Hi, Just re-plumbed the pipes running from our combi (Worcester Bosch 24i) to the bathroom due to a new extension. On the advice of a plumber, I ran 22mm copper cold feed all the way to the combi - to get the pressure up, and to future-proof in case we upgrade the boiler at some point. For good measure (though now I realise maybe not so smart), I also ran the hot pipes part of the way from the boiler to the bathroom in 22mm, the rest in 15mm. What I now find is that running the hot tap on the basin or bidet does not make the boiler fire up, unless I run them both simultaneously (though the flow is not bad). The kitchen tap, which is further along the same pipe does not have this problem. The bath and shower have a thermostatic mixer. When this is turned on, it runs hot for a bit, then the thermostat seems to reduce the flow, and the boiler goes off. Is the section of 22mm pipe to blame for this, by somehow reducing the flow through the boiler, or should I look elsewhere? Thanks!
That was my first thought, but honestly can't think where that would be happening - unless the thermostatic mixer on the shower is responsible? The cold feed on that is reigned back to around 1 bar - which used to balance it, but if the HW pressure went up due to the bigger cold feed which is also now direct from the mains, could the pressure differential overpower the thermostat and cause crossover? Thanks!
Odjob, as Mr Plum says, running the hot from the boiler in 22mm won't cause that issue. (All that will do is make it take a teeny bit longer for the hot water to get to the tap as it has to displace a little bit more standing cold water in that pipe first.) If the basin hot tap doesn't turn on the boiler, it must surely be down to one of two reasons (that I can think of...); the first is that the tap isn't coming on far enough. What is the flow from that hot tap like - a relative trickle or a good 'full' flow? If 'full flow', then it can't be the tap that's at fault. So that leaves possible reason two - the flow switch on the boiler ain't working as it should be. The shower might be a different issue - I guess it's possible for the thermostat to turn the hot flow down so far that the boiler switches off? But that wouldn't be normal behaviour. So, if the hot basin tap has a decent flow, and provided you haven't done any weird plumbing that we cannot anticipate, then I guess that leaves a dodgy flow switch in the boiler? (I'm guessing the actual cold mains supply to the boiler is good? You haven't done anything weird there, have you - a half-open stopcock...?)
another thought I had,,,before the extention or before you went combi, is there an old hot water draw off that was blanked off somewhere, this could be acting as a big shock assorber on the hot circuit,,,,I know its a shot in the dark,,but its a possible answer to your dilemma
Hi, Flow from hot taps is pretty strong, and the boiler flow switch was working fine the day before re-plumbing, so not sure it could be that, unless I damaged it somehow? Debris in the pipes? There are a couple of dead legs on the hot which are waiting for future kitchen and guest loo connections. Probably about 12m of 15mm pipe - would that be enough to act as a shock absorber?
I assume the boiler has an isolation valve for the hot water. Isolate it there and turn on the hot tap, if you still get water there is a pipe crossed, or the shower is allowing the mains water to back feed down the hot. Sounds silly, but put your ear up to the shower body and listen for water flow.
12m of 15 pipe would make an exellent hiderance if in the right/wrong place, put a vent on it bleed it and try it then,
I chust don't get the dead-leg cause, tbh. At least not if it's the type of flow switch in my GlowWorm - a wee paddle wot is span aroon' by the water flow. A 'dead leg' ain't going to affect the actual flow, and it's the flow wot turns on t'boiler. Unless I'm missing summat?
Odjob Run the hot water for a few minutes then check the boiler to see if the ch is warming up or check at the rad tails to see if any are warming up (they shouldn't be).
Ok, - turned off hot water to bathroom and hot tap runs dry (no cross-over) - fiddled with restrictor on the shower (doesn't change anything) - I got rid of the dead leg, by connecting a tap at the end and draining it through - no change. - there is a tap just next to the boiler, when I open this, the boiler fires immediately, though the temperature is not what it should be. - the
- I've looked the whole set of pipes over, and I can't see anything out of place Any more clues? Thanks!
Might seem like a shot in the dark but could it be youve increased your flow rate more than the boiler can handle? Maybe try closing the isolation valve feeding the boiler to half and see what happens? If you've upped your flow rate to 20ltrs a min n it can only heat up 12 or something daft it might be firing but not capable of heating thatch water that quickly?
so if the pipes are ok, its the boiler, has it got a diaphram or flow switch? its gotta be the first thing to check
Hi Tom, I think you're right and there's a man coming in the morning to try to figure that out - a bit outside my comfort zone...! Thanks for all your help!
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