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Central heating problem

lordsummit

Moderator
I'm struggling to fathom this. We had a new water supply recently. It's wonderful, only us on it, so no surprise drops in pressure. We're getting fantastic hot water now from all taps and showers, except for the main bath. Turn the tap on to full, and the water gets hot, then after about a minute it goes cold. Turn the tap so less water comes out, and a constant flow of hot water happens.



The boiler is a three year old Worcester Bosch CD30, specced to be more than we'd ever need. Watching the display it starts at whatever number it starts at, 40ish if it's the 1st time it's been used, 60ish if it's been on recently. Normally it stays in the 70s, but with the bath tap running, it goes up through the 70s, 80s and 90s, eventually cycling through U1, U2 and U3.



Is this going to cost me a fortune and need ally floorboards ripping up?
 
I'm struggling to fathom this. We had a new water supply recently. It's wonderful, only us on it, so no surprise drops in pressure. We're getting fantastic hot water now from all taps and showers, except for the main bath. Turn the tap on to full, and the water gets hot, then after about a minute it goes cold. Turn the tap so less water comes out, and a constant flow of hot water happens.



The boiler is a three year old Worcester Bosch CD30, specced to be more than we'd ever need. Watching the display it starts at whatever number it starts at, 40ish if it's the 1st time it's been used, 60ish if it's been on recently. Normally it stays in the 70s, but with the bath tap running, it goes up through the 70s, 80s and 90s, eventually cycling through U1, U2 and U3.



Is this going to cost me a fortune and need ally floorboards ripping up?

No expert, but the flow rate to the bath tap is probably much more than to any other tap, so the boiler has to output more heat for a given temperature.

That's why reducing the flow rate gives you hotter water, but at a slower rate. The boiler can only produce a given amount of heat. If you ask it to heat, say 20 litres/minute, the temp will be a lot lower than if asked to heat up 10litres/minute.

Chris
 
As Mescalito says it's probably down to excess water pressure. If you have had a new supply connected it's likely that the incomig pressure has increased and may require reducing to avoid the problem you describe. Have you tried turning the main stop valve down a bit?
 
I'm struggling to fathom this. We had a new water supply recently. It's wonderful, only us on it, so no surprise drops in pressure. We're getting fantastic hot water now from all taps and showers, except for the main bath. Turn the tap on to full, and the water gets hot, then after about a minute it goes cold. Turn the tap so less water comes out, and a constant flow of hot water happens.



The boiler is a three year old Worcester Bosch CD30, specced to be more than we'd ever need. Watching the display it starts at whatever number it starts at, 40ish if it's the 1st time it's been used, 60ish if it's been on recently. Normally it stays in the 70s, but with the bath tap running, it goes up through the 70s, 80s and 90s, eventually cycling through U1, U2 and U3.



Is this going to cost me a fortune and need ally floorboards ripping up?

Sounds like the boiler is overheating and shutting down as it can't get the water up to the set temperature.

Two options, either:

a) reduce flow rate in to the boiler as suggested by turning down the main stop cock until it maintains heat at full flow on the tap.
b) reduce the temperature setting on the boiler itself so it doesn't overheat in an attempt to heat up the water.

b) of course may mean that the bath tap may only get "so warm".

I'm not an expert but that would be my guess. Have you checked to see what the U1, U2 & U3 codes mean in the boilers manual?
 
Just a thought but is your hot water tank a thermal storage type? These store hot water for the radiators but take a cold water feed through a coil to heat the hot water i.e. work opposite to 'normal' tanks. I had one and nobody even the installer could figure it out but I did!

With this type of system the boiler temp is controlled by the hot water tank thermostat. So the boiler thermostat must be set to maximum whilst the tanks thermo is set to the desired level. If you don't do this the tank will call for heat and once the boiler water reaches the set temp it shuts off but the tank is still calling for heat so the water continues to circulate through the boiler heat exchanger wasting/radiating heat until the temp drops sufficiently that the boiler once again fires up. Hence the boiler will cycle on and off.

Worth checking........

Every time BG serviced the boiler it would recycle again they hadn't a clue and always reset the boiler temp downwards..

Cheers,

DV
 
Thanks guys. Flow is supposedly within limits, there's no tank, and turning down the hot water temperature doesn't alter anything! U1 etc refers to the boiler overheating. Guess it's time for an engineer. Hoping that the three months of trying to resolve the low pressure issue hasn't damaged something.
 
Weird - I have exactly the same set of issues!

Care to compare notes once our respective plumbers have visited?


Jason
 
Same here.

Combi boiler for rads and hot taps/showers.

Always enough for the shower, but the flow for the bath filler is much more and it struggles to get it hot at full flow. Just needs to be turned down to get a hotter fill.

Also very noticably worse in winter as the 'incoming' is much colder.

Not a big deal once you get used to it. Just start filling your bath a bit sooner.
Or settle for a shower. :)
 
I guess the flow may be set too fast. We have a combi boiler in our pad in Cornwall. The bath water is very hot but the flow rate is low even at full throttle. So takes a while to 'fill' a bath. Doesn't cut out though.

Cheers,

DV
 
Plumber visited today. Issue found to be a partial blockage in the secondary heat exchanger. Quick flush and all working as it should now. He recommended a Magnetic filter and to ensure the inhibitors are regularly checked.

Jason
 
To follow up ... we now have a magnetic and other solids trap and the system is full of gunk dissolver and working fine. In a month or so the water + gunk dissolver will be replaced with fresh water with plenty of inhibitor!


Jason
 


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