DHW - Indirect (solar?) tank feeding on-demand/tankless

[Cynnergy]

Hi all,

Looking into options for DHW for my cabin on an off-grid island on the BC coast. There is some electricity but not enough to run an electric tank.

Currently the hot water is made by a coil in the oil stove which heats an indirect tank. See pics in this thread: https://www.hearth.com/talk/threads/can-i-convert-this-oil-stove-into-an-efficient-wood-stove.94600/

VERY inefficient, expensive, and packing stove oil in the boat is a pain. I was thinking of hooking up a propane on-demand heater in addition to the indirect tank. Long-term (when the indirect tank needs replacing) it would be nice to have solar hot water feeding the tank too.

I've read on here some pretty positive reviews to on-demand hot water heaters, but I don't know how they would do with the incoming water being pre-heated - none of the manufacturers seem to encourage this and I don't know why. Does anyone have any experience with this sort of system?

Also, how reliable are they? I don't want to have a maintenance headache with some fancy hot water heater that I can't fix myself. We have very soft water though, which I think is good.

 

[GaryGary]

Hi,
Most of the newer tankless gas units will accommodate preheated water -- they have a burner that can fire at a variable rate, and they take into account the temperature of the incoming water and the flow rate and add just enough heat to get to your target temperature.

I used to use a Tagaki Jr with our solar water heater, and it worked fine.
If the water in the solar tank was already up to temperature, the tankless would kick on for just a few seconds and than drop out.
If the water in the solar tank was part way up to temperature, the tankless would just add enough heat to get it up to the target.

It even had a remote control that we mounted in the bathroom so that you could dial in the temperature you wanted

Just look for a tankless that advertised the variable firing rate or says its compatible with solar.

Gary

 

[Cynnergy]

That's great, thanks Gary! Nice to have some more reassurance. The Bosch 'applications manual' (pg. 27) http://www.bosch-climate.us/files/201206191703450.THERM_Applications_manual_06.2012.pdf shows some thermostatic mixing valves used to keep pre-heated solar water (at the right temp) out of the on-demand heater. Do you think this is a good idea? Maybe water that's too hot will damage the unit?

 

[GaryGary]

That's great, thanks Gary! Nice to have some more reassurance. The Bosch 'applications manual' (pg. 27) http://www.bosch-climate.us/files/201206191703450.THERM_Applications_manual_06.2012.pdf shows some thermostatic mixing valves used to keep pre-heated solar water (at the right temp) out of the on-demand heater. Do you think this is a good idea? Maybe water that's too hot will damage the unit?

Hi,
So, it looks like the 2nd mixing valve is what a lot of people call and anti-scald valve -- if the solar tank is really hot, it keeps you from getting scalded by mixing some cold with it. I think this is not a bad idea, particularly if you have kids in the house, but I don't use one. We find that even if the tank is up to 165F (the high limit), the water starts out much cooler in the shower, and while it eventually gets up to the 165F, there is plenty of time to manually adjust it down. Codes may require the anti-scald valve.

As you say, the other one keeps fully heated solar water from going through the tankless. Not sure what the logic is. It may just be there to keep the tankless from coming on at all -- on mine, the tankless would come on for a few seconds even if the solar water was fully heated. Its hard to imagine that anything in the tankless heater would be damaged by solar heated water at (say) 165F? It might be worth asking Bosch why they put the valve in?

Most of the solar differential controllers allow you to set a maximum tank temperature. When the tank gets up to that temp, it just shuts the collector pump off to keep the tank from going higher. I think this is a good idea and gives longer life for things like tank liners and some pumps. It does leave your collectors stagnated, so they have to be good for that -- but, they need to be good for stagnation anyway.

Gary

 

[Cynnergy]

Good to know, thanks Gary. There's no max tank temp on our indirect tank at the moment (although our new shower set comes standard with a mixing valve at the bath tap), so I think I might ask DH to plumb it in this way. Oh, and continue my search for a cheap thermometer that I can put on the tank. I don't think tank temp is a problem with the oil stove on low because it takes a LONG time for the tank to heat up (most of the heat dissipates into the bathroom as it's not insulated), but if I needed to roast a turkey or something with the stove on high for a few hours I might have to run some hot water out of the taps to keep the T&P valve from releasing...

 

[jdp1152]

I currently have such a system. Solar feeds an insulated 120 gallon stone lined tank. Well water comes in at 40 degrees in winter and a sunny day can boost that to 90. Summer goes from 40-45 to 130. Then feeds into a tankless on my oil boiler. It's all getting dismantled next week. Replaced the water with a heat pump electric. Pretty old system...20 years or so. Old guy that owned the place before me was a physicist and set it all up. There is a woodstove with coil that feeds to the tank as well as the radiant baseboards upstairs.

 

[Circus]

Consider possible freezing while investigating DHW. My DHW is a thermal siphon (no pumps or electricity). The tank is mounted at the peak of my heated bedrooms cathedral ceiling. The roof collector is full of antifreeze which heats the tank via a heat exchange.