Which tankless to go with thermosiphon solar HWH?

15 years ago

My wife is not happy about the performance of the backup electrical heater on our solar hot water system. The backup will heat the water, but it takes an extremely long time. And once the water is used, it takes forever to recover. For marital happiness I'm considering installing a very small gas tankless unit where the previous tank unit had been (gas line present, roof vent still there, all the pipes are accessible). With a tankless unit in place, my intent would be to shut off the electrical backup integrated into the tank.

What type of unit should I be looking for to service a two bathroom, single level home where the water will often be preheated by a roof-mounted thermosiphon system? Small footprint is essential for me -- the motivation for the solar system was to increase usable space in our laundry room.

I live near San Diego with very rare freezing temps.

Thank you for your advice.

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Comments (4)

jake2007
15 years ago
I take it that you mean that there will continue to be a storage tank for your solar system... just no electric resistance heat for backup.

There are some details that you will need to add, such as the max gpm that you need to supply at one time (how many faucets, appliances and showers? Any of them high flow such as a multi-head shower?) Also, what is the anticipated inlet water temp? I suspect that you will be looking at a 75k - 100k btu/h unit (probably closer to the low end of that) Unless you are using a condensing unit you will be venting with stainless steel and it gets expensive to go through the roof rather than the sidewall. Particularly in your case, don't bother with a condensing unit unless you need to vent with PVC to make it affordable.

There are several good brands out there... the only one I would avoid is Bosch. Be sure to check min flow rates and other specs on the one that you are considering. I have used the Rheem/Paloma unit and it works great.

You could install this outside also and not worry about venting at all. The outside units fit in a box that's mounted in the stud cavity so they are reasonably flush with the finished wall.

Is there any design issue - re: the solar - that I'm not considering?
mungo
Original Author
15 years ago
Yes, there is a storage tank on the roof.

We are a two bath home. The showers are all single head types -- no body sprays.

The inlet temp will vary. Sometimes the solar unit will provide all of the hot water. If we use it all up, it will get refilled with cold water though at whatever the winter/summer water temps are around here. If it is somewhat cloudy, the water will be lukewarm. It is unpredictable.

Sidewall venting is not out of the question, nor is outside mounting.

Besides flow rate (lower is better?) what other specs to pay attention to?

Thank you. Your comments are much appreciated.
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jake2007
15 years ago
Tankless heaters are designed to heat a given number of gpm at a specified temp rise.

For Example, in Indiana, the inlet water temp at the coldest is about 35 degrees so in the worst case, it takes about a 70 degree temp rise to get it to shower temp of 105.

Looking that up in the manufacturer's table, the Rheem RTG-53PVN (145k BTU/H) will heat up to 3.5 gpm at that temp rise. (that's one shower (2.5 gpm and one bathroom sink faucet (1 gpm)). If you wanted to be able to do, say, 2 showers at the same time (5 gpm)at a 50 degree temp rise it would probably be good to go with the next larger unit (199k BTU/H) that would heat as much as 6.6 gpm

These are variable gas valves, so they only use as much gas as they need to reach the temp rise for the amount of water that you are using. Buying a slightly larger unit isn't going to be a problem.

One of the issues to look at is a low flow rate to kick the unit on... usually .3 - .5 gpm. Lower is better.

I've mentioned the Rheem unit. I don't have any reservations about it but do your own looking around. There are other heaters out there with different BTU/H ratings that may be a better fit. Look for an energy factor of .80 or better.

Here is a link that might be useful: Spec sheet for Rheem tankless
mungo
Original Author
15 years ago
So they started installing my tankless system yesterday and plumbing the solar unit to work as a preheater. Plumber mentions "I noticed that the hot and cold are plumbed backwards on your solar tank". Doh! I think this accounts for some, but not all, of the poor performance I have been experiencing.

Tankless is still going in, but I thought folks looking into to solar units should get the complete picture of my situation.

Lastly, I've learned that if you're using solar pre-heated water with a tankless system, you really should include a mixing valve between the solar unit and the tankless to eliminate the risk/danger of super-heating the water. In my case, the mixing valve will be set at around 100 degrees, with the tankless unit at 120. If it works right I should not be getting temps over 120 in the house.

Which tankless to go with thermosiphon solar HWH?

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Which tankless to go with thermosiphon solar HWH?

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