Where should a NRV go? Combi starts when cold taps run.

[diyjunky]

I too have this problem. New Glowworm fitted in a recently re-developed property with a water meter about 30ft away in the street. I have no idea if there are dead legs but I there could well be as there apparently used to be a immersion tank and also wash basin in a bedroom.
Trawling round the web the consensus of opinion seems to be to fit a mini expansion vessel and a non return valve on the cold water inlet to the boiler.
Agile (aka Tony) above has suggested that the NRV should be fitted in the DHW outlet. A few others have also suggested this. Fitting it on the outlet would be so much more easy due to the configuration of the pipes under the boiler.

I was wondering why there was a split in opinion as to where the NRV should go ?
Any comments would be greatly appreciated as this issue seems to be very common.

 

[Madrab]

I too have this problem. New Glowworm fitted in a recently re-developed property with a water meter about 30ft away in the street. I have no idea if there are dead legs but I there could well be as there apparently used to be a immersion tank and also wash basin in a bedroom.
Trawling round the web the consensus of opinion seems to be to fit a mini expansion vessel and a non return valve on the cold water inlet to the boiler.
Agile (aka Tony) above has suggested that the NRV should be fitted in the DHW outlet. A few others have also suggested this. Fitting it on the outlet would be so much more easy due to the configuration of the pipes under the boiler.

I was wondering why there was a split in opinion as to where the NRV should go ?
Any comments would be greatly appreciated as this issue seems to be very common.

Please start your own thread, re-using old threads can make it harder to help as it can confuse things.

Typically tho, the reason for a boiler winding up is it thinks there's a demand for HW by the internal sensor registering a movement in the cold water supply when a close by tap is opened and closed. The NRV of the cold inlet would help avoid that. Also the mains water supply should be checked to ensure the pressure isn't too high.

 

[diyjunky]

What about putting a NRV on the hot water outlet of the combi as Tony suggested, would that also solve it ?
(it would be very much easier plumbing)

Thank you.

 

[Madrab]

Maybe not, depends on what's setting the sensor off. If it's the actual water movement then yes it probably would but if it's the shock wave from the cold valve closing travelling up the pipe to the flow sensor in the boiler then probably not.

 

[diyjunky]

You make a good point, thanks.
I suppose I could fit it on the hot water outlet and just see if it does the job

Edit: Just thinking about it more, fitting on the input would solve BOTH scenarios - is that correct ?

 

[Madrab]

Just thinking about it more, fitting on the input would solve BOTH scenarios - is that correct ?

Yup

 

[oldbutnotdead]

You might want to look at an expansion vessel on the DHW if you are fitting a non return valve on the supply. Better by far if you can track down and disconnect the inevitable dead legs full of air typical of most gravity to combi conversions.

 

[Johntheo5]

Some manufacturers specify a EV on the mains to the combi to protect the PHEX especially the ones with preheat.
Dead legs full of air are often quoted as the cause of the above problem, ironically one could say that a EV is a lovely dead leg full of air but as its installed before the combi probably absorbs the shock/flow.

 

[diyjunky]

I read a article about water meters and their inherent non return valve. It seems that the distance between the boiler and the meter is significant. A longer run will allow heat to be absorbed and therefore not cause too much expansion thus not damaging the hydro block in the boiler. A short run, say in a flat with the meter outside the residence, can cause an issue. This is where the small expansion vessel 'thingy' would be required on the cold water inlet to the boiler. If a non return valve was fitted close to the boiler, water expansion would occur as the pipe would be warmer due to the shorter distance. This is where an expansion vessel would be needed as a compensation for the short run of pipe.

Summary as I understand it:

1. The expansion vessel is a compensation for the non return valve that's within the water meter. You might need this.

2. Fitting a new non return valve on the cold water inlet will stop the water bounce and thus the false combi boiler start up. This will defiantly require a mini expansion vessel as a precaution.

The above issues are separate problems and yet related by the expansion vessel.

I hope this is useful (and correct)