Solar hot water panel placement when South face not available?

[Steeler.Fan]

I am building a home in a year round, warm, sunny location. My installer recommended two 4x8 south facing panels and 120 gallon storage tank with electrical element for back up heat. I will have substantial electrical loads so I need to save my limited south facing roof pitches for my PV panels. My greatest HW consumption will come from showers and baths; morning showers (after our regular daily exercise), late afternoon showers (before dinner) and bedtime showers/baths for children. Therefore, rather than putting both of my solar collectors on the west facing pitch, I am proposing putting one panel on the East roof pitch and one on the West pitch. Because I have read that W & E facing panels will produce 10-15% less energy than south facing panels, I will install 4x10' rather than 4x8' panels. I realize that splitting my panels between the E & W facing pitches will require me to install another circulation pump. I want to avoid heating water in the evenings/nights or on cloudy/rainy days then circulating electrically warmed water to my panels in the morning. Therefore, I will add a separate 80 gal electric HW heater as a back up heating source, see my attached proposed layout. I will use a Heliodyne Delta T Pro controller, which can manage 2 different pumps with a different differential for each.
Any comments or suggestions are welcome.
190307 MKW Solar Thermal Diag.jpg

[peakbagger]

Its pretty rare for anyone actually installing SHW these days. PV panels and Heat Pump Water Heaters have pretty well replaced them. The problem with SHW is you end up with all sorts of hot water during the summer when you do not really need it and in the winter when you need it you dont have enough. With PV and HPHWH you use the PV to offset the electric bill of the HPHWH which is far more efficient then a regular hot water heater. Also works as a dehumidifier.

FYI unless you need a storage tank Rheem makes what looks like a regular hot water heater but it has a cooper coil wrapped around the interior tank so no need for a heat exchanger. or a storage tank although with your proposed demand I think you need to go very large on the tank. An external HPHWH is probably a better fit than an integrated one.

[Ampster]

I agree with peakbagger on that. I installed a HW system in 1982, sold that house and installed another one in 2007. Since then I have installed PV solar and Heat Pump Water heaters in two homes I own and a third in my sister's all electric home. I am in California and the economics are optimum there for PV solar. Just my point of view from where I am standing.

You may wish to continue your hot water solar collectors but I would suggest doing the math about resistive vs heat pump efficiency as a backup heat source. If you are in a warm climate you will enjoy the benefits of the cool air that can supplement your cooling loads. I particularly like the Rheem which has connections for 8 inch duct that let's you pull heat from hot spots and send cool are where needed. It is also easily programmable so you can run it at optimal times suitable for your needs. I can also remotely put mine in vacation mode which I forget to do until I am an hour into my vacation. Hot water is also an efficient energy storage medium, especially if you ar using hydronic heating.

[J.P.M.]

I cut my teeth on solar thermal in the mid '70s. It was the original reason I changed careers and wound up a P.E. It changed my life. I'm a fan. As something of an analogy, I'm also a fan of steam powered locomotives (and backing the Big Boy restoration). I ride legacy steam trains whenever I can, but for regular travel I either drive or fly.

But, as much as I like solar thermal, particularly for DHW applications, if I was to do a solar DHW application today, it would be with PV and a heat pump water heater, particularly in the warm & sunny climate you write of.

Back of the envelope stuff:

Solar thermal systems as you describe them, and as your diagram seems to show, can have an annual year long system eff. of ~ 30-35 % if the system is well designed. That's, say, 35 units of heat energy to the load, with the load including tank standby/piping losses, for every 100 units of solar energy that hit the solar thermal array. My solar thermal system operates at ~ 40 % annual eff., but it's a pretty optimized design.

As for a PV+ HPWH system: I'm also in a sunny and warm climate. Since my PV system started up, 10/17/2013, through this morning before sunup, it has produced ~ 48,649 kWh of energy (~ 1,865 kWh/m^2). I estimate the P.O.A. irradiance on the array by conversion of GHI irradiance measured at the array for the same period to be ~ 288,365 kWh (~ 11,052 kWh/m^2). That gives me a gross conversion eff. since startup of 48,649/288,365 = 0.1687.

