Weird water temperature drop in shower

josieT

I have an odd issue but very consistent. In one shower, the hot water will flow for 6 minutes. Then it goes cold for about 2 minutes. Then it stays hot after that (at least 20 min that I ran the test. ) It seems to happen for only 1 shower. It happens very consistently whenever some runs the shower in that one bathroom. Other bathroom and faucets are ok.

I have a new combi boiler so at first suspected it's the boiler. However I don't have a reference for the old boiler since I never used it. So I can't say for sure that it's the boiler and that it used to work.

Is it a possible plumbing issue?

I'm going to do some more testing of the faucets to make sure it's only the one shower. But curious if anyone has seen this. I don't have an indirect tank just the combi. Given the combi has continuous supply. It seems unlikely that the hut hot water is running out.

I am not running any other appliances like dishwasher or washing machine nor any other bathroom. So it's not an issue of limited GPM of the combi.

RobG

A quick test is to just turn on the shower and go to the boiler and put your hand on the domestic hot outlet pipe. If it goes cold, it is a boiler issue. Possibly a flow sensor? Its a place to start.

Zman

Definitely check the boiler first.
Beyond that it could be a problem with a mixing valve or a cross connect.

4Johnpipe

Is there a hot water recirculation line? A missplaced return T could be the culpret. Also as mentioned it could be the cartridge in the shower valve.

Rich_49

2 hot things with a cold thing in between sounds like a sandwich . Triangle Tube CC120 or 125