Anyone have any experience with solar mixing valves?  I m wondering if there have been any problems with throughput in low-demand situations.

Are you talking about thermostatic mix valves on the top of the tank?

Yeah. On top of the tank. Thermostatic valves designed to limit flow of water that may be too hot. Do they limit flow even at low flow rates?

Posted By ICFHybrid on 26 Jan 2014 02:58 PM
Yeah. On top of the tank. Thermostatic valves designed to limit flow of water that may be too hot. Do they limit flow even at low flow rates?

I have a solar-heated 50 gallon tank that I have set the maximum temperature to 155 F. On the outlet of that tank, there is a mixing valve to mix cold water with the hot to limit the outlet temperature. That tempered water than goes through a tankless water heater to add heat if needed.

When the solar tank is relatively cool, then I have a problem with the tankless water heater that I think has nothing to do with the solar system. If the flow is too low, then the heating rate required for the tankless heater is below its lower limit, and the water delivered to the faucet is relatively cool. To get hot water, I have to meet the minimum flow requirement to get the natural-gas fired tankless water heater to fire.

I guess since I have a tankless water heater downstream of the mixing valve, I cannot address your question.

Lee, that is a common problem with most tankless heaters. The Navien with the buffer tank solves that to some extent, but none are as good as a tank.

Yes, there is a low flow limit to most thermostatic valves. It is not a published thing but it is there. The worst problems I have is with commercial solar systems where the large mix valves just cannot react well at all.

I have used Danfoss and Honeywell mostly but they all seem to have problems.

I concur with Mike. Tankless and solar thermal seems like a great idea, but there will be times when the two technologies fight each other and your shower is the collateral damage. I lived in a house with a similar setup before i started working in solar. I would bypass the tankless unit during my showers if the solar tank temp was high enough.

Lee has it right. The issue is when the solar tank is providing water just lower than the set point of the tankless unit (solar tank is at 115, and the tankless is set at 120F). Then add to the fact that we all "temper our shower water at the fixture also. So a 2.5 GPM fixture might only be calling for only 2gpm of HOT water. (The problem is worse if using very low-flow shower heads)

That 2gpm only needs a fraction of the heat that the tankless unit can provide, even at its minimum burn rate, so to prevent scalding it shuts down. So you can get situations where the tankless unit starts up to meet the load, throttles down to the minimum burn, then shuts down when it realizes it is overheating the water. Then the cycle repeats.....

We advise people not to get tankless water heaters if pairing with thermal around here, so I have not kept up on the newer models of tankless units. There may be models out there now that are smarter and can handle that situation with more grace, not forcing the shower occupant to do the hokie pokie with the ever changing shower temperature.

Mike, as for commercial work, have you tried using a "Hi Low" thermostatic mixing valve? They are usually good for flow rates from 1-50+ gpm. We haven't had any complaints of poor performance when that style mixing valve was used (admittedly only a handful of projects). Bradley, Lawler, and I'm sure others make these hi-low valves. Sometimes they're a single unit with two sizes of mixing valve assembly in the body, sometimes they're a factory assembly of a low flow, and separate high-flow mixing valve plumbed together with common plumbing.

I've tried Lawlor usually for the bigger stuff and I have also tried put 2-3 of the biggest Honeywell resi units in parallel when possible with mixed results (LOL). The best situation is an active mixing system but they are not considered foolproof enough for code, AFAIK.