More back of the envelope stuff: I'd somewhat conservatively est. a heat pump water heater will probably operate with an annual C.O.P. of better than 2.0, but use that for talking #'s. That means if I get a HPWH to replace my solar thermal collector system, I might expect it to produce ~~ 0.1687* 2.0 = 0.338 kWh of heat energy to a load for every kWh of insolation that hits my PV array, or an annual eff. comparable to the solar thermal unit.

Kind of looks like a rough first est. of PV+ HPWH at ~ 33 % annual eff. is about a wash w/ 30-35 % eff. for a solar thermal unit.

Add to that, and believe me on this, just like a diesel electric locomotive vs. a steam powered locomotive, the PV unit will take less babysitting and maint. than the solar thermal unit will require. The only fly in the ointment w/PV + HPWH combo may be that the HPWH part may have some reliability issues. Things seem to be improving in that respect, but I'd suggest homework to see what the latest is on that score.

Lastly, You can probably expect to pay a fair amount less for the incremental bump in PV size and a HPWH than you will for a solar thermal DHW system of the sort your diagram shows.

Take what you want of the above. Scrap the rest.

[Steeler.Fan]

Well, you guys have me reconsidering my planned SHW system for my new home. Fortunately, I got permission from my county to install a HPHWH instead of the otherwise required solar DHW system for new homes. They gave me a variance because we don't plan to be there full time and dealing with all of the heat generated while we are not there is a pain.
I like the 80 gal Rheem duct ready HPHWH, which is available thru a local Home Depot. Perhaps I can run the 8" air intake duct vertically thru the ceiling and up to the peak of the attic. That would allow me to take in much hotter air than most users. I would run the horizontal cool air exhaust into the non air conditioned garage where my EVs will be parked. With warmer air than usual supplied and cool exhaust air removed from around the heat pump, it should be even more productive and able to heat water more quickly. Rheem states that their 80 gal HP, will allow 7 persons to take 8 minute back to back showers. I would like to raise the tank temperature to 140F rather than 120F and that may give me more hot water. Of course, I would need a cold water mixing valve to cool water entering the house down to 120F. I like the HP's vacation mode, which holds the tank at 65F while the house is not occupied. I also like the ability to control it remotely and receive messages remotely. My home will sometimes have 8 occupants and I hope that my above described installation can provide enough water. If not, perhaps I will need to add a well insulated 40 or 50 gallon electric HW tank, which can use the HP preheated water to provide a boost.
Any comments or further suggestions?
Thanks!

[Ampster]

The Rheem also has a hybrid mode so you get the benefit of high recovery when needed and efficiency of heat pump the rest of the time. As I mentioned I consider the HPWH an energy storage device and also run it at high temperatures with a mixing valve for safety.
I also cut a hole in the ceiling of my garage to pull the hot air from the attic. There are only two of us so I run the heat pump at 105 most of the time and at 130 or more during off peak. I have never had a cold shower. I also charge my EVs during off peak. HPWH and EVs allow me to leverage my solar PV investment.

[LucMan]

Well, you guys have me reconsidering my planned SHW system for my new home. Fortunately, I got permission from my county to install a HPHWH instead of the otherwise required solar DHW system for new homes. They gave me a variance because we don't plan to be there full time and dealing with all of the heat generated while we are not there is a pain.
I like the 80 gal Rheem duct ready HPHWH, which is available thru a local Home Depot. Perhaps I can run the 8" air intake duct vertically thru the ceiling and up to the peak of the attic. That would allow me to take in much hotter air than most users. I would run the horizontal cool air exhaust into the non air conditioned garage where my EVs will be parked. With warmer air than usual supplied and cool exhaust air removed from around the heat pump, it should be even more productive and able to heat water more quickly. Rheem states that their 80 gal HP, will allow 7 persons to take 8 minute back to back showers. I would like to raise the tank temperature to 140F rather than 120F and that may give me more hot water. Of course, I would need a cold water mixing valve to cool water entering the house down to 120F. I like the HP's vacation mode, which holds the tank at 65F while the house is not occupied. I also like the ability to control it remotely and receive messages remotely. My home will sometimes have 8 occupants and I hope that my above described installation can provide enough water. If not, perhaps I will need to add a well insulated 40 or 50 gallon electric HW tank, which can use the HP preheated water to provide a boost.
Any comments or further suggestions?
Thanks!

