My Solar Thermal Hot Water Adventure

[Solordabber]

So I live in a home, a typical Northern California ranch style house built in 1960 before there was any thought about energy conservation and it has a two car garage and a 50 gallon gas water heater on the far left side of the house and two bathrooms on the far right side of the house where all the bedrooms are. This, of course, means that taking a shower involves a full minute plus wait before getting into the shower. The wait is even longer in the winter as the copper water lines were uninsulated.

Then a coworker at work (we're hvac techs) was telling me about his tankless electric water heater he installed in his home and it gave me ideas about how to deal with this long wait for hot water not to mention the waste of water going down the drain. I had recently had solar photovoltaic panels installed on my roof and the cost of natural gas had greatly increased in my area so I installed a 27K watt tankless water heater in a closet sort of between the two bathrooms. I have 3/4 inch copper going to the water heater but only 1/2 inch from the heater to the bathrooms for quick response. It takes no more than 5 to 10 seconds for hot water at each shower head. Then, of course, I insulated all the water lines under the house.

It worked great! We no longer have to wait over a minute to jump into the shower and my cost is reduced due to my now lower electric costs, my lack of heating a tank full of water and my lower water usage. But, I have six people in the house and watching 16K watts going through the meter when someone was in the shower and the wash machine was going kind of bothered me and since it was sill in the springtime, my NEM credit deficit with the power company was increasing!

Then I started thinking about preheating the water with solar thermal panels. I found two Alten 4'x8' used panels for sale for $200. I had to drive 50 miles to get them and when I got there I found that one of them had freeze damage but ended up buying them anyway for $150. To me it was a bargain since I know how much new panels cost. I tested the good one by leaning the panel against my garage door and connecting it to my sump pump and it that in 40 gallons of water in a clean plastic trash can. In one hour it heated the water to about 125 degrees F and I was very impressed with the panel but concerned that the sump pump might not handle such warm water and quickly removed it.

To be continued : - )

[J.P.M.]

I'll wait for the to be continued part.

[Solordabber]

Continued...

So then I connected the panel directly to my old gas 50 gallon water heater. The water heater was by passed, so I reconnected that. I connected an old grundfoss type pump I had and controlled that with a time clock. It worked great with tank temperatures over 100 degrees until the first overcast day we had. That day the panel brought the tank temp down to outside air temperature. Not good!

Then I ordered a Misol 110 volt solar temperature controller for $78 from Amazon and it worked great! It took just under 2 weeks to get it which is fast considering it probably came from China. It has two tank temp sensors and one panel sensor and it only turns on the pump on when there's a 15 degree temp differential of the panel over the hot water tank. My daily tank temperature gets up to 130 degrees now during the summer. When taking a shower, I get 100 degree water (my tankless heater setpoint) for about a minute or two then the 130 degree water starts coming in and I have to adjust the shower valve to mix in a little cold water which I love because I know I'm on clean free heated water (at least energy wise).

Then last week I found a guy giving away two Alten 3'x8' solar hot water panels on Craiglist because he had his house re-roofed and he was thinking of using the space for photo voltaic panels instead. Perfect! I responded with an email and ran over there that night to get them. He said he had over 36 responses in the first hour of the ad and I happened to be the first guy to answer!
He said he mentioned that there was copper in the panels in the ad to peak the interest in recyclers thus the big turn out. Imagine what those recyclers thought of me "wasting" those panels by only pumping water through them!

So that's where I am today, with a single Alten panel leaning against my garage door heating my water and two panels waiting to go on the roof possibly during this 3 day weekend. I'm hoping to evenutally find a used solar water storage tank with a heat exchanger that's larger than 50 gallons. I know with the extra solar panels on the roof I'll need a tempering valve but I'm trying to decide what to do about freeze protection. I guess with a heat exchanger I could just add glycol to the secondary loop or I thought about installing electric freeze protection cable around the piping and panels on the roof. It shouldn't have to come on very often as I'm in the San Francisco Bay Area and it seldom freezes here but I haven't decided. Either that or I could just do a drain back system. Any thoughts?

[inetdog]

Solar preheat is problematic with tankless, since they have a minimum burner output and so product of flow rate and delta T has a minimum value or they shut down completely.

[Ampster]

Over thee years I have done two solar hot water systems. They both worked great but considering the complexity of the plumbing and the space on the roof, I have concluded that I would rather have more PV panels on the roof and use a Heat Pump Water Heater. That makes even more sense in my current location because I do have frost and I would have had to do a drain down or glycol system. That would have added even more complexity. Some people will argue that HPWH are too complex or wear out. I have not had that experience yet.

[Solordabber]

Solar preheat is problematic with tankless, since they have a minimum burner output and so product of flow rate and delta T has a minimum value or they shut down completely.

