We had a new shower unit fitted in the loft connected to the combi boiler on the st floor. The shower has a thermostatic control which means it’s designed never to run too hot if taps are being used elsewhere in the house, however the problem is that, once on and running hot/warm as required, it then suddenly runs cold for several minutes, far longer than it should if the hot water system is simply heating more water. We’ve tried running taps in other parts of the house but this doesn’t make the shower run cold at all. Could the cold water be due to the shower fitting being faulty itself, or simply due to being poorly fitted? The boiler is still on when this happens. Any possible explanations please.
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you should have an adjustment on the mixing valve trim, possibly adjustment not set correctly, they can be tricky, usually requires owners manual , also a piece of debri could be jammed in one of the ports, this can happen usually when the water is shutoff for awhile when you had it installed, you could shut the water down pull the cartridge out of the shower valve then turn water back on and flush the hot and cold ports and valve body, then reinstall the cartridge. this may help. good luck
Source(s): im a plumbing contractor in portland oregon www.gardenhomeplumbinganddrain.com
Does this only happen when you first turn your shower on ? or after a few minuites in the shower ? if it happens when you first turn the shower on its normal to get a spell of hot water then it to go cold with a combi boiler. its to do with the small volume of water your boiler keeps heated up, once thats used it runs cold till your boiler fires up fully and starts heating the water. For a “Plumber” to say you cant run a shower off a combi i think needs a career change. You cant run a power shower off a combi. A thermostatic shower is completly fine off a combi. which proberly about % of homes now have.
It is not recommended for a shower to be connected to a Combi Boiler because you draw off hot water faster then the Boiler can heat it. For a showe, your hot water must come from a hot storage tank. This is mentioned in the makers installation instructions. Whoever installed the shower did not know what they were doing.
Source(s): Plumber UK