I am in the process of designing a solar water heater for my home. The plan is to circulate glycol anti-freeze through the collector and a heat transfer unit(tube in tube design). A 40 gallon water heater tank will be used for water storage. I am hoping that convection will circulate the water through the heat exchanger and back in the storage tank(if not, I'll pump that too). The house will pull out of the hot side(top) of the storage tank. Here is where it gets tricky. I would like to run a tankless water heater to supplement the water heat, but only when necessary. I have found propane tankless water heater units that stay in sleep mode(no pilot) when there is no flow. When there is flow, they ignite(via battery) and heat water with propane to a constant 45 degree rise from ambient. What I would like to do is use valve, similar to a tempering valve or mixing valve, that would open the flow through the tankless water heater enough to supplement the flow from the storage tank and maintain a preset temperature to the house. My first thought was that I could do this with a temping valve, but they seem to only work by adding cold water to hot to prevent over heating; a heat reduction. I want to increase the heat of the water by adding hot water to warm water.


Has anyone done this? Does anyone know of such a valve? Or does anyone have another solution. The idea is to maximize the use of the solar heated water and only add heat with propane as needed. I have seen the Bosch tankless water heater that can run in series with a solar water heater, but that unit maintains a pilot light and costs $600.


Any ideas?

You will need a mixing valve. The water in a typical solar storage tank can get up to 160 degrees.

I would skip the tankless and get a small tank LP water heater.

Maybe something like this. Lowes

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The valve needs to be between the solar storage and the back up heater that way if there the stored solar is too hot the valve injects cool water to bring the temp to set point using even less stored solar water and if not up to set point the back up can take over. Just be sure the tankless says it can take hot water input, not all of them can. As for the heat exchanger it will work! I run a closed loop glycol system with a 4'8" tube in shell exchanger on a 40 gallon storage tank ( make it as long as possible )it has no pump and I can see on the data logger that water flows so fast it will stir the entire tank.The exchanger tubes are 1"outside and 1/2" inside. One possible mistake I made was to use copper on the top connection???? I think it's transfering too much heat out at night, this winter I will replace that section with plastic and see what the data logger tells me.

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Thank you both for the replies. Good tip on the tankless water heater being able to take hot water as an input. Its good to hear that the convection design should work well. My design is to use 4' of 4” PCV and coil 1/4” flexible copper tubing at an 1-1/2” radius inside of the 4” PVC for the for the glycol to flow through from the panels. I figure the PVC to help insulate the water and allow for plenty of flow from the convection and the copper to be a good conductor of heat. I understand the need for a tempering valve for reduced the heat of the water during the day. That is exactly what the tempering valve is design to do. My main concern is in the morning, when I wake up to take a shower, will there be hot water? My guess is that the water in the storage tank will have cooled over night. Thus, the tempering valve will not mix any cold water with the warm water from the storage tank as it is not a scald situation. I would like to sense that the water in the storage tank is cold and trigger a valve that diverts the water through the tankless water heater to provide hot water. Likewise, during the day, when there is plenty of hot water in the storage tank, I do not want the hot water heater to run at all. I see this as an efficiency improvement from simply running a gas heater in series. I was hoping that there was a similar “tempering” valve for mixing hot water to water that is too cold, but maybe not. If not, I'm thinking that I could use a controller that triggers on low temp level. That is, if the water coming from the tank is <80 degrees F, then open the valve to divert the water through the tankless water heater. This also means that the tankless water heater will not necessarily be taking hot water in, but cold to warm water. As the tankless water heater will increase the temp by 45 degrees F, it will then flow through the tempering value and mix cold water as needed to maintain a consistent and non-scalding hot water to the house. Any suggestions on a cold water “tempering” valve or a controller for triggering the water to divert through the tankless water heater?

Randy For some reason I thought you wanted to bypass the back up heater? then I figured out you do! Dont cause yourself problems. Bosch makes an on-demand and proubably others do too that sences the water coming in and if it's warm enough the water just flows rite on through with out the heater fireing up. Too cold and it fires up to do it's job. As for your tank cooling over night my temp drop is 4~5 degrees per 12 hr period so if you can warm it you will have hot water as long as there are not more than two conserveing people in the house.(40 gal is pretty small)
Look up Bosch 1600 ????? I think? Made for solar back up heating.

HuntedHeads, sorry for the confusion. I did not want to use a tank-style water heater in series as it seems inefficient to maintain the water in the water heater tank. Thus, all you are achieving is to pre-heat the water for the water heater. Maybe that's a lot with a minimal loss of heat over time. My thought was to only heat the water with propane if the solar water heater was not hot enough. I figure that the water in the solar heater storage tank will not be too cold, thus, a powerful tankless water heaters is not needed. The Bosch AquaStar 1600 would work. What I am looking at is a little tankless water heater that does not have pilot flames at all, but simply ignites the burner when flow is detected. It seems like a more efficient setup and is a faction of the price. Thus, by using a 3-way valve and a temperature controller, I could divert the flow through the tankless water heater when its not hot enough. On the other side of the water heater is a tempering valve that would mix in cold water, if necessary, to maintain a constant hot temperature. At the moment all I don't have is the temperature controller. I looking for just a controller with a temperature sensor that triggers a relay on a threshold. I see lots of differential controllers, but not a simple temperature controller. Maybe I'm not calling it the right name. Its great to hear that the drop in temperature will be very little over night. I see that you have gone the extra mile in insulating you tank. That seem like a good idea. Do you use a differential controller for the glycol side? I see the EL-Sid pumps can run off a direct PV and act as a controller by only pumping when the sun is out. Any idea if that works in practice?

You might be able to configure a diff controler to divert water with electric flow control valves but they are expensive and another thing to malfunction. I had a whole house on demand heater in the old place and the fire would drop down with a slow flow and get bigger with a high flow the thing I worrie about is if it's not made for solar back up and you route water from the solar tank through it at say 110 you might have a on demand steam generator? I'm not sure if a standard on demand heater is capable of limiting the output temp or if they just adjust fire rate according to flow rate? Something to investigate for sure. As for the controller for my system I use a (hillbilly controller) I wanted it simple, and mechanical. I have done house maint for over 30 years and never remember replaceing a fan control switch on a furnace so that is what I installed on the collector it's self, Just a heavy duty snap switch thats adjustable. as for the solar pump I have to think it is a great idea and even greener!