Combi Boiler Comes On Briefly As Cold Water Is Run

[pbar]

I've just had an extension to the side of the house, and in the process my Worcester 240 combi boiler was moved elsewhere, close to a new small toilet. I've noticed now that when the new toilet is flushed, after the cistern is filled, the boiler 'does something' (activates in some way) for a couple of seconds and then stops. I was told that it's because the toilet is so close to the boiler and is taking water from it. But it happens no matter what cold water is run, cold taps upstairs, the outside tap, etc. I can't remember if it did this before, as it was under the stairs. Please can anyone tell me is this normal behaviour. And if not, what can I do please. Thanks!
It tends not to happen when the water is run slowly, and stopped slowly. But opening and closing a tap quickly causes it, as does the toilet flush/fill.

 

[gas4you]

Sounds like a dead leg in the water circuit

 

[pbar]

Thanks very much for your reply, it's appreciated. In laymans terms, what is that exactly. Is it when air is trapped in the pipework, especially in areas where they might have been capped off.

 

[baxpoti]

Fit a check valve in the cold supply to the boiler.

To comply with water byelaws 90 and 91 you'll also need a mini expansion vessel to accommodate the thermal expansion of the water.

 

[pbar]

Thanks for your replies, I appreciate it a great deal. The valve you mention seems to be more of a workaround according to my many searches on the net, as opposed to actually getting to the root of the problem.
Where the boiler used to be, I've checked under the floorboards and noticed 3 pipes capped off. Guess these are what's known as dead legs and are the cause of air in the system which is the root of the problem.
I'll speak to my lazy plumber tomorrow, most of you guys (reading other threads) seem to agree that these dead legs are bad practice. Stagnant water, air in the pipes, etc. One post mentions Legionnaires due to stagnant water in dead legs! And I've made the massive mistake of telling my wife this.
Is it common practice however to leave pipes like this.

 

[pannierstan]

Only if your name is Baxpoti obviously.

Any installer worth his salts will make sure to remove them and not find a work around for it!

Stan

 

[gas4you]

if unavoidable I always fit an air vent instead of just capping off

 

[ChrisR]

pbar - it's only dead legs in the tap water pipes which are relevant here, not the heating ones.

 

[Agile]

Fit a check valve in the cold supply to the boiler.

To comply with water byelaws 90 and 91 you'll also need a mini expansion vessel to accommodate the thermal expansion of the water.

But if you put it on the hot water out immediately after the boiler you will not need the expansion vessel.

Tony

 

[pbar]

pbar - it's only dead legs in the tap water pipes which are relevant here, not the heating ones.

I'm not sure I understand what you mean here. Surely all of the cold water pipes are connected, there's only one supply coming into the house after all. So capped off pipes which used to run to the boiler are connected to every tap, every toilet, etc. in the house. I've spoken to tech support at Worcester last night, and he said the same as you guys here, it's air trapped in the pipes, usually in a dead leg caused by capping off previously used pipes.
The boiler before never had to have these non return valves, etc. so I don't see why it should need anything extra now to have it working as flawlessly as it did before.

 

[gas4you]

Chris is stating that it is nothing to do with the c/h pipe work being cut and capped anywhere

 

[ChrisR]

So capped off pipes which used to run to the boiler are connected to every tap, every toilet, etc. in the house.

Wrong!!!

The tap water pipes are not connected to the pipes which go to the radiators. The water in the radiators does not come out of the taps, it's a closed circuit.

 

[pbar]

Aahhh, I see what you mean, thanks for the explanation. The boiler pumps water to the rads which is a closed circuit and has no connection to the cold water taps, toilet, etc.

If that's so, how come trapped air in the capped off pipes (which used to run to the boiler in it's old location) is causing the problem I have. (Boiler coming on briefly when a cold tap or toilet is run.)

There are 3 capped off pipes in total, 2 I think are cold, and one hot to the touch when the heatings been on for a bit.

Thanks again so far for all you responses.

 

[pbar]

Right, I've had my plumber round today and he's got rid of all the dead legs, cut them right back and put elbows where the T's were. But the problem remains, boiler comes on for a few seconds when the cold water is used.
Been on to Worcester tech support, they seem to think it happens now (and didn't before when the boiler was located elsewhere) because the boiler is not the first thing connected from the mains water supply. It used to be, but now it's in the middle somewhere.

So, he's going to fit a non return valve. I'm just worried that as it worked perfectly before it was moved without one, why does it suddenly need one to get it working properly now.
Unless (and perhaps someone could confirm) it's because it's not now the first thing connected to the mains water supply any more.

 

[Rob]

Have someone measure the pressure. Im sure a pressure reducing valve will solve this if all dead leags have been eliminated.