Hot water goes back down the cold pipe

[geoff2ey]

Strange behaviour of a shower mixer run off a combi boiler: when I run the shower and try and use the mixer valve it only runs cold if the hot is completely off. If I try mixing hot into the stream the temperature rises to full hot only, which is uncomfortable, and when I switch back to full cold I get a few seconds of hot water out of the cold side, implying the hot is pushing back down the cold pipe. You can feel the cold pipe warming up. I've had this the other way around elsewhere, where cold pushes back down the hot in gravity HW systems and the cold is supplied off the mains, but as I said this one is off a combi boiler. Any ideas?

 

[Puddle]

Hot water supply from mains through combi boiler,cold water supply from tank supply,ie low presure
This would cause mixing proplem as described
as flow rate resricted at shower hot mains supply forced down cold
On good showers internal non return valves stop this

 

[geoff2ey]

Hot water supply from mains through combi boiler,cold water supply from tank supply,ie low presure
This would cause mixing proplem as described
as flow rate resricted at shower hot mains supply forced down cold
On good showers internal non return valves stop this

Thanks for that - I had thought that the cold tap was supplied off the mains as well as the combi boiler but there is a roof tank on the block of flats so I'll check out the cold water pressure. This flat is on the top floor and would fit with your diagnosis if cold taps are fed from the roof tank.

 

[D34no]

just check that hot is connected to left side of the shower valve and that valve hasn't been fitted upside down m8. this is probably your problem, come across it quite a lot.

 

[marleyco]

had same problem few weeks back, put check valves inline but made no difference, i think seals in shower needed replaced as it used to work fine. they wanted a new shower, so i got one only for them to tell me it works again. check valves? or just temprementle.