on demand solar pre heat
ws9876 Solar Expert Posts: 440 ✭✭✭
about gas on demand water heaters....anyone do a solar pre heat that works good??what did you do and what components did you use??I hear that the hot water can screw up the tankless sensors??
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Is your tank-less water heater able to accept elevated inlet water temperatures? The water heater needs to be able to throttle down to almost no flame if you are going to feed it >100F water.
There are down stream mixing valves, but I don't think that will work with a non-compatible demand water heater.
I have a tankless gas water heater which takes preheated water from our geothermal system.
It works fine, but has some quirks. In particular, the tankless heater can only turn the flame down so far. So if the preheat water is too warm (but still below the water heater set temperature), it can over-heat the hot water.
Since the amount of over-heat also depends on the flow rate, it's possible for the water temperature to change suddenly and by a lot if the demand changes. I have three teenagers in the house, so this is sometimes a problem.
I have an Eternal "tankless" (it's a 2 gallon tank to prevent cold stacking) that has a multistage burner, winter it's fed with 90F preheated water from our stove (80 gallon thermosiphon tank over firebox) and summer it feeds from a 20 gallon roof top Rheem Solaraide. I do have a mixing valve to limit the feed water to 120F because in the summer, the Solaraide gets well over 140F
Been working fine, on it's 2nd year now.
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I have used simple flat plate collectors, circulating into a unwired electric tank, with aquastat control for the pump, all feeding into a demand water heater. As has been said, it can overheat in the summer, but that is easily overcome with a tempering mixing valve. In the summer, a 36 sq foot collector will produce about 40 gallons of 135f water.
Same as above, when solar heated water gets above 90F you have to have the tankless cranked up (131F) or it will cycle on and off, annoyingly. If the inlet is at of above 100F I shut off the tankless...warm enough for me. Summer is great, little propane burned, but winter's another story. That said, 2 sunny days and the holding tank will be stratified with 120F water at the top. If your shower is only 3 minutes long you can use all solar, any longer and the cooler water makes it to the top of the tank (for no-propane hot water).
With a tempering valve you would just keep the firing temp high enough to not cycle on and off, so high temp water enters tempering valve, exits at your prescribed temp and then you further adjust with your shower/sink controls (haven't gotten that far yet, no tempering valve).
Yes, I have exactly such a system. It works OK, but has some quirks that are mostly related to the tankless water heater. The solar system is a single Viessmann Vitosol 200 F flat plate collector 25 sq. ft. absorber area circulating a propylene glycol/water mixture through a heat exchanger in a Heat Flo HP 50-gallon stainless steel tank. A tempering valve is used downstream of the pre-heat water tank to limit outlet temperatures to about 120 F. This is followed by a Tagaki T-K3 natural gas fired tankless water heater, which is rated to accept preheated water. (Most tankless water heaters are not rated to accept preheated water.) The firing rate range is 3.2 to 58.3 kW (11,000 to 199,000 Btu/hr).
The showers work fine at any time of the year. The sink faucets have to be operated at a high flow rate in the winter time to get the tankless water heater to fire and produce hot water. In the summer, the pre-heat tank usually stays warm enough that the hot water is usable even if the tankless water heater does not fire. Natural gas usage in the summer for cooking and hot water is usually 0 to 2 CCF, which at a cost of $0.72/CCF (not including connection fee) results in modest natural gas bills.
This configuration allows the solar system to operate at peak efficiency. If the supplemental heat were applied to the storage tank, then hotter water would be circulated through the collectors, reducing their thermal collection efficiency. The trade-off is that warm water entering the tankless water heater can require high water flow rates to get the tankless heater to fire.