Recently had a full service, new seals etc. on our combi boiler, it's a Vaillant I think. Everything fine. Located in the kitchen.
At the end of the kitchen we have the bathroom. Also have a toilet and wash basin upstairs.
For some reason the hot water pressure on the hot tap on the bath and the wash basin in the bathroom is ok, then goes off to a dribble then nothing. Wait for some time and it will build up again, then go to nothing.
The hot water in the kitchen tap and the wash basin upstairs has good constant pressure. Just the bathroom, which is around 15ft from the tap in the kitchen and 12 ft from the boiler.
I had a similar problem a few years ago with a shower unit (not pressured and fitted to a combi - roughly the same distance). It worked fine (2-3 years or so) until our boiler got a bit old. The hot water would be fine for about 20 seconds then cut out (or go cold). Great! The water to the ajdacent basin taps was OK but anything needing a bit more water was a challenge. If being pushed upstairs in your case - this might be the 'challenge' part.
You don't mention how old your boiler is. The offending item at our pad was about 10 years old and only produced 5-7 litres of hot water a minute. We changed it to one doing 14+ which solved the problem for us. Get a 15 litre building bucket and time the hot water flow, perhaps? Apparently, if a boiler is stretched for demand, it closes off to prevent damage (our explanation not technical) then tried again.
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