Pressure regulation of UVC PRV

[ProDave]

So not long back I fitted the Telford 300L heat pump col indirect HW tank. It comes with a pressure reducing valve. There is a cold water tap off point so the hot and the cold taps all get the same pressure regulated feed.

 

Worth pointing out our mains pressure is in excess of 6 bar and has a fantastic flow rate.

 

I have had a nagging doubt since installing it that the flow rates are not brilliant.  Showers are okay at about 14L per minute, not fantastic but adequate.  It's the bath flow that may prove an issue as that is not a lot higher.  That may be the fault of the floor standing bath filler that cam with long inbuilt 1/2" flexis so that is bound to limit flow a bit.

 

But now cut to the chase and get to the point of the thread.  Today I borrowed one of the pressure gauges from one of the UFH manifolds and hooked it up to a pressure regulated cold water feed.

 

Sure enough, it measures a static pressure of 3 bar as it is set to.

 

But turn on say a shower (remember 14L per minute) and the pressure gauge drops to 2 bar.

 

To me, it seems like the PRV is doing a poor job. With our high mains pressure and flow, that will not be the reason for the drop. It would appear that at 14L per minute the PRV can only maintain 2 bar.

 

This is measured right at the PRV so it is not pipework causing the presure drop, and even if I just run a hot tap, so no flow through the cold feed where the gauge is, the pressure still drops.

 

Before I discuss this with Telford technical, I would be interested if anyone has done a pressure measurement and what sort of pressure do you thing it should maintain? I would expect a drop but not down to 2 bar.

[PeterW]

 They are a Caleffi unit and you can change the flow regulator. 

 

I would check you’re getting 2 bar dynamic  water pressure prior to the PRV first as it could be a 6 bar static until you get a flow. 

[ProDave]

Just moved the pressure gauge. Mains pressure is 6.5 bar and you can't even see the pointer move at all when you run a bath or shower, so mains pressure or flow is not the problem.

[PeterW]

Looking at the graph the pressure drop at 15 litres /min is about 0.5 bar at most. 

 

They can be adjusted upwards so you can set it to 3.5 bar if you are losing pressure downstream. 

[ProDave]

So is mine faulty? should I be asking for a replacement?

[PeterW]

Not necessarily - they are factory set for an average use at 3bar or 300kpa at 10lpm which is fine usually. 

 

Standard fluid dynamics  says that if you increase the flow you decrease the pressure so you need to decrease the pressure restriction. 

 

Graph below shows that for 14lpm you would need to decrease the restrictor by approx 0.45bar or 45kpa

 

 

 

[dpmiller]

check the strainer for crap.

[Nickfromwales]

I mentioned a job in a very expensive property, where their upstairs UVC turned into an indoor water feature due to incorrect installation. When I got there I put my pressure gauge on the outside tap and it went off the scale and destroyed my gauge in an instant. Over 10 bar I recon, deffo not less than 9 thats for sure, coming in on a 50mm supply. You could have run a hotel off it. 

Cause was;

No balanced supply to the cold side of the mixer outlets, so UVC was getting back-fed <10bar whenever the mixers were opened slightly. Result, EV ruptured internally, then slowly rusted out, and then the EV burst catastrophically. 

When I corrected the issue by putting a second ( primary ) PRedV at the stopcock, all was well. I then got a phone call from the guys wife saying the showers were miserable compared to how they we're before ( no bloody surprise there luv !!! ). Same issue you have, made worse when a couple of outlets were opened simultaneously. 

Cure;

I opened the primary PRedV up by another .5 bar ( from 3 to 3.5 ) and that increased the dynamic flow considerably. I explained to them that the wonderful pressure and flow they had been "enjoying" is what caused the system to fail and the house to be flooded, so they then accepted the lesser but still adequate flow rates. 

Try adjusting by 0.5 bar and see how that affects things, but don't forget you need to adjust it with a balanced hot or cold outlet flowing slowly to get an accurate reading. With the outlet flowing, adjust slightly, close the outlet and let the pressure settle. Repeat until you get it spot on.

Once happy, adjust the EV pre-charge pressure to suit the new inlet pressure. 

 

1 hour ago, dpmiller said:

check the strainer for crap.

Always a good place to start.   

[ProDave]

I am sure there is no crap in the strainer. Before the tank was fitted I had all the hot and cold pipes plumbed, and a temporary link, done with compression fittings from cold feed in, to hot and balanced cold out. That was well flushed through to get all the flux residue and other crap out of the system.

 

When the tank arrived I simply removed the temporary link pipe, and connected the cold in to the PRV in with compression fittings, so no chance of crap getting in.