Your on the correct track with the extra PV and HP water heater. Raising the water temperature to 140 degrees will seriously decrease your efficiency and increase power consumption. The auxiliary resistance heater may be energized more often at that temperature. The refrigerant used in the HP (r410A) has condensing temperature around 110 degrees so depending on the ambient air entering the HP the heat of rejection may limit the the heat output in the HP mode requiring the electric resistance heater to make up the difference. That being said look into Sanden CO2 HP water heaters. I can't see you needing more HW than the 80 gallon Sanden will supply.
The other option if 80 gallons is not enough HW for you would be to install another tank in series with a re circulation loop that would circulate the HW from your HP to the tank during low use hours to give you extra capacity.

[Steeler.Fan]

Well my contractor called me and this is getting complicated. I neglected to mention or account for the fact that besides the 4 showers in 4 bathrooms, my wife designed the 2 master baths to each have a bath tub! A quick internet search result indicates that usual baths may consume 10-35 gallons but filling a bath tub to the top could take 70 gallons! I need to make a modification quickly because my plumber is coming in 2 days and will be running water lines!
The simplest solution would be to add a 2nd 80 gal HPWH.
Another solution would be to add a simple solar HW system consisting of two 4x10 panels on my west facing roof and a 120 gallon solar storage tank with electric back up. Most baths would be taken in the evening and the peak solar HW would be produced in the afternoon.
Any other thoughts or suggestions would be appreciated!

[inetdog]

Well my contractor called me and this is getting complicated. I neglected to mention or account for the fact that besides the 4 showers in 4 bathrooms, my wife designed the 2 master baths to each have a bath tub! A quick internet search result indicates that usual baths may consume 10-35 gallons but filling a bath tub to the top could take 70 gallons! I need to make a modification quickly because my plumber is coming in 2 days and will be running water lines!
The simplest solution would be to add a 2nd 80 gal HPWH.
Another solution would be to add a simple solar HW system consisting of two 4x10 panels on my west facing roof and a 120 gallon solar storage tank with electric back up. Most baths would be taken in the evening and the peak solar HW would be produced in the afternoon.
Any other thoughts or suggestions would be appreciated!

Don't forget that you will typically not be filling your bathtub with 100% heated water. But additional storage would not hurt, except for the higher standby energy losses from the larger tank.

[J.P.M.]

Well, you guys have me reconsidering my planned SHW system for my new home. Fortunately, I got permission from my county to install a HPHWH instead of the otherwise required solar DHW system for new homes. They gave me a variance because we don't plan to be there full time and dealing with all of the heat generated while we are not there is a pain.
I like the 80 gal Rheem duct ready HPHWH, which is available thru a local Home Depot. Perhaps I can run the 8" air intake duct vertically thru the ceiling and up to the peak of the attic. That would allow me to take in much hotter air than most users. I would run the horizontal cool air exhaust into the non air conditioned garage where my EVs will be parked. With warmer air than usual supplied and cool exhaust air removed from around the heat pump, it should be even more productive and able to heat water more quickly. Rheem states that their 80 gal HP, will allow 7 persons to take 8 minute back to back showers. I would like to raise the tank temperature to 140F rather than 120F and that may give me more hot water. Of course, I would need a cold water mixing valve to cool water entering the house down to 120F. I like the HP's vacation mode, which holds the tank at 65F while the house is not occupied. I also like the ability to control it remotely and receive messages remotely. My home will sometimes have 8 occupants and I hope that my above described installation can provide enough water. If not, perhaps I will need to add a well insulated 40 or 50 gallon electric HW tank, which can use the HP preheated water to provide a boost.
Any comments or further suggestions?
Thanks!

