Flow rate drops off too much - unvented cylinder

[JDLDIY]

Hi there, I don't think there's a resolution to this problem, but just in case...

We had a Megaflo fitted, when I run the bath tap now, with both hot and cold running, initially the flow is great but then it backs off a lot and takes a long time to fill the bath. My theory is that the cold mains flow is going in to refill the Megaflo, so resulting in an overall reduction in flow rate at the tap.

Does this sound about right, is there any adjustment which might improve things or am I just stuck with it? I had hoped that having the Megaflo installed would result in good flow rates and pressure for both hot and cold.

Many thanks for any advice.

 

[holty]

how olds the cylinder,when was the filter last checked.is the stop tap fully open.was pressure and flow measured when installed.

 

[JDLDIY]

The cylinder is now 9 months old, where's the filter?

I'll check the stop tap but I'm sure it's fully open. Not sure whether pressure and flow were checked when it was installed but I remember the plumbers saying water supply was sufficient for it to be installed.

 

[muggles]

There is a filter on the mains feed to the cylinder, certainly if the problem has become worse over time then I'd say that's blocked. Get your installer back to clean it

 

[JDLDIY]

Thanks guys, we'll get that checked.

So, apart from that, would you expect the pressure/flow to drop off after a few seconds with both hot and cold running? Does my theory sound reasonable that the cold supply is now doing 3 things i.e. refilling the cylinder/pressurising the hot and running to the cold side of the tap?

 

[Agile]

Measure the flow rate at the bath hot tap and it should ideally be close to 20 litres per minute or more. Measure it initially and then after five minutes when the air bubble in the cylinder has been exhausted.

Either the connecting pipework is restricted or most likely the pressure and flow were NOT adequate when it was installed!

This is always a problem! If yoiu engage inadequately trained and experienced people then you end up getting a cowboy job.

Do you really know who installed the cylinder and are they qualified to fit unvented cylinders and was it notified to the Building control?

They are legally liable for the shortcomings in your installation. Get them back and find out who they are and where they live.

If the incoming supply was inadequate, I like 20 li/min dynamic with residual 1 Bar, they you can sue them to provide an upgraded water supply to the property. Thats usually about £800-£1400. Or Simon D will fit an accumulator for a bit more if you have enough space.

If only customers would choose properly qualified people then cowboys would be eliminated !

Tony

 

[holty]

yes basically you have the initial pressure and flow when static. but the flow soon depleats when theres a partial blockage.as it seems your cold should be balanced off the combo valve this also will be effected.

 

[bengasman]

Thanks guys, we'll get that checked.

So, apart from that, would you expect the pressure/flow to drop off after a few seconds with both hot and cold running? Does my theory sound reasonable that the cold supply is now doing 3 things i.e. refilling the cylinder/pressurising the hot and running to the cold side of the tap?

Unvented cylinders don't get emptied or filled other than at installation; the water goes through it, not in and out.

 

[Agile]

the water goes through it, not in and out.

I think that the water does go in and out!

In the inlet and out of the outlet !

 

[JDLDIY]

Unvented cylinders don't get emptied or filled other than at installation; the water goes through it, not in and out.

Sorry, I'm not with you there.

yes basically you have the initial pressure and flow when static. but the flow soon depleats when theres a partial blockage.as it seems your cold should be balanced off the combo valve this also will be effected.

Could you expand slightly on that please holty? What's a combo valve?

Measure the flow rate at the bath hot tap and it should ideally be close to 20 litres per minute or more. Measure it initially and then after five minutes when the air bubble in the cylinder has been exhausted.

Either the connecting pipework is restricted or most likely the pressure and flow were NOT adequate when it was installed!

This is always a problem! If yoiu engage inadequately trained and experienced people then you end up getting a cowboy job.

Do you really know who installed the cylinder and are they qualified to fit unvented cylinders and was it notified to the Building control?

They are legally liable for the shortcomings in your installation. Get them back and find out who they are and where they live.

If the incoming supply was inadequate, I like 20 li/min dynamic with residual 1 Bar, they you can sue them to provide an upgraded water supply to the property. Thats usually about £800-£1400. Or Simon D will fit an accumulator for a bit more if you have enough space.

If only customers would choose properly qualified people then cowboys would be eliminated !

Thanks Agile for your many points. I'll be checking the flow rate with a bucket as soon as I can - then I'll probably be back here for more advice! They weren't cowboys and I have the necessary certificates etc. Quite a big firm, good reputation locally, did a lot of plumbing for me.

 

[Agile]

I am glad that they were not cowboys and have a good reputation.

Perhaps something has gone wrong like the filter getting blocked. However, in that case it would have been better when first installed and although asked you have not replied about that.

We often see on this forum that bigger firms put less experienced staff on their jobs who dont always do such a good job as a working boss!

Tony

 

[holty]

Could you expand slightly on that please holty? What's a combo valve?

id love to, but iv not got the time nor energy to write an thesis, on the resistances of flow and pressure due to blockage,lets just keep it simple youve either got crap incoming pressure, or flow. or both, for an unvented. or the filters blocked.a combo valve is the combination(composit) valve which incorperates a series of controls including safety, and thats were the filter is.

 

[matt1e]

Unvented cylinders don't get emptied or filled other than at installation; the water goes through it, not in and out.

I think that the water does go in and out!

In the inlet and out of the outlet !

I think Agile is right and Bengasman is thinking of thermal stores Ala pulseacoils etc

 

[simond]

The apparent boost at the start of the drawoff is the compressed air in the top of the cylinder giving the water a small push. This is evident after a period of inactivity and is caused by the expansion of the water in the cylinder compressing the designed airgap.

The flow rate after this small boost will then revert to the natural flow rate of the water main you are connected to.

You could have a partial blockage somewhere but it is only 9 months old.

As Agile says, it is only too common to find an unvented cylinder connected to a relatively poor performing water main. As he also says there are ways of improving the supply, but none particularly cheap.

Your water supplier can vary your service at any time and also local use of sprinklers etc can make a real difference.

 

[fatplumber]

The apparent boost at the start of the drawoff is the compressed air in the top of the cylinder giving the water a small push. This is evident after a period of inactivity and is caused by the expansion of the water in the cylinder compressing the designed airgap.

This isn't the whole story. You may have excellent pressure but if you have an inadequate mains flow rate, then the air bubble will act as an accumulator. So, you can geat a few litres at a good flow rate but once the accumulated pressure is depleted the flow rate will drop to that of the incoming main.