Is it possible to change boiler priority?

[Jon]

I'm new to all this so please bare with me...

I'm having a problem with a combi-boiler and need help trying to identify the problem -

The boiler appears to be working fine - i.e. it's able to heat the radiators and water. However when heating water for the shower it seems incredibility sensitive to changes in water pressure, if someone were to turn the tap on in the kitchen the hot water flow in the shower completely stops. Also, if the heating is on it seems to frequently cause the hot water in the shower to fluctuate.

I'm wondering is it possible to control the boiler so that it gives priority to heating water? Also, is it possible to prevent the affect of turning on a tap in another room?

TIA

[Kalico]

What type of shower valve do you have? A thermostatic valve would help.

Not sure, but getting a 22mm pipe for the hot as near as possible to the shower, with other services branching off in 15mm, might make a difference.

My plumber disagrees, saying that it makes no odds since it is only

15mm coming from the boiler, but to me it seems common sense that it would help smooth the flow of hot from the boiler. Sure someone will tell us either way.

Replace 'spam' with 'org' to reply

[Kalico]

What type of shower valve do you have? A thermostatic valve would help.

Not sure, but getting a 22mm pipe for the hot as near as possible to the shower, with other services branching off in 15mm, might make a difference.

My plumber disagrees, saying that it makes no odds since it is only

15mm coming from the boiler, but to me it seems common sense that it would help smooth the flow of hot from the boiler. Sure someone will tell us either way.

Replace 'spam' with 'org' to reply

[Mark Hewitt]

As the combi depends on a constant flow of mains water to heat up then it's not easy to balance out changes to the water pressure due to someone/something else using the water supply. Unless you have an elaborate mechanism of storage tanks.

All you can do is to make sure you have a thermostatic mixer on your shower, so if the water pressure drops, at least the temperature shouldn't fluctuate too much. I know with my combi when someone else uses the water the pressure drops equally across the hot and cold.

As for the heating, then I don't believe it should do that at all.

[Doctor Drivel]

The best thing to do is run a dedicated 22mm pipe from the stopcock to the combi, even if the mains is 15mm. Tee off the cold to shower just before the combi on this 22mm pipe. At the stopcock tee off to all the other cold outlets and insert a controlling gate valve here. Also insert in-line service valves at taps and throttle them back. Then the combi and shower will get priority.

Putting 22mm on the outlet side can work but only when feeding the shower, and 22mm must be on the combi feed. Again this makes sure the shower gets priority.

[tarquinlinbin]

Alas one of the major drawbacks of many combis is finally revealed. Many combis just arent up to the job of supplying two taps at once with decent flow rate. As others have stated, check if the mixer vavle is thermostatic,if not then fit a thermostatic mixer,thats about all you can do apart from fitting a water storage cylinger to ensure a decent bulk hot water supply.

Remove antispam and add 670 after bra to email

[Doctor Drivel]

Bzzzt Wrong. A higher flowrate combi would solve it too.

[Mark Hewitt]

It's my understanding that Combi's can only ever supply hot water at the maximum flow rate of the mains supply. Which I think mine certainly can. So putting a higher flowrate boiler on wouldn't necessarily help

[Christian McArdle]

If your mains is that bad, then a mains pressure hot water system is probably a bad idea in the first place.

Christian.

[chris French]

In message , Mark Hewitt writes

And/or the maximum rate at which they can heat it. The mains flow in my old house with a combi the mains flow rate was a fair bit more than the boiler could heat. So the rate of heating was the limiting factor (as it is in most combi installations I suspect.

If you shower really does stop (as opposed to just get bit slower) when another tap is turned on then it suggests to me that the mains flow isn't up to the job maybe. In the above mentioned installation, (with a

100,000 btu combi) if someone tuned on the tap in the kitchen or the bathroom the shower flow would reduce, but it was still usable. 15mm water supply, bath or kitchen teed off of the same pipe to the shower. >

[Doctor Drivel]

Unless you have 76kW MAN boiler

[john.stumbles]

I wonder if the hot water flow from the boiler is all it could be. How old is the boiler? Is your water hard and, if so, do you have an effective (i.e. phosphor-dosing, not electronic or magnetic) scale inhibitor?

(I'm not sure if it being scaled up would cause the problem of the heating affecting the suply of hot water, but if it is scaled up you should definitely sort that out before trying to address other problems.)

You could partially turn off the service valve (if you have one - or fit one if you don't) on the kitchen and other hot taps so that they don't reduce the flow to the shower so much.

[Ian Clowes]

That would mean twice as much water sitting in the pipe between the boiler and the shower. If this is also used to feed a basin tap then anyone wanting a bit of warm water to rinse their hands will have to run off twice as much water.

IanC

[John Rumm]

Even with boiler powerful enough to handler more than one tap at a time there is still the problem that the change in flow rate will in many cases also cause a change in output temperature of the combi.

[tarquinlinbin]

Yes i know your a Combi evangelist doctor and i do know about higher flow rate combis. You tend to pick things like that up after 25 years in the industry.

Remove antispam and add 670 after bra to email

[Doctor Drivel]

It seems you never picked it up over 25 years.

[Ed Sirett]

There are a surprising number of half broken boilers around. A lot of combi boilers suffer from scale build up which restricts both the effectiveness and flow rate of HW through the boiler. Another common 'half' fault is a partially defective water flow detector which means that only a largish flow rate will make the boler fire and then it can only make luke-warm water.