Steam boiler, mixing valve issue

pete1969

I have steam boiler and domestic hw comes off the coil straight to a mixing valve. I am constantly having mixing valve issues.  This is a 7 family buikding.  I need help. I just put in a solar caleffi w a 210 input rating and it has gone into anti scald mode. I assume bc the temp difference was 50 degrees or more between hot in and out as per specs.  I do not have a storage tank the coil runs straight tot mixing valve. Any info would help 

dopey27177

One item is needed.

Do you have a circulating pump on the h.W. return line?
If not install a circ pump with an aquastat control set at 110 degrees F.
Set and adjust the discharge temperature from the mixing valve to 120 degrees F.

Jake

pete1969

Just to clarify this is for domestic hot water. There is no return line as this is not heating its domestic hot water.  There is no pump

Zman

@Erin Holohan Haskell
Could you please merge this with the other one?

pete1969

Idk how to do that. Maybe delete this one??

Zman

I think he was referring to a domestic recirc line.
Some pictures or a drawing of how this is piped would help, as would the model and pipe size.
Does it do this under all flow conditions or is it worse during low flow?
@hot_rod will surely jump in at some point.

Zman

Erin will take care of it. Double posts get confusing if folks respond to both.

pete1969

I will send diagram of piping. I’m going to assume it reaches highest temps when there is no flow and temp builds up at hot in but idk for sure. Caleffi solar mixing valve Ses any difference in temp 50 or more will activate any scald 

hot_rod

What is failing on the valves?  

Could the coil be scaled that you are not getting enough flow? When coils plug hot side pressure drops and you lose the pressure differential that the valve requires and it will fail cold.

Yhis usually happen under high DHW demand conditions. Any idea what the total gpm you need for 7 units?  That is a 3 Cv valve. Flowing 10 gpm or so will have quite a pressure drop

Erin Holohan Haskell

Zman said:

@Erin Holohan Haskell
Could you please merge this with the other one?

Merged them. Thanks for the heads up.

pete1969

I believe you need 7-8 gallon coil for 7 family and that’s what’s in there. 

pete1969

As of right now i throttled down on cold into mixing valve so tenants have hot water.  I adjusted he  temp to 130 when boiler was making steam for the building. 

dopey27177

In most cities in the plumbing code or building code there is a requirement for a return on the domestic hot water system if the farthest hot water fixture is more than 50 feet developed length form the hot water maker.

If you solved your problem manually by restricting the flow of cold water into the mixing valve that indicates the mixing valve is not doing its job.

Jake

pete1969

Water fixtures would be within 50 feet and i don’t think my city requires that and buikding is probably 75 years old.  I just put that mixing valve in caleffi solar and i assume bc i have a steam boiler when it runs for steam the temp gets way to high at the coil water IN at the mixing valve causing it to go into anti scald mode.     Can someone recommend a mixing valve considering the piping is correct.  Their is a steam loop trap before going into mixing valve. 

hot_rod

To properly troubleshoot this, so yet another valve doesn't fail, you need to know:

Inlet temperature at the hot port of the mix valve when boiler is steaming. Water pressure on the cold side, and a pressure gauge before the hot port.

My suspicion is you will see a considerable pressure drop on the hot (inlet to valve) pressure gauge, compared to the gauge on the cold side when the valve is under load. If so it would indicate the coil needs descaling. Has that ever been done?


The important numbers on the tech sheet, see below
Minimum flow
2:1 ratio between H & C pressure
30° differential between hot inlet and mixed out

1) If you don't meet minimum flow the valve will not fail, but the temperature will vary, warm, warmer, cooler, the valve is hunting because there is not enough flow around the copper thermal capsule inside for it to move accurately and maintain mixed setting.
This is where a recirculation loop can help to meet the minimun flow at all times. But not all buildings have that option, like yours.
If for example only one .5 gpm hand sink is flowing, expect to see the temperature bounce around. This is the case with all brands of TMVs.

2) 2:1 pressure ratio. Lets say your building has 60 psi incoming water pressure. If the hot supply to the mix valve falls below 30 psi the valve will fail to mix. When a tankless coil is partially plugged and you have a high DHW draw that is a common problem. We also see that when water softeners are used on only the cold supply to a water heater. Only with a pressure gauge or two could you determine this.

3) Hot inlet, to mixed outlet, 30°. I doubt this is a problem, unless the boiler is off and not generating hot water at least 30° above whatever mixed temperature you have set, 120F, I assume? Temperature gauges, or a point and shoot temperature gun would answer that question.

IF you have issues only at low flow, a high- low station might be the answer. Basically a small 1/2 valve in parallel with a larger 1" or so sized to the highest expected flow rate. Flow moves thru the smaller, more accurate low flow valve until the building is under high DHW load, then both valves are flowing.

Or you might consider an EMV "electronic mixing valve", instead of the TMV thermostatic mixing valve. It could eliminate some of the control challenges. Most EMV brands are high Cv and depend on external sensors and electronics to regulate temperature, not the internal, old school "heat motors"