 

I am not bothered about the flow rate particularly, it's just when I checked and saw the pressure drop so much, I thought that can't be right.  We had pretty much the same setup at the last house so same PRV and I recall the baths filled a lot quicker, and one of those was plumbed in 15mm which didn't seem to restrict it much.

 

@PeterW you talk about "decrease the restrictor by 0.45 bar"  What is this resrictor you talk of?  This is a pressure regulating valve that works by feedback and should (within limits) maintain a fixed pressure at the output. Are you saying as well as the pressure adjustment there is a restrictor?

[Nickfromwales]

I think has saying what I said, but more technically put. Open the restrictor ( spring ) turnscrew by 0.5 bar, found in the centre of the cartridge top. You may have to remove the dust cap to do so as I dont know exactly which one you have.  

[ProDave]

So you are saying adjust the static pressure to 3.5 bar to get the dynamic pressure up to 2.5 bar.

 

Won't that have implications for the warranty?

 

Question still remains, if it should drop by 0.5 bar at 14L / min but is actually dropping by 1 bar isn't it "faulty"?

[PeterW]

Are you getting an actual measured 14 lpm..?? Look at the graph and the 3/4 has a reasonably shallow curve but would still have a perceived increase in pressure drop from the valve. As they are spring driven, the preload on the spring will vary the actual response so your “dynamic” pressure is the operating pressure at the coil. These tanks have 6bar fail at the control group, 7 bar at the combined overheat pressure relief, and will comfortably take 10 bar without too much trouble as long as it’s not permanent. 

[ProDave]

So here is what I have done.

 

I tried setting the PRV so that the dynamic pressure with the bath taps on full, was 3 bar. That resulted in the static pressure rising to 3.8 bar with the taps all off.  So I have compromised and set the static pressure to 3.5 bar and that gives a dynamic pressure of 2.6 bar with the bath taps on.

 

This has given a noticable improvement to the bath fill speed but strangely made hardly any difference to the shower flow which is still at 14 litres per minute.  The shower flow is measured with the sceintific method of timing how long the shower takes to fill a bucket of known capacity.

[PeterW]

Shower will be limited by the head - have you tried just the hose ..??

[ProDave]

Happy with the shower flow. Don't want the ladies emptying the tank too quickly

[joe90]

Very interesting this thread, just started using my showers and a little disappointed in flow/ pressure so will play with mine and see what we have.

[Temp]

Bit late to this thread but …

 

We have several small reduction valves on basin taps to reduce splashing. They were all fine when first installed but all started to reduce the flow rate after a year of so. Adjusting them sorted the problem. Not sure if they just age a bit or what. We don't have a lot of scale.

[Nickfromwales]

9 hours ago, Temp said:

We have several small reduction valves on basin taps to reduce splashing

Flow restrictors or actual inline valves?

 

These are what I normally fit, available with a choice of litres per minute restriction 3 / 5 / 7 / 9 etc

[ProDave]

11 minutes ago, Nickfromwales said:

Flow restrictors or actual inline valves?

 

These are what I normally fit, available with a choice of litres per minute restriction 3 / 5 / 7 / 9 etc

Would those fit into the end of a quarter turn isolator?  I could do with restricting the basin flow so it does not overflow on full tilt.

[Nickfromwales]

Typical install is;

Incoming 15mm copper / other supply pipe.

Then 15mm 1/4 turn isolation ( full bore will be the quietest but a normal ballofix will usually suffice ).

Then out of that with a short section of 15mm copper.

Then into a 15mm x 1/2" chrome copper to iron fitting.

Then your chosen ( manufacturer supplied ) flexi goes onto the last, rather than straight onto the ballofix which some do but its wrong ( sharp cone fitting as opposed to the flat face of the iron, therefore no damage to the flexi rubber seal ). 

 

The insert flow restrictor goes into the 15mm side of the iron fitting, as the ballofixes simply dont have the depth after the olive.

[PeterW]

A lot of taps have the ability to change the flow restrictors. 

 

I’ve fitted some of these recently - come complete with the flexis and have a pair of brass studs rather than the one on a normal mixer tap. 

 

They have replaceable restrictors on the tap nozzle itself - the ones I’ve fitted are 7lpm. 

[Nickfromwales]

Here you go Dave

4 Lpm will have the girls out of the shower in no time at all  

 

[Temp]

6 hours ago, Nickfromwales said:

Flow restrictors or actual inline valves?

 

They are Inline pressure reducers set to 3 bar at the factory.  Something like this without the gauge...

 

https://www.screwfix.com/p/prv-with-gauge-x/41943

Edited June 2, 2018 by Temp