Unvented - how many showers/performance?

[fezster]

I have a megaflo unvented cylinder which feeds 3 bathrooms. In particular, it feeds 3 showers which have pura thermostatic valves:

http://www.purabathrooms.co.uk/catalog/index.php?route=product/product/datasheet&product_id=24158

The MI state the valve uses 32 litres/min at 3 bar. Two of my showers have 200mm heads and the other has a 250mm head.

Individually, these showers perform wonderfully. Pressure and flow is great. However, the first of these showers (closest to the cylinder, with the larger 250mm head) definitely has a noticeable drop in performance if either of the others are turned on. It's still adequate, but given my incoming mains is 32mm MDPE and over 60 litres/min flow, I'm surprised they don't all work without a drop in performance together.

All the showers are fed using a mixture of 22mm polyplumb and copper.

Are my expectations too high? Is it the pipe route which is the problem? Trying to get my head around what (if any) solution there could be in the future if I ever undertake any work.

Thanks.

 

[fezster]

So the reducing valve was pretty clean.

When he did the pressure test on the outside tap (he had the same gauge as me), initally he got the same 2 bar reading. He noticed when the thread on the gauge's connector was fully tightened onto the tap, the bit inside the connector wasnt snug against the tap (if that makes sense). So he wrapped some PTFE tape around it to get a snug connection. Then the pressure read 2.5 bar, and slowly crept up after about a minute to just under 3 bar. So I guess it's ok.

Is the gauge quite sensitive in that way to being tightly up against the tap (even though no water is escaping). Never used one before so didn't realise it made a difference.

 

[mutley racers]

If the gauge is creeping up and it is an outside tap then there will be a double check valve in it so you won't be getting the correct dynamic pressure I believe

 

[mutley racers]

It is really best to do the pressure test from a washing machine connection and open the kitchen tap

 

[Staffy85]

whats the pressure before the PRV with nothing running? and then what does this pressure drop down to with the shower running?

 

[fezster]

Thanks mutley, I think you are right about the double check valve (I read that elsewhere also).

Staffy85 - pressure on outside tap before the PRV is 4 bar, and drops 0.5 bar if the kitchen tap (also before the PRV) is switched on.

 

[PhoenixServices]

You want readings from outlets after the prv

 

[Howsie]

You have standing and dynamic pressure.

 

[fezster]

Done some measurements. It's not as easy as it looks. Ended up getting drenched!

Measured 3 outlets at a time, and repeated the test twice. By far the highest output is from the outside tap. This is also the hardest to measure using a 1.5 litre jug!

Results:

First test:
Outside tap - 1.2 litres in 2.9s.
Utility - 1 litre in 9s.
Ensuite shower - 1 litre in 13s.

I make this to be 30 litres/min.

Second test:
Outside tap - 1.4 litres in 3s.
Utility - 1 litre in 13s.
Kitchen - 1 litre in 10s.

which gives 38 litres/min.

I then measured just the outside tap in a bucket, and got 9.4 litres in 18s, which is 31 l/m.

I think overall it's probably about 40 - 42 l/m.

This points to my overall flow not being enough to drive 2 showers according to the shower spec. However, I also measured just the shower on it's own, and got 3.8 litres in 18s which is only 13 l/m. Nowhere near the 32 l/m the instructions state.

With the outside tap running, the shower gives me 1.4 litres in 11.5s which is 7.3 l/m. This felt similar to when I have a second shower running.

Not sure what to make of all of this. Is the pipesize to the showers the restriction? Or is the mains the restriction?

Not sure if I mentioned before, and Im wondering whether it's pressure rather than flow which is the problem - but I measured pressure at 4 bar static at outside tap, and drops to 3.5 bar dynamic with another outlet open. This is bypassing my 3 bar PRV. I tried to measure pressure on shower, but Im not sure it's entirely accurate because of the attachment I had to use. It read less than 3 bar static. There's approximately 15m of 22mm pipework to the first shower.

I have a thread on diynot on this also, in case anyone experiences deja vu.

 

[rpm]

I have a thread on diynot on this also, in case anyone experiences deja vu.

Go wash your mouth out with soap!

 

[fezster]

Go wash your mouth out with soap!

Sorry if that offends! Getting desperate with frustration. Want to work out what is going on here.

 

[fezster]

Yet more tests today. But here's the most useful ones (I think):

1)
Upstairs bath hot and cold - 20.6 l/m
Just hot - 15 l/m
Just cold - 15 l/m

2)
Bath taps + sink mixer tap + ensuite mixer tap - 29.6 l/m

This tells me the upstairs is getting up to 30 l/m. Yet the bath tap with hot and cold only delivers 20.6 l/m. It's a mixer tap but with individual hot and cold controls. The sink taps are mixer taps with a single control.

I also took the heads off the showers and measured 18 l/m in ensuite and 21 l/m in bathroom individually. Combined I measured 27.5 l/m.

This is driving me mad! Could my PRV be faulty and only allowing a max of 30 l/m. Where's my other 10 l/m going??