Thanks for alerting me to that. I'll be watching for any problems. So far with the two temperatures being so close I'm not seeing too many problems but when winter comes...

[Solordabber]

Over thee years I have done two solar hot water systems. They both worked great but considering the complexity of the plumbing and the space on the roof, I have concluded that I would rather have more PV panels on the roof and use a Heat Pump Water Heater. That makes even more sense in my current location because I do have frost and I would have had to do a drain down or glycol system. That would have added even more complexity. Some people will argue that HPWH are too complex or wear out. I have not had that experience yet.

I've thought about this too and I can see the advantages here. The only reason I did this is because of the distance from the garage to our bathrooms and so the small tankless heater looked attractive. But if all this doesn't work out, I may look for a place to put a heat pump water heater at this end of the house!

[J.P.M.]

Solordabber:

What others have said, plus :
- Without freeze protection, know your water panels and piping will freeze at some point.
- Wrapping piping will do nothing for the panels and, as a practical matter you'll never wrap the panels with heat tape.
- Get familiar with how flow controllers work.
- For direct systems - that is where potable (and freezable) water circulates directly through the collectors - controllers are and have been available for a long time that not only regulate flow during daylight hours but will also initiate flow during freezing or close to freezing conditions. They cost ~ $100 US or so. For mild climates that logic seems to work well if sensors and insulation between sensor and the outside environment are maintained. The lost energy is a small penalty in mild climates. My system operates in freeze prevention mode ~ 45 - 90 minutes/yr. in cycles of ~ 3 minutes each.
- I'd also reinforce Inetdog's comment about preheating systems not working well with tankless systems.
- For that matter, I'm not a big fan of tankless systems for residential applications. There are better, cheaper and more reliable ways to effect almost as much in the way of energy savings that take a lot less babysitting than using tankless systems.

[inetdog]

Thanks for alerting me to that. I'll be watching for any problems. So far with the two temperatures being so close I'm not seeing too many problems but when winter comes...

The problems, if you experience them, will come when the input temperature (solar heater output) is closer to the set point temperature and you are using a faucet or low flow shower head. The degree-gallons-per-minute temperature rise you need from the tankless becomes lower than the minimum burner output. If you are using an electric tankless, this may not apply as the electric heating element can be more finely regulated.

[Solordabber]

The problems, if you experience them, will come when the input temperature (solar heater output) is closer to the set point temperature and you are using a faucet or low flow shower head. The degree-gallons-per-minute temperature rise you need from the tankless becomes lower than the minimum burner output. If you are using an electric tankless, this may not apply as the electric heating element can be more finely regulated.

Yes, this is an electric tankless water heater. Maybe this is being confused with the older style electric tankless heaters that turns on each element with a contactor rather than a solid state device (like mine) that slowly ramps up each heating element as needed?

[J.P.M.]

Yes, this is an electric tankless water heater. Maybe this is being confused with the older style electric tankless heaters that turns on each element with a contactor rather than a solid state device (like mine) that slowly ramps up each heating element as needed?

At this time, if you have mostly eliminated the wait for hot water w/ the electric tankless, I'd leave it at that.

Sort of after the fact, but if it was me, given what you've described, after insulating the lines as you did, to address what reads like the main the main concern about wait and water loss, I'd have skipped the tankless and put a plain old, cheap, simple and easy 20-30 gal. elec. resistance powered tank in the closet between the bathrooms, slaved it to the bathroom hot water fixtures, insulated the crap out of it and plumbed the input from the 50 gal. heater at the other side of the house. No/small wait/waste for/of hot water, easy plumbing, lower initial cost, few maint. problems. Super insulating the tank would have cut the standby losses to maybe 2-3 kWh/day. You might have needed to run a 220 V line. Penalty over tankless: A bit more tank standby losses. Benefits: probably lower initial cost, probably easier maint. and probably better reliability from dumb but known/proven tech. along with likely easier maint., especially in a few years when the HX in the tankless fouls up and/or fails.

I'm a big fan of solar thermal and have a solar thermal water heater on my roof as well as having designed more than a few over the years. I think I might know a bit more about their design, operation maint. and economics than the average bear. IMO, your solar thermal scheme, while workable, won't get you as much as you might think, with a lot of the savings, such as they may be, offset by the PITA factors most folks don't know about or want to deal with that are needed to keep a solar thermal system in operating order.

Keep what you have and get low flow shower heads.

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

[inetdog]

Yes, this is an electric tankless water heater. Maybe this is being confused with the older style electric tankless heaters that turns on each element with a contactor rather than a solid state device (like mine) that slowly ramps up each heating element as needed?

Actually, I was thinking of a gas fired tankless, as the full on load of an electric tankless puts a real strain on the electrical system and electric costs a lot more than piped natural gas per therm.