dramatic change in shower temp after combi boiler fitted

[Albumen]

I am not a plumber.

New combi boiler fitted. Vaillant 831. Was a system boiler + hot storage tank, header tanks etc. before. Temperature in the shower (mira themostatic) a lot colder now. In fact to get anything like a decent temperature it has to be all the way over to the hot side, way past the safety click button thingy.

But the actual pressure in the shower is a lot higher. In fact almost at hotel bathroom shower pain level.

It seems the cold pressure is far higher. Just did a test. Turned the bath hot tap on (bathroom h&c are all on the same pipes), decent flow, decent temp. Turned the bath cold tap on and the hot flow plummetted. Same in bathroom basin, utility room sink. Can\\\'t really tell in kitchen sink, as it\\\'s a mixer tap.

It looks like when a cold tap is turned on it robs pressure from the cold feed to the boiler.

Can\\\'t see the plumbing layout exactly without some pain, but the cold feed to the boiler is in 15mm and is teed into the cold feed to the kitchen sink. So it\\\'s after at least one other cold feed, and maybe after all the other cold feeds. But is that how water pressure works? Isn\\\'t pressure just pressure, wherever it is?

So, do the cold feeds need altering so the cold feed to the boiler comes first? Or is there some sort of regulating valve which can split a cold supply so I can balance up how much each half of the system gets?

Cheers

 

[corgiman]

shower is not combi friendly and unbalanced

get a combi friendly shower or balancing valves for the existing one

 

[Albumen]

Thanks. Would you say that the drop in flow from the hot taps when a cold tap is turned on is normal? If filling the bath, for instance, and both taps are on full, there's a lot less from the hot than the cold. Yes, I know I can just balance them up, but I'd like the boiler to be getting more from the mains than it is at the moment, when a cold tap is on.

I'll look up balancing valves in the Screwfix catalogue. Or is there some source of info/prices/fitting instructions you can reccomend?

Cheers

 

[corgiman]

nope when did you have the combi fitted and did the installer check the mains pressure and flow rate before he suggest it be fitted?

 

[Albumen]

The combi was fitted 2 days ago. No, the plumber didn't check the water pressure before he suggested it, as far as I remember.

I pointed it out to him yesterday, he said it was because the hot water temperature on the boiler wasn't at full. It is now, but to be honest it's not made a lot of difference.

Does the order that things are connected to the mains supply make a difference to how much pressure each of them gets?

Many thanks.

 

[ChrisR]

Yes it can, importance depending on how good your mains pressure is. Ideally the CW pipe should go to the combi first and the rest after that, but they rarely do. I have moved just one, and it did make a big difference, but that situation wasn't typical. Pressure is pressure, but only when there's no flow. There's resistance in pipes which reduces the pressure at point of use. Opening a tap on the pipe on the way to the combi will reduce the pressure in thepipe right down. You might have 3 bar mains, but if it's the kitchen tap wide open it could reduce the pressure at the Tee where the pipe goes off to the combi, to less than 1 bar.

Test your mains flow - Cw taps all on together (kitchen sink and outside at least) and add up the flows. You might get 20l/min. Your combi (I haven't checked) will have a flow limiter set at 13l/min or so. It has resistance. So if you open a cold tap wide, your cold will get more than the hot even with equal pipe resistances.

Then there's the mixer/shower hose.
Take that CW at 3 bar again. Out of the end of the shower head, if you pointed it upwards it might go up what, 5 metres? That's 0.5 bar pressure. Further back up the hose , where the hot joins the cold, the pressure might be say 1 bar. So the combi has 1 bar on its outlet, and something much less than 3 bar on its input, depending on the pipe resistance to it. maybe 2 bar if you're lucky. So the combi has only 1 bar across it.

