Combi boiler - Hot water 'heat dump' too hot

[Phil Addison]

HELLO

> > I have recently had a combi boiler put in, and whenever the hot tap is put > on there is a really hot burst of water (too painful to use). It then goes > cold for a while, then heats up to the temerature I set on the boiler > thermostat. I have two very young children and worry about them turning on > the hot tap in the loo or bathroom. Has anyone else found a solution to > this. I asked the engineer who came out to sort out a lighting problem > today, and he said 'other people don't moan about it' which left me feeling > fed up and no wiser. He did explain to me how it happens several times, > each more slowly than the last, he was a very rude man! he had no will to > try and reassure me, but when pushed did say it would be 64 degrees > maximum - so not scalding. Any ideas will be gratefully received. > > Regards > > Tania

Water temperature fluctuations seems a common problem with combis in houses I have visited. Can anyone help with a solution to this problem that I received via email from the FAQ website?

Thanks

Phil The uk.d-i-y FAQ is at

NOSPAM from address to email me

[tarquinlinbin]

Yes,why oh why do combi installers chuck out a perfectly usable cylinder,especially in large family houses?. My view for family properties is to keep the cylinder,feed it from the combi,obviously make sure things are well insulated/controlled.

[IMM]

This that is your view you should change it, as the cylinder will burst. Find how water systems work. here are high flow combi's around, that will do a large family.

An advantage of a combi is that it liberates space tanks and cylinders take up.

[Andy Wade]

Have you ever though about taking remedial English lessons?

[Andy Wade]

Have you ever thought about taking remedial English lessons?

[IMM]

No need to as I am brilliant at English and life in general.

[Dave Plowman (News)]

Strange. I know of one system where the combi heats a tank in the normal way, and the water heating side is used just for the kitchen. Not sure what the benefits are, as the owner is as mad as you.

And the cost to achieve this, assuming a good storage system already exists?

Not everyone lives in a rabbit hutch.

[Christian McArdle]

As well as weird effects due to the combi design, another possibility if the combi DHW output and the CH pipes run in very close proximity (particularly if boxed in) is that the DHW is being heated by the CH pipes whilst sitting adjacent to them, to a much higher temp that the actual DHW output. When you turn on the tap, this slug of hot water comes out.

It is only a suggestion, and probably isn't the problem, but could be fixed by separating the pipes with a bit of insulation.

Christian.

[Andy Wade]

The evidence for that seems to be a little thin on the ground...

[Christian McArdle]

It can be good in a large house to reduce water lead times, particularly if the boiler is in the kitchen. The hot water cylinder is placed near the bathroom to provide lots of hot water quickly, whilst the kitchen runs off the combi, because you'd have to draw off 10 litres of water before anything came through from the distant bathroom. It also provides drinking hot water in the kitchen, which is very useful when cooking. I use my hot tap all the time to fill kettles and pans. (It's a heat bank, not a combi, but still drinking water).

Christian.

[IMM]

< snip tripe >

Please do not comment on fields you have no experience of.

[IMM]

[John Rumm]

Should we award the same respect to your other posts as we do to the one above?

[Phil Addison]

How about attempting top answer the actual original question?

Phil The uk.d-i-y FAQ is at

NOSPAM from address to email me

[IMM]

combi,obviously

It was answered. It will burst.

[IMM]

I doubt you would recognise a good post from a bad one.

[John Rumm]

Short term memory propblem again? go read the OP again.

[Ed Sirett]

I installed a similar system for my nieghbours directly opposite my house. S-plan for the house, combi HW for a sink in the garage below the house. They like baths and it would have been hard to gain access to the HW pipes under the kitchen floor to supply the garage sink.

The cost IMHO is less significant the space taken up in the kitchen (usually). The space to be lost in the kitchen would usually be more precious than the space gain in the loft and/or airing cupboard.

Round here a lot do.

[Dave Plowman (News)]

Right. Suppose it makes some sense. Any combi should be ok for just the kitchen tap. Could it still be used if the central heating side was drained down for any reason?

[Ed Sirett]

IME experience there are several reasons why (ordinary) combi boilers have poor regulation of DHW temperature.

1) Drawing off water from elsewhere especially in the same dwelling. 2) Thermostatic mixing valves fighting the boiler.

but mainly:

3) When the DHW exchanger becomes scaled up the heat transfer rate becomes limited, that's bad enough. Eventually the heat transfer rate drops below the lowest modulated setting of the burner (typically around 10-12kW on a popular 24kW unit), at this point the control system, in order to modulate the burner further has to start switching it on an off. Given the considerable thermal inertia of the components a fluctuation of the output temperature begins.

1. Can be mitigated by good design and practice. Namely don't fit a combi where the supply is inadequate. Take it's supply upstream of other draw offs (even consider putting a slight restriction on the feed to everything except the combi.
2. This is not a common problem and shoudl be mitigated by getting 1 and 3 right.
3. Descale or replace secondary heat exchanger.