Lochinvar DHW Overheating

bberge

I have a Knight WHN-111 with a Squire SIT-50-LV DHW tank, installed about 6 years ago, and it has worked fine until last week, when the domestic hot water temperature suddenly shot up to 169 degrees. From what I can tell, the system is treating the DHW as a zone on the radiant heating system and all of the DHW settings are not being followed. The control panel still shows that DHW system is set to “Normal” rather than “zone,” but it’s behaving like zone. Has anyone seen this?

Ironman

Some pics would help.

You could have a bad aquastat.

When you say “control panel”, are you referring to the boiler’s display or a zone control panel?

Alan (California Radiant) Forbes

You could have a failed check valve. I just replaced an integral check on a Grundfos DHW pump that has the same symptoms. 

bberge

By “control panel” I meant the boiler’s display. I’ll start with the check valve, but will upload pictures if that doesn’t work. I appreciate everyone’s feedback.

HomerJSmith

I assume that the DHW tank is sensored. Maybe the sensor is out of tolerance. You might want to check the programing and reset the parameters.

bberge

Photos are attached. We’ve replaced the check valve in the DHW pump, the DHW is still overheating. It feels to me like the DHW pump is running whenever the system is calling for heat. Does anyone know why that pump would come on even when the DHW is already plenty hot?

EBEBRATT-Ed

@bberge

This isn't the right fix it's just a band aid, but you could temporally put a strap on aquastat (Honeywell L6006C1018) on the outlet pipe of the hot water tank and wire the circulator through that control

HomerJSmith

How do you know that the tank temp is 165 deg? A 3 sec shower?
I would like several closer pics of the whole sys. especially the connections to the indirect tank. What do the 3 pumps do aside of the recirc? The indirect has a sensor that is connected to the Lochinvar boiler. You can check the ohm reading of the sensor for accuracy.
Pics of the boiler display in resting mode and in hot water demand mode. Check the boiler programing. Do you know how to do that? There doesn't appear to be a hot water mixing valve on this unit, only a hot water recirc.
The boiler controls the indirect pump and the distribution pump. The kind of pump and the operation is in the programing. This boiler is normally set up with a distribution pump and a indirect pump. Both pumps must have a check valve (usually in the output of the volute) and the indirect pump is priority and when the tank temp reaches 125 deg the boiler shuts the indirect pump down. If you don't have the two check valves you will get a cross connection. Look for the check valve in the indirect pump. It may have failed.

If the tank has an aquastat instead of a sensor, it will have a zone valve. The problem could be a failed or mis adjusted aquastat or failed or partially open ZV.

HomerJSmith

A zone valve setup:
A sensor setup:

bberge

Thanks to all for your input. Re the sensor, I have assumed it’s working, since the boiler display is showing the high temps on the DHW. Re the 3 pumps, the black one circulates to the DHW tank and the other two distribute heat (one appears to pump outflow and the other inflow). The check valve in the DHW (black) pump has been replaced. And yes, I have checked all of the boiler’s program settings, including all that relate to DHW; all are set to the manufacturer’s recommended defaults. Some additional photos are attached, but because the DHW is so hot, I don’t have one of the display showing hot water demand mode.
The DHW heats up when the system calls for heat to the house. It seems to me that the boiler is turning the DHW pump on when it shouldn’t. I don’t know why, but at this point I cannot think of any other explanation. And yes, taking a shower is more of an adventure than it should be!

bberge

p.s. It is a sensor setup, not zone valve. And it has worked just fine until about a week ago, when the DHW temp went rogue.

bberge

Follow up: today we used pipe clamp thermometers to trace the heat as the DHW overheated. The bottom line is the check valve in the pump, which I replaced two days ago, failed. Per the photo below, we’ve now installed another check valve in the DHW loop, and the proper temps are holding. The lesson I’ve learned is don’t trust the pump’s plastic check valve, even though the first one worked fine for 9 years. I appreciate everyone’s help.

HomerJSmith

Good job.

tim smith

We have found the plastic checks get a little groove in piston over a period of time from vibration due to flow and then stick. I kind of wish they would offer a high quality stainless check at addl cost of course as the metal would hold up better to the vibration.

hot_rod

I’d like to see pumps with high temperature HT checks. Some of the white ones are rated at 149F continuous temperature.
I have seen a few brands or models with a brass seating insert.