[code:1]
You may have:

(0.5 bar across the combi)
[3bar] [1.5bar] [1bar] [0.5bar]
----------RRRR------|-R-----------x----------|-R---------------
(CW main) | | (outlet)
| |
|-RRRRRRRR---------------|
(combi)

Where this would be better:
(2 bar across the combi)

------|---RRRR--------R----------------------|-R---------------
| |
| |
|---------------RRRRRRRR---------------|
(combi)

R indicates resistance. I can pick holes in this as others will, but you get the idea I hope.

.
[/code:1]

Solution - put a resistance - flow restrictor probably best, at points x all over the house
This may cause some argument. I'll let it run!

 

[chrishutt]

put a resistance - flow restrictor probably best, at points x

No argument with what you're saying Chris, but from the practical point of view what do you or anyone else suggest to form a restrictor?

Ideally a pressure reducing valve I suppose, and these aren't too expensive these days, but a bit OTT for this purpose? What about those little isolation valves? They can be turned almost shut but then generate a lot of noise. Traditional stop taps are quiet when turned almost shut, but are a bit bulky.

I've heard of isolating valves with built in (and replaceable) flow restrictors, but these seem a bit pricey for widespread use and aren't widely available AFAIK. I would like to be able to source the little flow restrictors (as fitted to some combis, shower valve inlets, etc.) on their own with a diameter (15mm) that can be slipped into the outlet of an isolating valve.

 

[Paul Barker]

What I don't understand is why a fitter with enough nouse to fit a quality combi doesn't check what shower you have. I also don't understand why if he explained to you the pluses and minuses to a combi versus conventional open vented domestic hot water, you have come here with any questions at all?

I would have been persuading you to keep your existing type of hot water system, but if you didn't have a bs 1566 Cylinder I would change that.

It is the customer who has to persuade me to fit a combi not the other way around.

 

[Albumen]

What I don't understand is why a fitter with enough nouse to fit a quality combi doesn't check what shower you have. I also don't understand why if he explained to you the pluses and minuses to a combi versus conventional open vented domestic hot water, you have come here with any questions at all?

I would have been persuading you to keep your existing type of hot water system, but if you didn't have a bs 1566 Cylinder I would change that.

It is the customer who has to persuade me to fit a combi not the other way around.

You can see from my original post that I am not a plumber. Neither am I stupid.

The pros and cons of a combi boiler have been explained to me before. I have considered them. And was thinking of changing sometime in the future. The failure of the old boiler forced us into making a rapid decision. We had no immersion heater, so apart from kettles no means of heating water, and only one gas fire in the house, and not a very hot one at that. We both work and have 2 young children. Perhaps I made a rash decision. But I'm not going to have the combi boiler taken out now.

I don't remember the plumber checking what sort of shower we have. Though I don't think the shower is the entire 'problem', just the place where the symptoms are worst.

I came to this public forum for an answer to my questions. What's wrong with that? No, the plumber didn't explain the pros and cons of combi versus 'conventional' hot water systems. Except to say that he would always reccomend a combi.

I think that the installation was rushed, though of course we were grateful to have it fitted so quickly. And perhaps not enough consideration was given to the CW supply to the boiler. That's what I'm asking about.

If you have an opinion or experience on that precise question I'd be interested to hear it. But if your reply is just intended to tell me that I'm stupid then keep your opinions to yourself please.

The plumber is from Leeds, as it happens.

 

[Agile]

Although you are not stupid according to your own diagnosis, I am surprised that you have not realised that both hot and cold now come from the mains water supply.

The mains supply pipe has a certain resistance and even with an open ended pipe in the house can only supply perhaps 10 or 15 or 20 litres per minute.

This flow has to be shared between all water outlets. A conscientious plumber will fit restrictors so that a toilet cistern only takes say 2 li/min and does not steal all the hot water. Likewise a kitchen tap should be set for only say 5 li/min. Doing this will lessen the effect on the shower.

Before fitting a combi the installer should have measured the flow and if necessary recommended an ungraded supply pipe into the house. Alternatively balancing the supply to each outlet as above will lessen the effect of the limited supply flow.

