When a recirc loop meets anti-scalding valve in a solar thermal system

I get a lot of questions about the recirculation loop in solar hot water systems. The recirc loop delivers hot water instantly to a tap, instead of having to run a bunch of cold water; they’re used particularly for long-distance plumbing runs.

However, when a recirc loop is used with an antiscalding valve, which is mandatory on solar thermal systems, problems can arise. The anti-scalding valve — sometimes called the thermostatic mixing valve — is code for all plumbing systems, but it’s not always installed on non-solar systems. This issue comes up more often on solar thermal systems simply because they’re more likely to have anti-scalding valves.

What follows is an email exchange with Alan, a customer in the Sacramento, California, area.

Question from Alan: I have two HTP Solar Phoenix systems (119 gallon units) and four 4×10’ Solar Skies hot water collectors. I’m having a problem getting the domestic hot water system to work during the summer. I was wondering if you had a design for a system that would remedy the situation. I have a Taco anti-scald valve set at 120°F and the Phoenixes have a tank set-point of 155°. So, the anti-scalding device deadheads the recirc pump when there is no hot water demand in the house and when the Phoenix tank temperature is above 120°.

I’m guessing someone has designed and implemented some type of “injection” system or a separate blending tank system to resolve this issue.

Rod: Are you saying that the problem is in your recirc loop – that you can’t get a constant temp delivered through the loop with the tempering valve because of the way it is piped in with the recirc pump?

Alan: Yes, that is my problem.

Rod:  Here is a drawing I’ve created on how to make a recirc loop work with a Solar Phoenix. Your inquiry is exactly why I created these drawings in the Drawings Library.

Also, the early Taco tempering valves do not have the rating for solar temperature variation, particularly when you have more than a couple collectors in a hot climate. Sometimes they are OK, and sometimes the customer has a problem.

Many engineering drawings showing a building recirc loop have the recirc pump deadhead into a thermostatic mixing valve — or anti-scalding valve; this simply doesn’t work. Also, many water heater manuals will show how to connect a thermostatic mixing valve, but the installer will add a building recirc loop to that illustration  — and it won’t work either.

The mixing valve needs to be piped so that it has the choice of continuing the recirculation if the temperature is OK, or diverting through the tank to pick up a little more heat. Take a look at the drawing and let me hear your comments and other ideas.

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