Solar heating project equipment and calculations

[df0rster]

You can multiply that with a heat pump, but..............
Considering you can only get about 600W into a heat pump water heater, it is going to take a few! That is kinda expensive and an elaborate a system. And where does that heat come from.

I heat water with resistance PV "efficiently" by keeping it at power point with electronics. It can be low cost and respond instantly to changing clouds. No costly batteries inverters or charge controllers. My cost is only the panels. Considering the simplicity, access to cheap used panels and if there is space, it is not such a bad option. And you can always do something else with that power in the summer. It is simply a math/engineering problem. I designed a system for a guy in Lithuania doing concrete floor hydronic heating with 1400W of panels. Seemed pretty low and he was only trying to heat a little. I've never heard back.

I was going to mention the 4200 btu/h that Ampster just posted. Where are you getting the 600W number from? Maybe a different water heater? I can't find many of the water heaters that specify the rating on their heat pump or even their power requirement. They just give an amp size for the breaker which doesn't tell you anything about the heat pump.

In your second paragraph I think you are explaining using DC from panels to heat water? I guess you can cut the inverter losses but other than that I don't see what else you'd gain. But then I am not a solar expert so I could be missing something. The problem with me doing that is that this second solar array I am installing for heat is located about 250 ft away and I kind of need to backfeed the AC power into my main house panel for this plan to work. Otherwise I have to run another set of DC wires and can't use my existing buried AC feed to the shop.

The Rheem HPWH is actually not that expensive. I think they start around $1200 for the 40 gallon model with a $200 tax credit or rebate.

My other option I have considered are using solar collectors to heat water but has some disadvantages of the additional plumbing outside and the collectors are very expensive. It will cost about what I would spend on the solar setup & water heater just for a single vacuum tube solar collector that provides maybe 15k btu/h, and I'm not even confident in that number. With my panels and 6kw inverter I can provide about 17k btu/h using the HPWH. My total investment will be around $4000 for the HPWH setup and about $1k of that was already spent on the 5kw of panels. The rest is the heater, inverter and mounting hardware. Floor piping and all the plumbing and wiring is pretty much all in place.

It will be an interesting experiment for sure, and will qualify for the 26% tax credit. So at a net $3k investment I figure it will pay for itself in gas savings in about 3 years. Not even including the additional power in the summer when we don't need the heat, and if I can find a way to use the HPWH for domestic hot water during the non heating months it will save on hot water too.

Now all this being said, I would love to find someone who has actually tried this. I'd hate to do all the work and find that I was really only getting 2000 btu/hr or something less than what they advertise..

[mjs020294]

The Rheem HPWH is actually not that expensive. I think they start around $1200 for the 40 gallon model with a $200 tax credit or rebate.

The tax credit is $300 or at least it was in 2021. Personally I would recommend going with an hybrid tank at least the same size as you have now. They are little smaller than the sticker size and they don't recover as fast. We switched from a ten year old 50g traditional tank to the 50g Rheem. Although our electrical consumption for water heating has dropped from 12kWh a day to 2kWh a day the tank definitely runs low a lot more than a traditional tank.

[bcroe]

So when you say several times on the heat pump does that mean 25kwh is
really more like 50 or 100kwh of resistance heating?

The usual window units are terribly inefficient, not to mention noisy. My shop
air to air hardly runs summers here at 42 deg Lat. Its real mission is keep the
shop a cool working temp most of the time, and at least well above freezing the
coldest days of the year.

With the air warmed, the shop floor is never noticeably cold when working, a
lift keeps me off the floor. Often sheets of cardboard are thrown down to catch
drips, no cold there. But this may not work for a house.

Yes the Coefficient Of Performance for a Heat Pump can exceed 4 in some
conditions, so 25 KHh electrical input puts out 100KWh of heat. The COP
varies with the temps involved, but is almost always better than the 1 of a
resistance heater. Bruce Roe

[PNPmacnab]

I can't find anything directly specified but, the RHEEM units are specified with a heating element size and total wattage being 500W more than that. Being the heat pump and resistive element on at the same time. My NYLE HPWH is 600W. So someone with a 4,000W array is going to have to buy a lot of water heaters or go to a commercial unit. You can buy a lot of panels for $1200.

[Ampster]

I can't find anything directly specified but, the RHEEM units are specified with a heating element size and total wattage being 500W more than that. Being the heat pump and resistive element on at the same time. My NYLE HPWH is 600W. So someone with a 4,000W array is going to have to buy a lot of water heaters or go to a commercial unit. You can buy a lot of panels for $1200.

All the Rheem units used the same 4200 btu/h compressor regardless of the tank size. The specs are on the Rheem site and df0rster already posted those specs. Your Nyle is probably 120 volts and if I recall does not include a tank. There are multiple ways to skin a cat depending on the circumstances. You already know the benefits of a HPWH since you have a Nyle. The situation of df0rster is unigue and it does not appear practical for him to do exactly as you have done. I am going to convert my gas water heater to a 50 gallon Rheem which from a thermal efficiency does not make sense but it is a better allocation of my resources and a hedge against natural gas price increases. In my previous home with a HPWH if I needed more tank capacity I simply raised the tank temperature and used a tempering valve to limit the output temperature to 120 degrees.