Tony Glazier

 

[Albumen]

Although you are not stupid according to your own diagnosis,

Don't be sarcastic please.

"I am surprised that you have not realised that both hot and cold now come from the mains water supply."

The hot and cold water both came from the mains water supply before. The water authority doesn't pipe hot water into the house. Just that the route was different.

The mains supply pipe has a certain resistance and even with an open ended pipe in the house can only supply perhaps 10 or 15 or 20 litres per minute.

This flow has to be shared between all water outlets. A conscientious plumber will fit restrictors so that a toilet cistern only takes say 2 li/min and does not steal all the hot water. Likewise a kitchen tap should be set for only say 5 li/min. Doing this will lessen the effect on the shower.

No doubt. But I want to fill a kettle as quickly as possible.

Should I fit a restrictor on the cold supply to the shower?

My question is still - does the order in which cold feeds are taken off the mains supply make a difference to how much water each one delivers, if more than one of them is drawing water simulataneously.

That's what I'd like an answer to. 'Plumber should have....' doen't help me now.

Still, some of this explains why intelligent householders have such a negative attitude to tradespeople.

 

[ChrisR]

Should I fit a restrictor on the cold supply to the shower?

My question is still - does the order in which cold feeds are taken off the mains supply make a difference to how much water each one delivers, if more than one of them is drawing water simulataneously

Jezus H, Did you understand what I spent some time explaining? If not, then ask, please DON'T say nobody has answered your questions. It isn't by any means hard to understand.

You have been told about the way the total flow is limited by the mains, and how the pipes connected to it affect the flow at different taps along the line.
Also that you appear to have the wrong type of shower mixer to cope well with the supplies you have, and what to do about it. X marks the spot for a restrictor which will help balance the flows.
Less than a tenner: //www.diynot.com/forums/viewtopic.php?t=44720

You don't seem to be able to grasp that you previously had a water store, in the loft, so DID NOT have to take all the water from the mains at once.

But I want to fill a kettle as quickly as possible.

Your kitchen tap will probably stop your shower - that's what you paid for.
You might want the moon, but you've got yourself a badly installed combi on an inadequate mains supply. But you take it out on people giving you free advice...

Still, some of this explains why intelligent householders have such a negative attitude to tradespeople.

Where is the intelligent householder?

 

[Agile]

"""The hot and cold water both came from the mains water supply before. """

How can an intelligent person say that? Surely it must be clear to an "intelligent" person that the hot and cold in the bathroom came from a storage cistern in your loft???? That had about 50 litres and delivered it by gravity to your bathroom quickly but at low pressure through 22 mm pipes.



"""My question is still - does the order in which cold feeds are taken off the mains supply make a difference to how much water each one delivers, if more than one of them is drawing water simulataneously. """

Why dont you try applying Kirchoffs Law to help you understand your question.

Your obviously despise "tradespeople" as you call us! What should we think of electronics engineers who cannot understand series and parallel resistances?

Tony

 

[Softus]

The hot and cold water both came from the mains water supply before. The water authority doesn't pipe hot water into the house. Just that the route was different.

mikey, the basic point that you're missing is that the hot water now flows directly from the cold mains, merely passing through the boiler to be heated before reaching your hot taps.

Previously the hot water was stored, so, although it originally came from the cold mains, (and even then it was stored as cold water before being heated and stored as hot water), there's a vast difference between the two scenarios - it's this difference that you give the impression of failing to understand.

This doesn't mean that you're stupid, so please take a moment of calm and understand that we're not trying to get at you, just a bit exasperated that you haven't got the point yet.

 

[ollski]

This doesn't mean that you're stupid, so please take a moment of calm and understand that we're not trying to get at you, just a bit exasperated that you haven't got the point yet.

Oh I don't know, I think slating folks who you are asking for help and advice when they point out you may have originally been poorly advised could quite easily qualify as being stupid.