Comments ? Well, a few.

1.) On ducting, know and be careful/mindful that the HP works best when the ducting that handles the air flow in/out of the unit has the lowest possible pressure drop to ensure best/design air flow rate. Too long/small/flexible/ or otherwise flow restricting ducting will lower air flow and reduce performance.
2.) More on ducting, and maybe not a problem, but worth considering: Air introduced to a HP must come from somewhere and also get exhausted to somewhere after cooling. If air is taken from a new (tight(er)) conditioned space, or unless provisions are made for a (probably) outside air source, the HP may be choked a bit for supply air - the air has to come from someplace. If nothing else, tight spaces will not allow as much air ingress and so decrease flow when the HP starts sucking air from the conditioned space. Similarly, if cooled air is exhausted to a conditioned space, such as a garage, and the garage is reasonably tight, the HP may not be able to overcome the added flow restriction - sort of like blowing up a balloon that's already at max. volume.
3.) More on ducting: After you make sure ducting pressure drop won't cause problems, stop and think about what you'll do about duct condensation. Reason: The colder exhaust air will cool the ducting on its way to somewhere. As it does, any duct outside surfaces in direct contact with warmer (and moister) air will have water vapor condense on that surface. With no duct insulation, you will get condensate and it will drip. Think where the condensate will go before you start routing ducting. Putting ducting in confined, or inaccessible spaces is a formula for problems. If you insulate the ducts, make sure the insulation is first of all airtight and second not damaged by water. Fiberglass insulation is neither.

If you have the work done, don't expect the installer to know much of the above and if they do, don't expect them too tell you about it.
The above may seem a bit overly conservative. May not be a problem for you, but I've seen it and made the mistakes and too late thought about what can happen.

4.) If one of your goals in all this is to get a lot of showers done in a hurry, start by GETTING FLOW RESTRICTING SHOWER HEADS. Not a plug, there is a simple shower head that once had the name "Incredible Head". I think social pressures got on the mfg. to change the name. First off, besides having had a great name, they're cheap. Second of all, they use about 1.25 - 1.5 GPM. vs. 2-3 GPM for std. heads. Big box and others still have them but with less controversial names.

Bottom line: Reducing the demand on a water heater is the best single way to help ensure you get 8 showers and still have hot water.

FWIW, Rheem statements about how many showers on a tank are non quantifiable and so, simple B.S. How long per shower (more like how much water per shower), or what water supply temp. set point, or what flow rate ? At, say, 3 GPM and a, say, 6 minute shower, and the published Rheem 89 gal. first hour recovery will allow ~ 89gal./(3GPM*6 minute) = 4 - 5 showers or so, assuming you start with a hot tank. For 8 showers/hr., the heads need a flowrate of ~ 89 gal./(8 showers * 6 minutes/shower) ~ 1.9 GPM. Reduce the hot water demand and things go better. Restricting shower times with timers works but probably not as socially acceptable as low flow shower heads. Been there. Done that. Didn't come to a good end for me.

5.) Tempering valves are a safety issue, just get a good one. To my experience, most are not up to the task and either foul up with scale which then throws the temp. regulation off, or don't have good temp. regulation to begin with. The Watts valve was pretty bad for me in that regard. I switched over to Honeywell-Sarco after 2 Watts valves each lasted ~ 1 yr. before failing.

If you do get a cheap one, live with the lousy temp. reg. but make it accessible and easy to change out, and know that it probably won't do what you bought for for very long. Not a plug, but I've had decent luck w/the Honeywell tempering valve after the Watts valves. Mine has a temp. gage on it, but the accuracy of the gage is not good. I've seen a similar unit from TACO, but have no experience with that unit.

