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Hi @Teemok , My understanding of the conventional wisdom for household hot water use is 20 gallons for the first person and 15 for all others. Still, measuring hot water usage we've found that it can change by nearly an order of magnitude in different households. That's part of why I try first to reduce the load. People have always liked 1.5 gpm showerheads if they're well designed. They can make up for old, clogged up plumbing. I recently did a job where I was allowed to install 1/4" tubing from a manifold for the distribution. It takes no more than six seconds to get hot water, and the short runs have it in three. There is a lot we can do to make things efficient without sacrificing comfort. Or we can build monsters 👹 I've dealt with my share of those too 😑

Yours, Larry

@Larry Weingarten I wonder if 15gal. is enough? I guess it all depends on the clients intention and the end users expectations. While I have lived using 15 gallons or less of hot water a day, asking people to be that frugal here is an affront to their freedom to consume. A big no no even here in crunchy Nor Cal. I agree with the unpressurized storage and high coil HX approach. That could feed a 120 gallon tank with a mixer that's backed up by a cascade of 199K tank-less heaters. Could even low coil the big storage tank so that if the sun goes down or away and the big tank is too cold the tank-less could bring it up to a min. temperature for the next mornings peak load. And this is how I start to build a monster. 😁

Hi, Just some basic math… Twelve apartments with two people each, or 24 people. At fifteen gallons of hot water per person per day, that's 360 gallons, or three times the storage you have now. One approach would be to install an unpressurized storage tank of 500 gallons or more and put a large coil in the top of the tank as a preheater for your backup storage heater. This likely wouldn't be inexpensive. I agree with @Teemok that unless you can adjust the owner's expectations to line up with reality, this could become a nightmare.

I like solar thermal and have been doing it for about 46 years, but to function, it MUST be simple and designed for the people who live with it. In this case, I think I'd start by looking at what could be done to make the hot water delivery system more water and energy efficient.

Yours, Larry

Hi Mike,

If tossing best logic to the wind and doubling down on thermal is the unalterable will of the investor, so be it. I think if are they willing to fully understand it, monitor it and maintain it themselves, it could be a good thing.

Is the money worth it for you to be involved? The risk to you is: Can you make a system meet the needs in almost all conditions? Will you be expected to maintain it? Will the customer at some point in the life cycle expense of the system realize that the whole effort was ill advised from the start? If that happens, will they find you at fault for aiding and abetting in creating the creature they invested in? Stating it up front in writing that it's "not wise" and getting it signed before hand doesn't prevent the future unhappy customer syndrome.

12-2 bath units all linked with a recirculation system?

High volume daily use makes for a best payback scenario but substantial storage is necessary and that's not cheap in materials or sqft-age. If it's a mission critical hot water system, full backup capacity is a must. Double investment. And the two systems need to play well together to make the solar payback. Then there's parts failures, clouds, hail, dust, roofing and heat dumping.

Think hard about it, it's going to be primarily a heart thing. Logic is necessary but secondary only to make it function. I don't think any logic will make it a simple, reliable and good investment. It's the free panels trap. It has to make sense if the panels are free right?

Around me you can pretty regularly pick up solar hot water collectors on Craigslist for free.

When you consider the cost and complexity it makes more sense to put electric panels on the roof and use electricity to heat the water.

I know that's not what the customer wants, but sometimes what the customer wants is not in their best interest.

What DCContrarian says is generally true. But I have uncountable number of clients with solar thermal installations—mostly flat panel collectors, but some evac tube systems as well. The infrastructure is in place and seemingly robust, but often not maintained. Overheating, bad fluid, no pressure, no circulation, freeze ups are most of the common problems. The principal drawback is that it is a separate plumbing system that is often hard to access and it involves indoor and OUTDOOR plumbing. When they work…they make virtually "free" hot water. Your average, run-of-the-mill plumber has more than enough work maintaining conventional plumbing systems these days. They generally aren't looking for work on unconventional systems, let alone ROOF work.