Hot and cold flow with a combi - suggestions?

[David.WE.Roberts]

A friend has a BG combi boiler, at least 10 years old.

When you just turn the hot on you get a good powerful flow of hot water.

However when you turn the cold on you get a powerful flow from the cold tap but the hot dies down to a very weak flow.

I would have expected that the flow from both taps should be roughly equal with a bit less from the hot because of the constriction due to the heat exchanger on the boiler. However I wouldn't have expected such a marked difference.

The major effect of this is that if a cold tap is turned on whilst a shower mixer is on the shower goes cold. With a thermostatic mixer I would assume that the flow would slow down a great deal.

In the past she fitted an electric shower because this didn't suffer from the 'cold tap syndrome' in the same way.

Unfortunately I can't test the exact same set up at home because all our taps are mixer taps so I can't be sure that I am ever getting 100% hot however as far as I can tell our system doesn't do this.

[Mind you I went to a lot of trouble to design the pipework so that the combi got the best possible cold mains supply.]

Anyway, it doesn't seem right.

Is this a known problem with combi systems?

I will check where possible for simple things like a valve partially shut somewhere in the mains supply to the combi, but failing that what might be the best way to improve the hot and cold balance?

I guess it depends on where the feed for the boiler splits off from the cold mains.

If it is near the stop c*ck then I assume some kind of flow restrictor could be installed for the cold side.

If the cold supply to the combi is at the furthest end of the cold run then I assume that some kind of in line tap would need to be fitted to the cold feed to each set of taps so that the cold could be limited at each tap. Isolation valves would presumably do this, and probably not be a bad thing anyway.

The alternative might be to run in a new 22mm cold feed to the boiler from as close to the stop c*ck as possible. However major excavations under the kitchen may not be flavour of the month.

The house is being refurbished prior to rental so now is the time to fix anything major before new carpets and stuff go down and tenants are in.

Suggestions welcome :-)

Cheers

Dave R

[polygonum]

First question is surely to find out what the mains flow/pressure is like? Is it capable of supplying a decent stream through both cold and hot at the same time?

[David.WE.Roberts]

I haven't yet put a water pressure gauge on the cold supply or measured the flow per minute but from just watching it the flow seems O.K.

My main concern is the difference in flow rates between cold and hot.

If there was very poor pressure/flow surely both hot and cold taps should slow right down?

Cheers

Dave R

[polygonum]

If supply is capable of 10 lpm at reasonable pressure, if you take 8 lpm through cold (assuming that to be the easier path), then you don't have much left for the hot.

[David.WE.Roberts]

It is this 'easier path' thing that I'm tying to work out.

As I see it if there is more potential flow than can be handled by a pair of taps at full bore then no problem.

If there is enough flow for 1.5 taps would you expect one tap to run flat out and the other at 50%?

It somehow doesn't feel right that there is such a difference.

I will try it with two cold taps on at the same time to see what happens then.

I am trying to sort out what makes the route through the hot tap so much 'harder' that the cold tap gets the lion's share of the water.

Cheers

Dave R

[Onetap]

The combi boiler's plate heat exchnager, mostly, especially if it's scaled up, as they do in hard water areas. Plus all the fittings and pipes in the combi boiler.

I'd suggest fit flow regulators to the cold to reduce the maximum cold flow to match the hot. And or descale the heat exchanger.

You could get a similar effect by partially closing a valve to restrict the cold flow.

[Mrs Bonk]

This has been the way with every multipoint I have owned. It is well known in my house that when the hot tap is being used ( no shower but hair washing hose) no one must pull a flush or run the cold. We are used to it. If the washing machine is on and it goes into rinse cycle I make sure I'm not rinsing my hair! If I use the garden hose the hot tap will dry up totally. I can regulate the heat in the hot tap by using the cold - in winter the water is not quite as hot as I like so by turning the cold on slightly there is less going through the boiler so it becomes hotter. I thought with my new boiler this would be a thing of the past but looking at it logically I guess it is obvious this would not be the case. However, despite this flaw I would not change my hot water system to any other I have known .

[John Rumm]

It varies... some combis include automatic flow regulation to limit the temperature fall on high demand, and some don't (requiring manual setting of the inlet service valve to limit the maximum flow)

[David.WE.Roberts]

Interesting - if there is an inlet service valve and I can find it this may be one option to explore i.e. opening it up fully if it is partly closed.

No issues with water temperature at full flow at the moment.

Cheers

Dave R

[John Rumm]

You may find if you open it more (and it was correctly set) then you will then have problems at full flow...

[David.WE.Roberts]

Always a risk.

However it is probably easier to manage by turning down the hot a bit until the temperature is right than to try and balance the hot flow by playing with the cold tap as suggested elsewhere.

I presume (assuming I can find the damn thing) that I can do a rough calibration by adjusting it with the hot tap running.

Cheers

Dave R

[robgraham]

Picking up on the suggestion of the furring up of the heat exchanger, no one has asked the question - what was the system like when first installed; has it degraded ?.

I know nothing about combis, but this adds to my list of the disadvantages of them.

Rob

[John Rumm]

Yup sure. In fact that gives more flexibility anyway, and if you understand how the interaction between flow and temperature works its easy enough to adjust as you say.

The only difficulty is perhaps for guests etc who don't appreciate the interplay... (who will not doubt pop up uk.d-i-y at some future point claiming "its impossible to get hot water out of a combi" and all that normal twaddle ;-)

Or just open it full (if its not already) and see what happens.

Last time I had a combi the max flow from the mains was only about 17 lpm. The combi could manage most of that in the summer months so it was not too much of an issue to leave the inlet valve full on.

[John Rumm]

In reality it sounds more like a result in using one on an inadequate mains supply.

[David.WE.Roberts]

AFAIK it has been like this for a long time - the electric shower went in a good few years ago because of the problem.

However I think that because there seems to be a good flow of hot water when the cold tap is off the heat exchanger may not be too badly furred up.

How easy is it to de-fur a heat exchanger?

I assume that you would have to remove it from the boiler and then clean it and refit it.

Cheers

Dave R

[robgraham]

That flow could be misleading. Am I right in thinking that the HW o/p of combis is thermostatically mixed down to a safe temperature ?

Rob

[John Rumm]

Not usually IME. There is usually a temperature control that sets the upper limit temperature. With modern boilers as the flow rate reduces they will modulate down to prevent this (user selectable) limit being exceeded, and start cycling the burner should that not be enough to keep inside the limit. Older boilers without modulation had to do this sooner. Good ones that also store a couple of litres of preheated water can often deliver a stable temperature at pretty much any[1] flow rate using these tricks.

[1] Obviously "any" up to the when the flow rate increases to the point that there is inadequate power available to maintain the temperature. What happens then depends on the boiler. Ones will automatic flow regulation (used to be rare - but getting more common) should simply peg the flow rate at the point that it can just cope. Ones without will start delivering ever cooler water until some other factor limits the flow rate (like max available from the supply, or manual throttling either by the inlet tap, or by the user at the point of use).