6.) I'd reiterate my comments about HPWH's reliability - or maybe the lack of it. They are more complicated than your mother's water heater and can be expected to require a bit more attention. I have no user experience with HPWHs so I'm left to what I think I know from an engineering standpoint. Also, I tend to not take user published comments too seriously, except that maybe if a larger number of bad reviews show up, something may be amiss. With that in mind, seems to me that there are a troubling and relative high percentage of lousy reviews for HPWHs when compared to the relatively lower percentage of lousy reviews for more standard types of water heaters. Maybe the required/necessary (but probably not done) added attention to maint. causes some of the problems, or maybe the HPWH idea still has some kinks to work out. I'd still consider a HPWR if/when I need to make a change, but seems to me that Caveat Emptor may apply here, at least fr now.

[Ampster]

As I may have mentioned I have purchase 3 HPWHs and only had issues with the GE one which will not be an issue for you because GE sold that business. The Rheem's have worked well with no issue. Regarding condensation, you will need a condensate drain that your plumber can install when he does the rough plumbing. The cold air coming out of the unit is very dry because any moisture condenses on the coils, hence the need for a condensate line. I have not experienced an issue with condensation on the ductwork with the two Rheems. The energy savings is worth the risk that might arise from the complexity of their operation. They have the same basic mechanisms as refrigerators.

Regarding your concern about capacity I would have your plumber do the rough plumbing for two water heaters in tandem.. You can install one and see how it goes. If you go with two you can always set the temperature on the first one lower so you don't waste energy keeping two tanks at high temperature. That will also improve the recovery rate on the primary tank.

[Steeler.Fan]

Thanks for all of the great comments and suggestions! I was very lucky to get that variance that allows me to install a HPWH rather than SHW system. They changed the law recently and very few if any will get a variance in the future. They also don't allow someone with a variance to change their mind and add a SHW backup system without drafting new plans and going through the building department approval process. We are going to go with 2 tandem 80 gal HPHW as long as Rheem doesn't tell us tomorrow that it would void their warranty.
My plumber didn't understand the reasons that I want to be able to heat the tanks to 140F if needed and add cold water mixer. If we heat both 80 gal tanks to 140F and add cold water (~75F) via mixer valve, we effectively gain 71.1 more gal of 120F water. (160g*140F) + (Xg*75F) = (160g+Xg)120F, after rearranging and solving for X, X=71.1g!

[LucMan]

I'm assuming you didn't read or understand my comment on the temperature setting for Rheem HP water heaters.

[Ampster]

I'm assuming you didn't read or understand my comment on the temperature setting for Rheem HP water heaters.

I did but had to reread it to understand your suggestion of a second tank for storage which would only be needed during high demand times. A storage tank would be a lot less expensive but the unknown is the cost of the recirc pump, control mechanism and the extra plumbing. It would be worth considering, especially if Rheem comes back negatively on the issue of high running temp voiding the warranty.

[Steeler.Fan]

LucMan and Ampster, I read your posts. Not sure what you meant by, "The refrigerant used in the HP (r410A) has condensing temperature around 110 degrees so depending on the ambient air entering the HP the heat of rejection may limit the the heat output in the HP mode requiring the electric resistance heater to make up the difference." The intake air temp may vary from 100-140F since it will be from my attic.
I am going to install an Aoetec z-wave 40 amp heavy duty switch for each HPWH, which will allow me to remotely power on and off these units according to my occupancy calendar and # of occupants and will also give me energy consumption data. I hope to use that data to compare how much energy that I am using when I change the tank temps from 100->120F vs 120->140F. If my back up electrodes kick when I go from 120->140F, the increase in consumption should be greater than when I go from 100->120F.
Is my logic about this data correct? How much greater would I see if the electric heat kicked in rather than staying in hybrid mode?