I have a 3.5 year old 2 panel Velux solar system with 80 gallon tank in NC. I have a recirc pump that needs to be on most of the day - it is insulated but pretty long. The electric backup is rarely used for 6 months out of the year and I suspect is basically $150 a year. I am installing PV and the utility is giving me lots of money but I have to switch to a TOU-D rate. This rate is cheap per KWH (.05 or .067 based on time of day) but it imparts a charge based on peak usage for a 15 minute period during peak hours. This charge is $3.70 for 8 months and $5 for 4 months. With the 4500 watts the hot water heater uses, the peak charge could add up to $200 a year which would approach my total other electric use/peak charge. I could add a NG tank at roughly $2000 to eliminate the peak problem. This might be a 25 year payback at our usual $.10 a kwh rate. The peak charge makes it a 10 year payback. At our new rates, the actual usage is really no different in cost to NG. I could add some storage for the solar system. The panels are due south and unobstructed. Today, I maxed out at 12:30 so storage would definitely help. The extra storage could be heated during off peak for cloudy days - hard to figure out this one. Turn up stat based on weather report? What would be ideal is to have enough storage to never go into backup but I think that much storage might be cost prohibitive. I really just need enough storage for 10 hours of peak but then I need an algorithm for how to manage it. If I didn't have solar, I could just have 2 tanks and crank to 150 during off peak and use a mixing valve to 120. I can still do that, but that would negate most of the solar system. The solar is limited to 140.

What is the name on the controller? Most European controllers can be over ridden to go to a higher temp and as long as you have a thermostatic anti-scald valve there is no reason why you cannot heat to 160F or higher if the tank is rated. Also, what size is the storage tank. Typical is 80Gal for 2 panels.

It is 80 gal and a Velux system. The mixing valve on it has a range to 160 but the installers don't recommend going higher as they have had more leaks. I don't know how to go higher on the tank but I am willing to do it if anyone knows how to go higher. There is "Wilo" on the controller in addition to Velux. There is a large brass screw that looks to be a setting to match the size of the tank and there is a red plastic knob with 3 positions.

Wilo is the pump. There should also be a control (white and grey box) with an LCD display on it which is where you change any parameters. The mixing valve is there to keep you from scalding yourself and is probably set at 120F and doesn't need adjusting.
A picture is worth a thousand words.....

The controller says Deltasol BS. I got into a setup menu and change a parameter called "S Max" from 140 to 150. I decided not to go higher yet. The tank has a stat with 3 lines - 110, 140, and 170 which might make you think it would be good to 170. Of course, I don't know what the controller will go to and what would be safe for the hoses etc.

I use them all the time.....You can increase S Max to 160. It won't hurt it but that should be the limit for now. Does Your tank also do the backup heating with an electric element or is there another tank to do the backup? If there is only one tank (both solar and backup) and there is an element doing backup, It should be kept as low as you can put up with, typically around 120. You really don't want to use the electricity any more than you have to.

The electric backup is rarely used for 6 months out of the year and I suspect is basically $150 a year.

The energy guide sticker on a GE heat pump water heater says it costs $195 a year to operate. So, does that mean you invested in all that solar water heating equipment to save $45 a year, and you are about save even less once you invest in more solar equipment?

Just a joke. I spend twice as much to make my own beer and still can't make one as good as Heineken, but I keep trying. Everything doesn't have a pay back.

I've upped it to 160 - thanks. I went down to what seems like 125 today on the electric backup (rainy and 2nd day of no sun) and it was pretty cool at the tap. I had to go back to 130 or my wife will kill me. I maybe able to go down to 125 if I modify the mixing valve setting although I realize that it shouldn't matter. It is a single tank that relies on stratification but as I've found out, the recirc pump really messes with the stratification. I can get down to about 105 at the bottom of the tank but no lower. I am considering adding a 105 gallon Marathon tank to the system to improve the uptake but for now, upping the system to 160 would seem to be a huge increase in storage (almost double). Thanks again for the help.

How big is the house? You should seriously look at the longest distance from the farthest tap and the tank and decide if you need the recirc. Shut it off for a while, close the valve on that line and live with it for a few days to see if you notice a lot of difference. Recirc can really mess up the stratification on a tank depending how it is done. I don't know your local codes but it is frowned upon to try to adjust the antiscald valve. I would, however, look at the piping to the taps and if they are 3/4", think about dropping them to 1/2" and insulating them (assuming you can get to them). It may not be that hard and the volume of water in the lines and the time it takes to get hot water to the taps goes down quite a bit.

It is 5000 sqft - the pipes are 3/4 inch pex with foam insulation. There is no access to the vast majority of them - there is a finished basement. I've gone back and put more insulation on the few I can get to.

The plumbing is setup as a loop and it is 2+ minutes to fill the loop which is unacceptable. I don't think our codes even require the antiscald valve - it is more a byproduct of the solar system. The service guy had recommended for me to adjust it to my comfort (not that it means that is code compliant).

The energy guide sticker on a heat pump hot water heater certainly doesn't account for a recirc loop. Can you imagine the lifespan of those cheap things with a recirc loop? Certainly less than 5 years - if I thought they had a real lifespan in this situation, I would just use a heat pump as my backup.

Well, you could take out the antiscald and run hot to the taps and have a smaller antiscald on each important fixture. With the tank at a higher temp, some protection is needed regardless. With the recirc as it is, it may be necessary to have 2 tanks, not an expensive ordeal in this case.

Why would I take out the antiscald? I would think I would want to distribute the coolest temp tolerable to minimize losses.

The temp today rose over 140 (bottom of tank) but only to 142 or so - laundry and dishes done during the day and tank was at rock bottom temp after 2 rainy days.

