Domestic hot water is tepid at best.

[U.P.Boiler]

I'm baffled on why my domestic hot water is not getting up to the 120° degrees. Getting 105° at the tap. I have a Honeywell mixing valve it turns freely. The inlet and outlet as well as the heat plate are hot to the touch. I have hard water and am running treated water in the owb.

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[leon]

If you own an infrared thermometer you can check all the hot water lines
from the heat exchanger to to the mixing valve and beyond to the faucets.

It seems that you may have a plugged flat plate heat exchanger that needs cleaning
or the mixing valve is not mixing properly and the infrared thermometer will find the problem.

 

[Fred61]

I have similar mixing valves in my system. To adjust the temp. you must loosen the screw and pull the knob out for it to engage the stem.

 

[U.P.Boiler]

I have similar mixing valves in my system. To adjust the temp. you must loosen the screw and pull the knob out for it to engage the stem.

Yup. I understand the operation of the valve. That's not the issue.

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[maple1]

Follow Leons suggestions. You might have to spray a spot of flat black paint to get accurate readings - IR guns aren't accurate on shiny surfaces.

How old is this system?

 

[U.P.Boiler]

I'm gonna pick up an IR gun today and get some readings. The system was installed June 2015

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[maple1]

Mixing valves can mess up sometimes, but not usually that quick. Could be some dirt got into it - or into your HX. Usually takes longer for them to crud up & need flushing - but could be a possibility. I would bet one or the other, if it was working OK last year but now not.

 

[tom in maine]

A simple test is whether the hot inlet to the valve is hot (usually too hot to hold).
If it is and the outlet it tepid, it is time to replace the innards of the valve.
They do get messed up, especially with water issues.

 

[Bob Rohr]

I agree, check and de-lime the valve. In some areas those mix valves need to be delimed every six months.

Hard water and high tank operation increase the need to service them.

Some folks buy a second one, switch it out and so the removed one in vinegar or mild deliver acid.

Beware if you disassemble them, there is a spring inside that can puke the innards out when you take them apart.