Some "Pros" just installed 4x10 SunEarth collector w 80 gal tank here in our SW Florida home w 5 baths. So far we've endured lukewarm showers even w as few as two showers a day despite much playing with differential temp and shut off temp settings. There've been plenty of sunny days... If the tank's one electric element thermostat is turned up enough it's ok but we really don't want a really expensive electric water heater, LOL!! We're told we'll have more hot water than we need in a few months but that's cold comfort now, pardon the pun. Playing with the elec element thermostat indicates we have about 105-110F water in the tank at the end of the average day, the controller (Goldline GL30) has no temp displays for either panel or tank, but that's not enough by the time it makes it to the various showers through the attic (WH stuffed in far corner of garage). I've got R3.3 pipe insulation for the longest run to install tomorrow. In a former home I had a Paloma tankless that I loved. We have LPG (but not two-stage regulator) by the solar tank and I was thinking that might be a solution as a 'finishing heater' but it seems small tankless WHs really limit flow so several showers going occasionally will be an issue while a big tankless may not like small demands or low gas flow and we'll be back to tepid water. One of the techs suggested a 40 gal elec water heater on a timer as a 'patch' during winter months but that seems like the least efficient solution. Is there an LPG tankless designed for high flow and a small increase in temp, 10-20F? Should I just crank up the elec thermostat until we can get more heat into the tank on a regular basis? Does the small elec water heater idea make sense? Right now the differential temp setting is about 10F, it seems if it's much higher the elec element will do the heating or am I all wrong despite a long run from collector to tank? Any and all input will be deeply appreciated. Thanks!

Tempra 24 on-demand electric water heater. www.stiebel-eltron-usa.com
http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/MAZeroEnergy.htm

Definitely do not use a heating element in the same tank that is used for storing water from the solar collector. You could put a heat pump tank water heater after your existing tank - it's efficient ($/btu) and isn't effected by flow rate or temp rise (as tankless heaters are). And it could supply some "free" cooling.

Do insulate your hot water pipes. I hate waiting for a shower to get warm, so I'd consider a third stage just before the most often used shower - a small electric heater. I also believe in thermostatic shower valves and very low flow shower heads.

Posted By jonr on 15 Mar 2016 11:06 AM
Definitely do not use a heating element in the same tank that is used for storing water from the solar collector.

Why not Jon?  It almost sounds like his system's 80 gallon storage tank is configured for that type operation...

Every BTU you put into that tank with an electric heater is a BTU that you aren't going to be able to get from the solar panels. As with geothermal and a desuperheater, you want (for efficiency) a buffer tank and then some other heater after that.

I agree, I was just seeing if there was some other reason you were thinking of. From a practical user perspective, an electric element on a timer or daylight sensor seems like a viable way to make his solar water heating collection system a little more responsive to his needs while still keeping it relatively simple. Since solar is only available during the day, and sometimes only at a reduced level, an electric element on a timer or daylight sensor to only run after the sun is no longer a viable source would allow the solar collector to put as much heat into the tank as it can during it's viable hours, but if it cannot reach a viable temp, the electric can continue to top-off the tank to a useable temperature, hopefully by the time they want to use it.

Of course this can be done with another tank or tankless downstream of the 80 gallon solar to take the preheated solar water from the 80 gallon up to finishing temp. Any of those options add another level of complexity to the system. You need the space and the structure to support the added weight. Then you need the plumbing re-routed and connected. Then you need the electric and perhaps a gas fuel source. If it is a HP type water heater, you need the access to an air mass to pull the heat from, either by ducting it to the heater or placing the heater in a larger volume airspace. Any permits involved in any of these parts of the process?

I am guessing he was sold a system and expected it to ultimately save him money, but so far it has only been a cold shower...

something very wrong with your system if you cannot get enough hot water
I ran 2 x thermoax 20 tube(evacuated type of glass tubes) feeding a 300 litre tank --in scotland and it did all my hot water and also contributed to underfloor heating
the big limitation with solar thermal is size of storage tank -you must always have a tank large enough to take ALL output form panels for the full day and if you want to go even better ,,enough to store it up for a few days
even my 300litre tank would max at at 95c by lunch time in summer ,when no heating load on it .with still another 8hours of sunshine to keep storing energy .. I had a sensor failure --result was the plastic foam pipe insulation melted off the copper pipes + steam coming out of heat store --melting temp of the foam is 170c!!,so that shows what temps you can get even in a not so sunny northen latitude place like scotland
If iwas going to run an electric back up --then it would be a plumbing in electric shower type unit --or "stick boiler" as we call them --set it to 45c and then it will only come on when you run the hot tap allowing solar to do everything it can when it can .
I do alot of calculations inconjuction with "kingspan " and designed a fully solar powered underfloor heating system .the result for my home which is not a super insulated modern type the requirement would be 20,000litres of insulated storage =4 x 20 tube panels and that would store up enough energy from summer to last all year round.
started to do it with 5000litre tank wrapped in 6" phonelic foam (pir roofing insulation panels ) ---problem was the wife would not loose the garage space for tank -but if I ever to get building new home then it will have a big tank (ICFconstruction) in basement and panels on the roof .
warning!!! large volumes of water must be sealed unit to stop any chance of legionella .,unless you can be sure tank temp will never go below 50c--so sealed with large expansion vessel