Check valve on cold side of mixing valve, is it necessary ?

Holder

Hi,
We have a condensing boiler used for DHW and 2 CH systems 1. a concrete radiant slab and 2. Cast Iron radiators. Each system has their own supply pump, both returns join together with a circulator pulling back to the boiler.
The radiant heat line has a 3 way thermostatic valve (before the pump) to reduce the temperature. The radiant return tees off off one side going to the mixing valve, the other going back to the return line with a check valve in line before joining the cast iron return. There are no check valves for the cold and hot sides of the mixing valve. Is that ok?
My concern is that when only the cast iron system will be calling, the circulation pump will then be pulling the return lines which will cause hot water to flow through the mixing valve up the cold line back through the return through the boiler.
Is this a problem?
Is it therefore necessary to add a check valve for the cold side of the mixing valve?
Is there anything I am missing?
Thank you very much!

JohnNY

Yes. Mixing valves need check valves on their inlets. If you don't see them, they may be intergral to the valve. The Honeywell AM-1 series uses check inserts on the hot and cold ports. It's typically only a problem when there is no system draw through the mixing valve but that's probably pretty frequently.

Holder

Thank you @JohnNY ,
My plumber put in a Caleffi 521609A - 1" Sweat MIXCAL 3-Way Thermostatic Mixing Valve without the check valves at the tails. He didn't think that it was nesassary. Do we need a check at the hot inlet too?
Should we try to get the Caleffi tails for the mixing valve? Or is a inline check valve above the mixing valve ok?
Thanks to all

hot_rod

If you have a recirculation loops connected, yes you need the checks.

It needs to be piped like this, with a bypass to adjust recirc flow.

Or use a Caleffi 520 mixer which has 100% H shutoff and does not need a bypass and balance valve.

Holder

Is this also a issue with a closed ch system and a condensing boiler? The return is quite warm and there should be no issue with shock.

hot_rod

The tankless or combi boilers really work best with a small buffer tank for the DHW. Some brands now have a small, few gallons, DHW tank inside.

If not try and have 5 feet of piping or more between the DHW output and mixer to get more accurate temperature control.

Any system with a pumped recirculation loop needs checks inn the mixer as the pressure can be different from the added delta P.

Holder

DHW has it's own tank, so no cold water gets mixed in. The heating system is closed.
Back to the original question: are the check valves for the mixer absolutely necessary? Is it a problem that some of the CH hot water will circulate through the mixing valve right back to the return when the radiant pump isn't pulling?
And do I also need a check for the hot inlet of the mixer?

hot_rod

the valve will probably not provide a steady temperature output without the checks. It may or may not be noticeable or be an issue. Adding the checks as suggested will prevent he from pushing into the cold line and vice versa. This is why checks are offered and also options

Try it and see how the mixed output behaves, add checks if you have temperature drift when running DHW

Holder

I don't think this has anything to do with the indirect DHW system. The question is if I need Checks on both sides of the mixer inlets?

hot_rod

The installation manual shows checks and explains the need for them. Maybe pass it along to the installer. Some valves require the checks to meet the ASSE listing

https://www.caleffi.com/sites/default/files/file/38476_na_11.pdf

Holder

Why would it need a check valve at the hot inlet?

JohnNY

Because, and I'm getting the feeling Hot Rod may disagree with me, a mixing valve is effectively a controlled cross connection. At least when it's in a static position it is. During a DHW demand it becomes more like an old 2-handle shower valve but you've got hot and cold water meeting up in one device. Plenty to go wrong in there....and things will go wrong inevitably at some point. Adding check valves is at the very least a strong inexpensive safeguard against valve failure. I always install them and have had problems where I didn't. Every single handle shower valve, a type of mixing valve, has checks on the hot and cold.

hot_rod

Simple enough as John explained. A checked line is a one way street.

Holder

https://forum.heatinghelp.com/discussion/168876/outlet-sensor-short
The outlet sensor just went out, could it have to do with this?
I did realize that the flame was going on and off a lot since we put in this mixing valve. Or is it just the cold weather?