Combi Boiler water not hot

[waterbabe]

My daughter has a combi boiler but when trying to fill the bath the water runs cold it then runs warm then cold then warm by the time she has finished the bath is luke warm.
The pressure seems ok and she isnt running any water for anything else at the time. How can she get a bath with hot water in it.
It runs hot ok when running the water in the kitchen

Heeeelp

[gucciman]

if the water is fluctuating hot and cold then its nothing to do with the flow rate whatsoever.

it could be the hot water thermister but its more likely to be a blocked secondary or plate heat exchanger.

what happens is the dhw heat exchanger gets partially blocked with sludge, the water then passes through the heat exchanger a lot slower than normal so the water gets too hot, the boilers safey stats notice this and shut the burner down until the water cools, the burner then comes back on and the water gets too hot again, hence the hot water gets hot cold hot cold.

the soluton is a new secondary dhw heat exchnger as trying to clean them never really works. but if your central heating system is full of sludge then it may well block up again over time.

[coccinelle]

My combi boiler does this too always has done. When I put the tap on I get cold water, which then gets hot for a few seconds then goes back to cold before maintaining a steady flow of hot water. What you do is wait until the it's gone through the cold, hot , cold procedure and once the hot comes through, put the plug in. I know it means a lot of water down the drain but at least you get a hot bath. I have this procedure to do before a shower every morning so brush my teeth while waiting!

[tiggerblue10]

I have the same problem so I have to let it run a bit until the water gets to a decent temperature. Its very wasteful but I have no choice. Also tell her not to open the tap too much. For some reason if you have the tap open to full it cools down.

[coccinelle]

That's right tigger, I turn the tap on just enough for the boiler to kick in and get the hottest water possible, the slower the water goes through the boiler the hotter it is.

[waterbabe]

Thanks for all your lovely answers they were quite amusing to read I shall pass on the answers to her.

[gucciman]

what is happening in the above cases is when you turn the hot tap on you get cold water first because the water in the pipework has cooled down since you last used it, you then get a burst of hot water that was left in the boilers heat exchanger, it then goes cold again until the boiler heats it properly.

and yes a combi boiler only heats the water as it passes through the boiler, so the slower it passes thru the hotter the water, you will also find the water is hotter in the summer simply because the incoming cold water is warmer too.