hot water is just better than lukewarm

Postby shazwilliams » Thu Mar 08, 2007 10:03 pm

I have a combi boiler and the water upstairs is only just hotter than lukewarm yet the downstairs water can get red hot. The boiler is just a couple of metres away from the bath. Can anyone please explain why?
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Postby Cpt Nemo » Fri Mar 09, 2007 12:48 pm

Hi Shaz

Is this a problem that has just happened, or is it something that has got progressively worse over time?

One of the challenges with Combi boilers is that they will only heat a limited amount of HW in a given time, and the small print in the manufactures info will tell you that the quoted litres of HW per min are in fact for a 35 c temp rise, ie if your inlet temp is 10 C and you want water at 45 C, then you should get the stated output.

However, during winter months the in-let temp of the water tends to be a couple of degrees colder than in the summer and as a result the Combi has to work harder to get the water to the same temp - hence you get less volume.

In your situation I would suspect that your HW feed to the kitchen tap is fed via a 15mm pipe and you are pulling a smaller volume of water from the Combi - hence it is hot. On your bath you could have a 22mm feed (if the Combi was a retro-fit at some point) and that you are pulling a larger volume of water from the Combi which has less chace to heat up - hence comes out luke warm.

Type flow rates for kitchen taps are 3 - 5 litres per min, bath taps are 10 - 15 litre per min.

Hope this helps.

PS You can find out more about our Weekend Plumbing courses by going to our advert at the bottom of every project on the website.
Cpt Nemo
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Postby riley » Thu Mar 15, 2007 10:18 pm

i had the same problem with my combi. the only solution i have found is to make sure that the combi is set to comfort rather than economy and turn the dial for the central heating right down when you are filling the bath. bit inconvinient but at least the water is hot!
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Postby thedoctor » Fri Mar 16, 2007 1:58 pm

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Postby DONFRAMAC » Thu May 10, 2007 3:49 am

This is the identical problem to one I recently correctly remedied. The upstairs combi was unable to maintain a useful output temp., when nearby outlets were opened. Excessive rising-main pressure was surmised, and the remedy was to slightly reduce the flow to the combi;-- tightened the riser slightly. (water maintenance can lead to a rise in local water mains pressure!)
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Postby DONFRAMAC » Thu Sep 06, 2007 4:23 am

I am about to have a gas combi system installed. With 28 KW, it will deliver hot flow of 11.5 litres/min, with a 35 degree temp rise.
The pressure of the hot water needs to be no more than 3.5 bar when a flow of hot water of 5 litres/min is running. If it is more, then a pressure reducer needs to be fitted.
It is expected to give a much more satisfactory shower, than any electric unit.
The only better shower I can envisage would take pumped water from a pressurised stainless steel hot indirect cylinder, and cold from a large loft header-tank, and feed the flow thro' a "pressurised system" blender shower, such as the Combiforce or Combiflow units with pressure or temperature stabilation.
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