When the PV comes, I'll get an energy use graph so I can see when the backup runs. I have nothing close in the house to 4.5 kw (except the dryer and over which should be easy to figure out).

David, I would be cautious as you change the mixing valve setup.I am sure your building code requires one .Most pex is rated at 180F maximum typically at 125psi.Pex piping has a thermal expansion of approxamately 1.56 inches per 100 feet per 100F change in temperature.This shouldn't be a problem but worth mentioning. Also as you raise the limit of the solar tank the total btu input into the tank along with water pressure and resulting temperature can lead to temperature pressure relief valve opening prematurely.Adding another solar storage tank may not be of substantial benefit until you reach the summer months. Sizing your water heating demands correctly will steer you to making the right decisions in your design.

Posted By acwizard on 27 Feb 2013 06:11 PM
David, I would be cautious as you change the mixing valve setup.I am sure your building code requires one .Most pex is rated at 180F maximum typically at 125psi.Pex piping has a thermal expansion of approxamately 1.56 inches per 100 feet per 100F change in temperature.This shouldn't be a problem but worth mentioning. Also as you raise the limit of the solar tank the total btu input into the tank along with water pressure and resulting temperature can lead to temperature pressure relief valve opening prematurely.Adding another solar storage tank may not be of substantial benefit until you reach the summer months. Sizing your water heating demands correctly will steer you to making the right decisions in your design.

The PRV issues is only there if there is a check valve on the water coming in and there is no expansion tank on the water line (there should always be a tank with a check valve). I have lots of tanks running at higher temps and while the pex does expand more, if the OP hasn't heard it yet, he may never hear it as it is insulated, but this is to be determined. The pressure rating is not an issue as the water temp may go to 160 or 170, the pressure rating is way over 125 at that temp and besides these are minimum agreed upon ratings. The tubing can take a lot more, not that I like to push those boundaries.

With the recirc line causing mixing in the tank, the solar is at a greatly reduced capability because the water temp going to the panels is higher than it would be with proper stratification. Recircs sometimes don't work well with antiscalds due to the low flow rates going through them (depending on where the line is tied in). I still would install a second tank which will get rid of any issues.

Mike , I agree with you. Not always are systems installed correctlly .I have seen plenty of T&P valves leak and ruin a lot of personal property over the 32 years that I've been doing solar. As far as the second tank, there is no doubt that it would increase thermal storage capacity.Without additional collectors the water temperature generated will see a reduction during the winter months.

My this does get complicated. First - If I am just turning up the tank temp, why would that effect the Pex since the mixing valve (aka antiscald right?) is on top of the tank. I would never remove the antiscald - but I might change its position. Second - there is no question the stratification is an issue. I'm still not sure that a second tank solves the problem but I suppose it depends how you plumb it. Of course if the solar just preheats the water and the recirc is on a separate tank, it solves the stratification but doesn't allow the solar to help with recirc losses. I honestly think our recirc losses are greater than our actual hot water use. Third - The solar panels are ideally placed so we actually do have generation exceeding storage in the winter. That may certainly be different with a higher set temp - this is actually something easy to figure out. Thank you all for your help so far - I only wished I had figured all this out earlier - I just realized we had a mixing valve only after it failed.

The PEX is just a plastic tube and plastic expands with temperature. The hotter the temperature, the more it expands per foot, so at a relatively constant 120F, the expansion might be something you may not notice but at 160F it might. With the antiscald on the top of the tank, the expansion in PEX, after the antiscald, is the same regardless of tank temperature. If the recirc comes back to the tank through a port in the tank there will be an effect on the stratification of water in the tank. Mixing will occur no matter what you do (and this affects the efficiency of the solar) .

If it returns to the hot pipe at the top of the tank, this is reduced to nil. I doubt you are losing enough heat to negate the use of solar, but one thing you can do is put a timer on the cecirc so it runs for a couple of hours in the AM and the same at dinner time. Grundfos and Taco both have these models. It is true that the solar won't help with the losses in a 2 tank system as described.

If you have solar generated heat exceeding load most of the time in the winter then you are very oversized for summer use and you should have a heat dump. I doubt this because, unless your hot water load is 10gallons a day or less, there just isn't enough sun hours and intensity in Dec/Jan/Feb to do the job. Add to this your $150 electrical bill which is about right for a conserving household when solar produces 60-65% of the annual heat.

I think some smarter controls are needed. A PV powered solar pump can be used and the back up put on a timer as well. Where are you located? I could explain some things.

The recirc pump is on a timer but our irregular use has it on from 5a-10p. The pump does return to the cold line which puts the bottom of the tank at 105 right now - obviously high but I wonder how much difference it makes. Today at 8am - the temp on the collector was 55. 8:30 - 100. By 9, the pump was running. So - without stratification, the pump would have turned on just 30 minutes earlier. I wonder how much heat that actually generates. What would be your recs for setting up the second tank? I think the optimal would be winter, the solar tank would just preheat and the recirc losses would be handled by the 2nd tank. In the summer, the stratification probably doesn't matter much and so the solar could handle the recirc losses. So summer - tanks in series with recirc. In winter, the tanks in parallel - with the solar tank being just a preheat.

The solar is not doing what it could when you keep the bottom of the tank warm. I would have the recirc come back into the hot line just out of the tank. Nothing is ideal when there is an antiscald valve but it will at least allow the bottom of the tank to keep cold like it should. The solar tank should be the preheat tank and the the backup tank can have the recirc. Obviously, there will be some losses with the recirc but i think they could be more than made up for by better production by the solar. A bit tighter use of the the recirc pump will help too.