opening cold tap has effect on boiler

[Simon F]

I think I read a similar thread on this but can't find it.

glowworm combi, turn on a cold tap and most times the fan ( I think fan just from listening )
runs for a minute or so. is this just a glitch that sometimes happens?
is the solution simply to fit a check valve on the cold feed, if so does it also need a mini expansion vessel?
or is there something else going on?
:stupid:

 

[Roger Welton]

I'd agree, check valve or shock arrestor should do the trick.

 

[Mike Jackson]

Dc valve and shock arrestor will cure the problem. I had this on a Worcester.


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[JCplumb]

Blue flag installed a boiler at my mums house about 5 years ago, it did this. When asked they explained that the boiler senses the use of the cold tap and pre-heats the DHW heat exchanger in anticipation of the hot tap being used. I liked the excuse to be honest

 

[JeffD]

ya lol ) like the jcplumb comment , double check on


ajdgas

 

[diamondgas]

Usually caused by air trapped in a dead leg on the hot water circuit somewhere Simon. Just checking but you mean a check valve and arestor on the feed to the boiler yeh?

 

[Simon F]

Usually caused by air trapped in a dead leg on the hot water circuit somewhere Simon. Just checking but you mean a check valve and arestor on the feed to the boiler yeh?

thanks for the tip about dead legs, might have a look at that.
its actually at a new place my mum is moving into, I am currently refitting all the bathroom,
but noticed the fan/pump in the boiler runs when you use the cold.
I did some tests today, the problem is only when the cold is closed quickly, if you slowly shut the cold tap its ok.

so yes I was thinking would a check valve on the cold into the boiler resolve it?
or would a shock arrester be better?
-:bucktooth: daft question - if both were needed which way round?

 

[hammers4spanner]

says in user manual whioch way round ,
think its prv,non return then shock arrestor

 

[Whn1]

How does a shock arrester work? What is it used for?

 

[hammers4spanner]

had a dead leg issue before on new heat install on council bash where after couple mins the hot just ran cold off and on now its been mentioned

 

[hammers4spanner]

How does a shock arrester work? What is it used for?

used for when the water is heated up on demand and expands ,,,where does it expand into??
takes pressure out of h/c

 

[REDSAW]

i think dcv just before the boiler 't' off will sort it

 

[Mike Jackson]

If you put a DC in you will need the shock arrestor to take up the expansion on the hot water as it won't be able to expand back down the pipe.

 

[Whn1]

used for when the water is heated up on demand and expands ,,,where does it expand into??
takes pressure out of h/c

Like a mini vessel?

 

[REDSAW]

If you put a DC in you will need the shock arrestor to take up the expansion on the hot water as it won't be able to expand back down the pipe.

but not on the cold!!!!!!!!

 

[hammers4spanner]

Like a mini vessel?

In a nutshell yes , as far as i'm aware most boiler mi's now ask for prv,check valve and shock arrestor

 

[REDSAW]

In a nutshell yes , as far as i'm aware most boiler mi's now ask for prv,check valve and shock arrestor

can you name any to hand?

 

[Mike Jackson]

Atag and Ideal both ask for them to be fitted if their is a water meter.

 

[hammers4spanner]

worcester ,ideal and fitted baxi other day which asked for this, think most others are following suit,
also another gsr fitter i know was pulled up for no prv ,non return and shock arrestor on a gsr inspection on the boiler he installed (dont know what model)

 

[diamondgas]

thanks for the tip about dead legs, ......
-:bucktooth: daft question - if both were needed which way round?

As mentioned already Simon, If you fit a check valve you'll need somewhere for water to expand into, even such a small amount as what's held in the p-hex. You fit the arrestor down stream of the valve. If i suspect a dead leg I'd be asking the custard if they'd previously had a cylinder fitted or any alteration on pipework that could account for a now disuesed piece of pipe where air could have collected?

 

[Simon F]

As mentioned already Simon, If you fit a check valve you'll need somewhere for water to expand into, even such a small amount as what's held in the p-hex. You fit the arrestor down stream of the valve. If i suspect a dead leg I'd be asking the custard if they'd previously had a cylinder fitted or any alteration on pipework that could account for a now disuesed piece of pipe where air could have collected?

the combi has been fitted in the last couple of years, presume it was from an open vent before.
the pipework is a mess, so would not be surprised if there is a dead leg up in the loft, its a bungalow with pipes going up from boiler and drops down to rads.
will have a look in the loft, thanks Diamond

 

[diamondgas]

You're welcome ... Let us know how you get on

 

[stevetheplumber]

dead legs tend to cause the boiler to run on after the tap is shut removing a dead leg will probably make the problem worse as it will be acting as a buffer
not come across the boiler needing all the extra bits yet but it makes sense my argument is if its needed it should be fitted to the boiler they wouldnt sell a combi with no pump

 

[diamondgas]

dead legs tend to cause the boiler to run on after the tap is shut removing a dead leg will probably make the problem worse as it will be acting as a buffer
not come across the boiler needing all the extra bits yet but it makes sense my argument is if its needed it should be fitted to the boiler they wouldnt sell a combi with no pump

A dead leg pocket of air acts both ways Steve. turning a cold tap on reduces the overall pressure throughout the water system, hot and cold. When you turn the cold off the water runs back into the hot water circuit as it compresses the air once more either making the turbine switch or lifting the diaphragm momentarily ... Regards fitting them they use too Myson Midas, Saunia Duvals, and the old Vaillants had the arrestors fitted as standard... I don't think manufacturers should accommodate for poor installation into their design though personally! When a boiler conversion is undertaken the plumber should know that dead legs will cause these issues and be aware that expansion needs to be absorbed somehow IMHO.

 

[Nostrum]

Already covered! If you look in the older vaillants, they all had shock arresters integral. I suppose it's cheaper to make them without these days but specify them in the book! Also they are physically smaller now so might be a bit awkward getting them in.

Dead leg acts like a shock absorber, so when you turn hot tap off the pressure of the water compresses the air trapped inside. It's the time it takes to compress the air that causes the lag between tap off and boiler shutting down, more air longer the wait.

Similar effect when first filling a system up, it takes ages to get the needle moving because you are compressing all the air within the pipes. Once there air is out a small blast on the filling loop will bring the pressure right up quickly.

EDIT:Just realised i've just ranted on about everything that was said in the post above! Sorry diamondgas