has been.


Aggravating_Foot_528 t1_j81q2em wrote

Did you have a 220v outlet already?


the_real_xuth t1_j81sd01 wrote

> My previous electric water heater

Based on this comment, presumably they did. But even if they didn't, adding a 240V outlet is not very expensive as long as its location has reasonable access to the breaker panel and you have available capacity. If you don't have available capacity then you really want to consider upgrading that in the semi-near future regardless because you will absolutely want it.


Aggravating_Foot_528 t1_j81t9ng wrote

Duh. I totally missed that they swapped out electric for electric. My house is gas so whenever I think about adding electric I think about the 220v line.

I have 200 amp service but it's unclear at what point do you need a 2nd box if you concert a lot of gas appliances to electric or add a plug in car.

And as you said I know physical box space makes a difference too.


the_real_xuth t1_j82t846 wrote

If you have 200 amp service in your house, the wiring to your house is at least sufficient for household electrical needs for the foreseeable future. However you may want more discrete circuits in your house than your panel currently holds or you might want to wire in a bunch of new circuits to a location far away from your electrical box, and then it is often worth your while to put in a sub-panel. Presumably if you have 200 amp service you likely have a decent sized electrical box. And even once all of the slots are full, you can get away with a few breaker slots being doubled up. But eventually there comes a point where the box is too crowded to safely work in it.

For instance when I moved into my new house, I wanted to set up a small wood and metal workshop in my garage. My garage is on the opposite side of my house from my main electrical panel so it was worth it to me to hire an electrician to install a subpanel in my garage so that I'm only running one large electrical circuit all the way across the house and through a foundation wall rather than a dozen smaller ones.

Similarly when I moved in, I had 100 amp service to my house and the main panel was both small and completely full. 100 amps would cover my immediate needs but I did want more circuits in the main house. So I could have gotten away with installing yet another sub-panel near the main panel. But I had no headroom for doing things that I anticipate will likely happen in the future (switching to a heat pump for heating and hot water, I can't imagine that my next car won't be electric, and likely other unknown uses). Thus I opted to just increase my service line to 200A and replace the original small panel (that likely was installed when the house was built in the 1940s) with a much larger panel.


MarvelAtTheSky t1_j81u4x8 wrote

So, your electric bill is lower, but if it is collecting that much heat in your basement enough to overcome its balance point on a consistent enough run rate to save ‘big bucks’, where is the heat energy coming from that gets to the basement for it to extract when it’s 30 degrees outside?

Your floors are providing the heat via your furnace heating the house. If you read my post I’m not giving hard numbers or saying they don’t have a use case, but the movement of heat energy is very complicated, so much so my work is done for the Department of Energy, ASHREA and ACCA and is only accepted when it’s corroborated with three other engineers work. If your loosing that much heat energy from the buildings thermal envelope, your biggest savings are insulating your house, in which case once you would, your heat pump water heater that has a high balance point of close to 50° would end up running almost entirely on its electric element.


shanafme t1_j81wa5z wrote

Yes, I went through the process of improving the home’s insulation prior to installing the HP water heater. HP Water heating is using about 50-60 more kWh in winter months than it was in the summer. Also using an air-source HP for general heating.


gullyBo1z OP t1_j82u0c5 wrote

I do see where you're coming from and also delved in Manual J calcs. Aerospace engineer and work in utility scale power production so I've had a fair share of energy modeling done. You're also right that the basement location will start getting colder and pull heat from the floor above to keep heating itself.

If we do end up using rockwool comfortbatt and insulating in the joist cavity, we might avoid that energy pull from the floors.

My main reason is to eliminate a propane consuming thankless heater provided by the builder. The Navien 240A2 has an estimate of 450ish $/year assuming 2.4$/gal of propane. The electric bill on the HPWH is est to be at 100-150$/year.

If we live in this home for 5+years then that is a lot of savings. Now that you mention the added bonus of placing it in a warmer location, I'm tempted to place it in the garage but the install charges would be high and the winter will make that garage way more colder than the basement.

This is a good start for anyone more interested in the deep research on the topic