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Anyone know much about Combi boilers? Bit of a weird one this:
Ours works, to a point. Heating is fine, hot water is fine. Except out of the bath tap. For reasons yet to be fathomed, the hot water out of the bath tap is only ever warm, not hot. So the other day I filled the bath from the shower head, which was hot. Until I knelt down at the bath, so the shower head was the same height as the bath tap, then the temp dropped to "warm"
Note - there are no other lower hot water taps in the house (its a bungalow after all)
All I can fathom is there's something about the lower back-pressure from the reduced head that is triggering the boiler to switch off.
Any thoughts?
Interesting, I have something similar. I cant quite get my head around the fact that the bath and shower upstairs (different fittings at opposite ends of the same bath, but presumably the same feed from the boiler downstairs).
Have you tried running the bath hot water at a very low pressure?
We had the same thing happen in our house, until the plumber told us that the boiler couldn't keep up with the demand of a full-flowing hot tub tap.
We were then able to get hot water, but only if we ran the water so softly as to be virtually useless.
Consequently, we now start running the kids' baths a long time before they are due to get in.
Not an expert, but if (big if) saxonrider is correct that it can't deliver the flow rate, then the correct fix would be to adjust the flow rate reducer that all combi boilers have. Usually as part of the diverter valve assembly IME.
Yup as above it's defo the flow rate. Think about it like an electric shower! the temperature is controlled by flow rate not the output of the heater element, apart from preset output states (1-11) obviously.
You need enough flow to fill the diaphragm and actuate the flow switch and not too much that the burner can't keep up, makes sense when you think about it.
Knackered cheapo taps can also allow a higher than usual flow rate when worn.
Hmm, that kind of makes sense, but the fact that the shower actually changed temperature when lowering it, without an obvious change in flow-rate counter-acts it. Although, assuming the feed-in pressure off the mains is constant, lower downstream head would result in greater flow. I'll go and see....
By jove, you're right!
Guess I need to tweak the valve enough to prevent too much flow to ensure a decent temp.
And then we need to replace the boiler!
I'm feeling a bit pleased with myself.
EDIT: Removed Mr Green. He's a bit pathetic.
Tried running the hot tap in the bath last night at half the normal flow and sure enough it got really hot.
Need to have a fiddle with the flow rate in the boiler then....
Turned the mains feed down a bit and the water upstairs is much hotter, presumably because the flow rate is reduced. BUT the shower is much less powerful. So I need to sus out how reduce the flow rate in the boiler rather than the mains.
When i fitted my bathroom i deliberate undersized the pipe feeding the hot tap to the bath to minimise this problem. You could also not open the tap fully which helps!
"Turned the mains feed down a bit and the water upstairs is much hotter, presumably because the flow rate is reduced. BUT the shower is much less powerful. So I need to sus out how reduce the flow rate in the boiler rather than the mains."
no you need to know how to reduce the flow in the pipe work after its been t-d off to the shower. anything you adjust in the boiler will have the same effect on the shower.
so smaller pipe or limit the tap flow rate.
this is a function of heating design and a common complaint about combis when the installer has cheaped out and designed to heating requirements instead of hot water requirements.
i had a house once with a tiny WB oil boiler - it wasnt actually possible to run a hot bath as the water in the bath would be cold by the time it fileld up.
My parents house has 2 mixer showers that can both be run at the same time off a WB 42KW combi unit which has a decent internal heat store they can also run a bath at near full chat out the hot tap.
So i went for a hot water output similar to that - how ever the heating system could have got away with a much lesser boiler....
Ok, I've turned the mains feed in the kitchen back up and the shower is running like it normally does.
However, the water heating problem has returned, i.e. doesnt get that hot on a fully open hot tap. But it get nice and hot if the tap is partially opened.
This suggests to me that the mains flow rate is quite high and thus the flow rate through the boiler when heating the cold water is too high, meaning the water doesnt get heated enough.
Soooo, [b]and I do hope this is correct[/b], I've located the main cold water inlet feed into the boiler, and turned the inlet valve down a bit.
This has reduced the flow of water being heated and therefore it gets hotter coming out the taps.
I've tested this with the hot tap in the kitchen and the temp increases from tepid to quite hot (can only just keep my hand under it hot). I can also see that the flow rate has dropped a little.
[b]Is this safe to do, or am about to melt my boiler?[/b] 😯
Your boiler won't melt, it'll lockout before it does that because the overheat stat will tell it to. You should be fine if you've only turned it down a little but. Some older boilers had a summer/winter knob underneath which basically restricted the flow of water into the boiler as incoming mains is a bit colder in winter so it would keep temperature. That's effectively what you have done.
Surely you are over complicating things. Leave your mains stop tap, and the boiler inlet as they are and just turn down the flow at the relevant tap to get water hot enough. This is standard on most combis (unless you have a large 30kw plus one).
Industry standard used to be a 35 deg C rise and 9 litres per minute. Water main coming in at about 8 deg, plus 35 gets you to 43, probably not quite hot enough for a bath but fine for a shower. Therefore to get the water hotter turn the flow at the tap down. Standards have improved slightly but still not great. Put a measuring jug under the tap and see how much 9 litres per minute is, it's not a particularly great flow and when you think you might have to get it down to 6 litres or so to get the temperature up.
Bigyin how is your shower now....... Sounds to me like you have done the same thing on a different valve.
Phew! Thanks northernmatt.
Obviously I did a bit of googling first before buggering about. I just wanted a bit of reassurance that I was doing the right thing!
Obviously its more noticeable in winter because the cold water is colder, so the temp increase from the boiler is supposed to be +35 deg C.
Its the first winter in our own house, so its been a bit of a learning curve setting up the heating etc.
All the rads were had the flow valves wide open, so some rads heated up fine and others were pretty chilly, so I had to learn how to balance the system. 🙂
Plus the boiler kept losing pressure on the CH side, mainly because the expansion vessel hadn't been pressurised properly. Since I've sorted this, it hasn't lot any pressure. 8)
trail_rat. Shower is fine now. Back up to temp and pressure much better.
I'd shut the main water feed off in the kitchen. So whilst I'd dropped the water pressure to a level where the boiler could keep up, I'd also dropped the cold water pressure meaning the shower was weedy.
This time i've just reduced the cold water feed into the boiler, meaning cold water pressure is unchanged everywhere else.