As I mentioned earlier, I am not trying to change your mind. However using df0rsters assumptions I think his concept is a better fit for his situation. You are making assumptions to skew the argument toward the solution that is specific to your situation and not his.

[PNPmacnab]

No, I just don't think a lot of people who think HPWH are the greatest thing in the world have put a lot of thought into it. All the NYLE were 240V.

[Ampster]

No, I just don't think a lot of people who think HPWH are the greatest thing in the world have put a lot of thought into it. All the NYLE were 240V.

But you have to agree that on a rank order of efficiency the HPWH is 3 to 4 times more efficient than resistive heat? When replacing an electric water heater with a HPWH it doesn't take much thinking to see the payback. Clearly you must have thought about that? For some, with high energy costs it doesn't need much more analysis because the savings are even greater than the EPA tags. Most people , do not have the DC switching and programming skills required to implement your solution. In the context of this thread, the OP's analysis is fairly thorough and he has obviously put a lot of thought into it.

[df0rster]

The usual window units are terribly inefficient, not to mention noisy. My shop
air to air hardly runs summers here at 42 deg Lat. Its real mission is keep the
shop a cool working temp most of the time, and at least well above freezing the
coldest days of the year.

With the air warmed, the shop floor is never noticeably cold when working, a
lift keeps me off the floor. Often sheets of cardboard are thrown down to catch
drips, no cold there. But this may not work for a house.

Yes the Coefficient Of Performance for a Heat Pump can exceed 4 in some
conditions, so 25 KHh electrical input puts out 100KWh of heat. The COP
varies with the temps involved, but is almost always better than the 1 of a
resistance heater. Bruce Roe

agreed. I just had a couple of free ones. I plan to look into a used heat pump or split AC unit to use in the shop since I'll have additional capacity in the summer. Either that or I could invest in crypto mining!!

[df0rster]

But you have to agree that on a rank order of efficiency the HPWH is 3 to 4 times more efficient than resistive heat? Clearly you must have thought about that? For some, with high energy costs it doesn't need much more analysis, especially for those who do not have the DC switching and programming skills required to implement your solution. In the context of this thread, the OP's analysis is fairly thorough and he has obviously put a lot of thought into it.

Yes, definitely more efficient that resistive heat. The heating elements in the water heater are equivalent to the back up heat strips in a heat pump system, and those things can spin a meter!! I think the HPWH will be at least as efficient as a standard heat pump used for heat and AC, depending on install, source air temp is the most important factor I imagine. out of all the options I have considered, I think a geothermal water heater would be most efficient. but I have priced those and they are cost prohibitive considering I already have a main source of heat. The HPWH is basically a way to utilize my free solar energy and keep my toes warm in the winter!

I'm excited to get my experiment going! But I won't be able to really test it out until next winter unfortunately.

So now I want to start an easier discussion on which inverter to purchase. I may start another thread but will ask the initial question here. GroWatt or SMA sunny boy? About the same price for a 7kw but Growatt is a hybrid and will take batteries which might be nice to have that additional backup at some point.

[Ampster]

............

So now I want to start an easier discussion on which inverter to purchase. I may start another thread but will ask the initial question here. GroWatt or SMA sunny boy? About the same price for a 7kw but Growatt is a hybrid and will take batteries which might be nice to have that additional backup at some point.

Given the layout of your property that could make some sense and give you some redundancy. As you know from discussions on other forums there is a strong feeling about using multiple packs for redundancy. My decision to go with a 3P16S pack was partly driven by the fact that I already had an expensive Orion BMS. I am in the process of adding more micro inverter capacity to a shed on my property which might test the limits of my Skybox. If I can't AC couple them I can always connect them to my service panel. I would just lose their generation when the grid was down.

As far as which inverter to buy I think there are a lot more threads on the DIY forum about using GroWatts and Sunny Boys than I have seen here. I know very little about either of them so I don't have much further to add. Keep us informed.

[PNPmacnab]

I think the fact that I couldn't get a correct answer of how many watts a RHEEM HPWH was proves my point.

[mjs020294]

I think the fact that I couldn't get a correct answer of how many watts a RHEEM HPWH was proves my point.

All the Rheem Hybrid tanks use the same compressor and element:

Compressor = 4,200 btu, max wattts = 500 (mine draws between 350-450watts)
Element = 4,500 watts

https://s3.amazonaws.com/WebPartners/ProductDocuments/855D8C69-ECD2-4E5C-9DB6-ABF24A6FDF4F.pdf

[Ampster]

I think the fact that I couldn't get a correct answer of how many watts a RHEEM HPWH was proves my point.

Now that you have the correct answer, what is your point?

[df0rster]

All the Rheem Hybrid tanks use the same compressor and element:

Compressor = 4,200 btu, max wattts = 500 (mine draws between 350-450watts)
Element = 4,500 watts


https://s3.amazonaws.com/WebPartners...F24A6FDF4F.pdf

500 watts is basically a refrigerator compressor. That doesn’t seem like much at all.

[mjs020294]

500 watts is basically a refrigerator compressor. That doesn’t seem like much at all.

Which is why its so ridiculously cheap to heat water using eco mode. Funny you should compare it to a fridge; since I installed my electricity monitor 24 days ago my fridge has used 45.62kWh and the water heater has used 27.88kWh. I expect the water heater to use a lot less electricity has the weather warms up outside.