Low hot pressure on tap in bathroom

terryplumb said:
Its very unlikely that you have a blocked pipe ,not impossible ,but highly unlikely. Did original tap ever work properly ? First thing is to focus on the iso valve that you have in vanity unit and go from there.
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Ok, we moved in and the tap didn't work correctly. Hopefully its the valve.

23vc said:
Could it be that your tap is not suitable for low pressure systems? I've definitely seen that before where cold is mains pressure so works fine - well I've seen it on showers and pretty sure same would apply
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It's a combi boiler. Is that a low pressure system?

If it was to be the valve, I simply turn off the boiler, to stop the flow of hot water and replace, right?
 
If you want to change the iso. Electric supply to boiler is turned off. I would isolate the cold mains at your mains water stopcock ,turn all hot and cold taps on to drain water out. It is possible to just isolate the hot water at the combi boiler but I am against advising any one inexperienced to do it this way. Much better you isolate mains cold stopcock. I have just had a thought that you may have misinformed us on your boiler ,some people think a system boiler is a combi. Do you have a hot water cylinder ? Or tell us make and model of boiler.
 
Ok, so I removed the pipe coming from the iso valve to the tap and tested the flow. It was still slow. I then replaced the iso valve with a full bore one. Still the flow is the same. I’m guessing I must have a blockage or a kink in the pipe or something else ‍♂️
 
Is there a shower fed by the combi , or is it cold mains electric ? Anyway ,your not getting pressure thru that pipe branch so upstream investigation required.I would start at boiler ,follow hot pipe toward sink looking for a branch pipe to upstairs ,where you may find an isolator.or is it a bungalow ?
 
The shower is fed by the combi. When we moved in we had pipes installed under the floor for the shower. The pipes used to come through the ceiling but still came from the combi. We live in a bungalow so have no water in the loft. The full bore valve seems to have improved the flow just enough to kick on the boiler so we are getting hot water now. The boiler is outside in the old coal shed and the pipes go under the kitchen which has been floored. It is accessible by crawling from the bedroom underneath the house but i'll leave that to a professional when we get the bathroom redone. We'll be knocking down a wall to increase the bathroom size so we can go under then and fix the problem with the pipe.

Cheers for your help
 
Sorry to keep this thread going but I've discovered something pretty weird.

It turns out that increasing the bore on the pipe hasn't fixed the problem. The tap, when moved to the hot position is still cold.

However, if i turn the shower on, the sink tap gets hot. I've no idea what the hell is going on.

Cheers
 
So you are really in the same position then ? Flow from hot tap is not enough to trigger boiler ,flow from shower is.so shower starts boiler up and tap then gives hot water. But is the flow at the hot tap still as slow ,its just hot now ?
 
terryplumb said:
So you are really in the same position then ? Flow from hot tap is not enough to trigger boiler ,flow from shower is.so shower starts boiler up and tap then gives hot water. But is the flow at the hot tap still as slow ,its just hot now ?
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Yeah, exactly. Must be some restriction in the pipe. Could there have been some water saving restrictor added to this pipe? Can i adjust the minimum flow rate on the boiler? Am i going to have to go under the floor to have a look at the supply pipe to the bathroom sink?
 
What is the flow rate at the hot tap ? Run hot tap on full for one minute into a container and measure how many litres it gave. Are you sure there are no isolating valves on the branch run ,before the one in vanity unit ? Could be anywhere ,kitchen maybe ?
 
Flow rate at the bathroom sink hot tap is 2.5 litres/min. Flow rate at the kitchen sink hot tap is 7.5 litres/min. I don't know if there are any more isolating valves. I guess I'm going to have to rip it all out to investigate as the pipes go down behind the vanity and under the floor
 
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Ok, so removed part of the vanity and found another isolating valve. Took it off and inspected it. It looked like it wasn't fully open so reinstalled it fully open. Still the same problem
 
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