Domestic hot water sometimes scalding

bhowden

I have a Sandon hot water tank that varies between 130 F and 165 F. It is plumbed with a Honeywell AM101C-US-1LF temperature regulating valve to regulate the temperature to about 115F. 98% of the time it works perfectly but every now and then the water comes out of the tap hot enough to burn a small child. The hot period lasts for 10 to 30 seconds and then goes back to the 115 F set point. This is with the tap flowing about 2 gallons per minute so well over the .5 gpm minimum. I think the valve is doing something as the water is well below the top tank temperature. I have replaced the valve with a new one and the situation improved a bit but it still occurs. Is there a better valve to use? I cannot imagine this being acceptable in a day care or other situation with small children.

Brian

Jamie Hall

That valve is pretty reliable... is there, by any chance, a very small hot water draw -- even a leaking faucet? That valve requires a minimum flow to regulate (they all do, comes to that).

Intplm.

Is something clogging the valve? How is your water quality? Does it need to be filtered?

bhowden

As far as I know, no leaks or drips but I will double check next time I notice it overly hot. Am I correct in assuming you are suspecting the leak is charging the pipes with hot water and that is the hot water we see? It is a straight run from the hot water tank to the kitchen tap and we always see the typical lag of cold water before it runs hot. When it runs overly hot it is fairly near the beginning of the flow. This is subjective but it feels like the water comes up to normal temperature for a few seconds and then runs excessively hot. The water is municipal and generally pretty good Certainly never any trouble with sediment or corrosion before. The screens in the aerators on the taps are always clear.

Are these the same valves used in things like day cares or is there a different valve for those applications?

Brian

hot_rod

Is there a recirculating pump on the system?That can cause temperature creep with thermostatic valves

In some instances it is wise to have point of use mix valves also at the fixtures. Day care and assisted living. Are usually set lower, to 110F

mix valves need to be de-scaled from time to time as a sticking cartridge can cause wide swings like that 

bhowden

There is no recirculation pump. Just a line from the tank and a line from the cold supply to the temperature mixing valve and then to the domestic hot water pipes. I am assuming you mean temperature mixing valves and not pressure balancing valves. Most of what is sold around here as "mixing valve" refers to pressure balancing valves for showers etc. As I understand it, those won't work in this situation as the temperature of the tank varies quite a bit. We have very soft water and no scaling problems anywhere else but I am willing to give it a try. Am I correct in assuming disassembly, cleaning / soaking in vinegar and reassembly? Does it make any sense to try running the valve through the temperature range to see if anything will "unstick"?

Brian

Larry Weingarten

Hi, Does your present mixing valve "see" the heat of the stored hot water, or does it normally live at room temperature? If it's always hot under no flow conditions, that could give you the burst of "too hot" water. A heat trap between tank and mixing valve could make things better.

Yours, Larry

hot_rod

A few things about thermostatic mixing valves, regardless of the brand. It takes a number of seconds for them to respond to a first draw. The wax "heat motor" inside needs to have a minimum flow around it to respond accurately. the manufacturer will indicate minimum flow for stable operation.

If you have a single, low flow faucet draw, below 1 gpm generally on a 3/4 valve it may not regulate accurately, look up the spec sheet on that model.

The other important spec is the temperature difference between the hot being supplied to the valve and the mixed outlet temperature from the valve. At least 20° hotter water supplied to the valve, some valves will list a 25 or 27° difference required. So if you want the mixed outlet at 120F, the tank should be at least 145°.

A cleaning or cartridge replacement may get you back to an acceptable operating condition.

If you replace, consider a Caleffi 520 valve, it was designed for low flow minimum, is quick responding, and a unique angle pattern, stable down to a .5 gpm flow, 18° differential.

bhowden

Several good ideas I will look into. The valve is about 9" below the outlet from the tank. (water exits the tank into a 90 degree bend down followed by 9" copper pipe into the mixing valve) I can keep my hand on the valve full time with no water flowing and any convection current heating from the tank water should be stopped by the 9" down pipe. The valve is warm but I am thinking that it is due to the heat conducted by the copper pipe and not convection of the hot water. The valve is rated at .5 gpm and I am testing at well over 1 gpm so I don't think it is minimum flow. It could well be insufficient delta T from the tank temperature. I have not been able to find the spec for minimum delta T for the valve but it could easily drop below 145 F at the outlet of the tank. I will see if I can do some data logging of the mixing valve inlet pipe.

Thank you for the recommendation of the Caleffi 520 valve.

Brian