Solar water heating + electric water heater


I would appreciate any information on how two hot water sources (solar water system and an electric water heater) can be used concurrently. Mid-morning I would like to be able to switch to the solar water system as the source of hot water, then early in the evening switch to the electric heater. What would be the most convenient set up that would allow the elderly to perform the switching?

Also, is there an automatic manner in which this can be accomplished per a daily schedule?

Many thanks in advance -


Indeed, with Taco zone valves, the appropriate heat exchangers, and a timer. Would you like a wiring and piping diagram?

Depending on the output of your solar water system they are often installed as preheaters to the conventionla water heater. In this manner, you do not need to switch between solar and conventional.

In my climate we try to shoot for around 80% of the hot water needs supplied by solar and the rest (20%) by conventional fuels.

To install a system large enough to supply 100% of the hot water needs just would not be financial feasable. Again this is for northern climates.

Yes, please, and thank you for taking the time to respond.


Thank you for your response.

  1. When the solar system is inactive (and I am not sure what inactive means in this case other than not having any sun), would drawing water from it have any harm? As I am not sure how the solar system functions, this may be a silly question (but then all questions from laymen are silly questions to experts!).

  2. When heated water from the solar system enters the electric water heater, would it have any disadvantages?

Thank you -
Software Engineer but no plumber!


I am not sure of what type of system that you are referring to.

I myself utilize a Solar Hot Water system for the domestic water in the home.

The circulating medium is Glycol thru the collector panels on the roof which is exchanged with the water via a coil within the water heater itself. The particular system is also connected to the oil fired boiler as a backup / support for the Heating system. Electric coils are also contained in the Heater so Electric backup is also an option.

Hey Paul, is your store front up and running yet?

  1. A closed loop solar controler will have a temp sensor at the panel and at the tank. If the delta T is greater than a set limit then a circulating pump turns on to circulate the transfer medium though the system. It is possible for a flat plat system to generate heat even at night when there is no sun. Sounds bizarre? If the delta T between the ambeint air temp on a warm summer night and the incoming water main temp are say 10 deg differance then the pannel can collect some heat. Not a whole lot but some. So no, pulling water when the system is not producing will not harm the system2) In my climate (as we don’t know where you are) the transfer medium that circulates though the panel is separate from that of your potable water. Heat is transfered from one to the other by special tank or an external heat exchanger

We typically will either use a water/food grade gylcol mixture or design the system as a drain back system. This is to prevent freezing in the colder months.

If you are in a warm climate area, I can not think of any reason why you couldn’t simply plumb an array in line from the water main. Perhaps stagnation, but you would have to hear from someone in a warm climate as I have no expeirance with systems in those climates.

Keep in mind that there are many types of systems, everything from Bobby Joe thowing a black garden hose on this roof to advanced evacuated tube systems and the methods of hookups can vary greatly depending on what you are trying to achieve and the area you live.

Just got back from another 5 hour meeting regarding it. The more people involved the more likely it is to succeed but boy do the wheels turn slower. Leashold improvements are now finished. Start to move in this week and next. Thanks for asking :slight_smile:

Paul, thanks for your response. I am not sure I understand what you mean by the following sentence, would appreciate clarification:

Thanks -



Thanks for the response and the pics. My solar hot water system has a coil inside its tank, but I do not use it because it is very inefficient and uses an unbelievable amount of electricity. This is why I purchased a separate electric boiler with its own tank.

I have to say that I am amazed at how little certain plumbers know about connecting two sources of hot water! It seems as if it is a novel idea they are hearing for the first time. I know not much about plumbing and would expect plumbers to know more than I do. Seems a single source of hot water is the status quo that is not to be disturbed. :wink:

Thanks -


Paul, does conventional water heater category include water boilers with electric heaters (not oil)?

I will be contacting the manufacturer of the said boiler to ask if it is OK to have hot water being output from the solar hot water heater be input to the boiler. This is what would transpire during the daytime when the solar hot water system would be discharging hot water (and it gets incredibly hot!). In this case, if I maintain the temperature of the boiler to be below that of the water output from the solar hot water heater, the boiler should remain passive and the hot water should only pass through the boiler’s vessel.

Is this mostly correct?

(I know that things described on paper do not always turn out exactly the same in real life.)

Many thanks -


</p>It basically means that water comes in from the main (city, well, ect) goes though the solar panel and then to the cold water inlet on the hot water tank. This is a rather crude method and does not have any protection from freezing, over heating, storage. It is less efficient mostly due to the lack of preheated storage. Ask yourself “what time of day are my hot water needs?” High noon is not the the most common answer. But that is when the panel output can be at its maxium.

It shouldn’t be that much. The circulating pump may be oversized for the panel output or its not installed to automatically turn on and shut of with a proper delta temperature. Or it could just be a bad pump.

Yes, by a conventional water heater I mean ELECTRIC, NATUAL GAS, PROPANE, OIL. The water heaters that most people have.</p>

I would be interested in thier answer if they said no. </p>

That’s correct …buuuut… the temp of the water from the panel will change thoughout the day. It is better to put the temp of the boiler to the desired thep you want. Then use the solar water to augment the demand on the boiler by preheating the water. </p>
Ideally you can use separate insulated large tank that uses a stratification/thermo siphon system to provide preheated water long after the sun has gone down.

Todd, What brand fo panel system are you using? And again, Where are you located. This would help a great deal.

Paul, many thanks for your detailed answer.

The more I consider your series solution, the simpler and more straightforward it seems.

I am currently off the Mediterranean coast (Turkey). The panel is manufactured by Baymak, a company that is owned by the Baxi Group of UK. However, when I called their tech. support line for information, they were completely clueless!

The electric boiler is manufactured by Beko.

I don’t think the panel has a pump. I believe it’s an option, and I may have to have it installed to get the max. efficiency out of the system.

I will post updates here.

Thank you again -


Paul, just one additional question: If the electric boiler somehow overheats because it is getting pre-heated water from the panel, won’t its (the boiler’s) built-in safety kick-in and mitigate the situation?

Just to clarify: The panel and its tank are located on the roof, and the heated water drains down into the house via gravity.

If the water temp from the panel is higher than the temp seting on the boiler then the boiler will simply not kick on. The heated water will simply flow though. If it got really high then yes, the safety valve should open.

Yes, an external storage tank using a thermo siphon method. I checked their web site which wasn’t help. They list 2 systems the 100 and the 200. I don’t run activeX on this computer so I may not be seeing some of the info they have on their systems. You may be able to see more than me.

I must conceed that I have not personally visited your part of the world but me thinks freezing is not a concern :slight_smile: :slight_smile: :slight_smile:

Your manufacture make a remark that they used a closed loop system so that there is no scale build up. Even if you used distiled water in your closed loop you would still get some scale - albiet very little.

You do need freeze protection and even if you added gycol it would likely break down in your climate. And when glycol breaks down it becomes acidic which is hard on your colector system.

They also make referance to being able to used preasurized water with the closed loop system. This makes me think they did not desing the system for very high preasure. I would check its rating before plumbing in-line. You may indeed have to keep it as a closed loop system.