Thermostatic shower with pumped hot and mains cold

[Tykey]

Looking for some advice please.

I'm ripping out our en suite to replace it, and I'm looking to replace the existing thermostatic shower with a new thermostatic shower tower. The cold feed is from the mains, and the hot will powered by a Stuart Turner Monsoon standard 3 bar single pump.

I understand that thermostatic showers need an equal hot and cold feed, so:

1) Is there a way to measure the pressure of the cold? Presumably this will vary slightly dependent on time of day, but I guess it's safe to say it's more than 3 bar?

2) Will a pressure reducing valve - such as this Honeywell one https://www.screwfix.com/p/honeywell-pressure-reducing-valve-x/7978j - do the trick of equalising the cold to the same as hot?


Another thought, the shower that I've just pulled out was also a thermostatic one, with normal feeds from the pumped hot and mains cold (i.e. no PRV). It has never worked since we moved in, as it was just cold. I always presumed it was just the cartridge that had gone, and never bothered trying to sort it as the plan was always to put a new one in. Could it have been the imbalance in pressure that stopped the old one working properly?

Thanks in advance for your suggestions.

 

[terryplumb]

You should take a cold feed from the same tank that supplies your hot water cylinder ,to a pump that will boost both hot and cold. Anything else mixing pumped hot and mains cold is asking for trouble. Do it once and do it properly.

 

[Tykey]

Thanks Terry.

So I'd essentially be looking at a standard pump for hot water servicing the bath and taps around the house, and then a twin pump dedicated entirely for the new shower. The existing pump is at the base of the cylinder, would the other need to go here too, or could it be closer to the shower? The airing cupboard doesn't have much space left!

Out of interest, why would anyone have put this shower in like this in the first place? It seems that there's no way to install a shower correctly with pumped hot and mains cold, so why would anyone have ever considered this set-up? Or did that used to be the norm but things have changed since?

 

[Tykey]

Also, I'm not trying to sound churlish, but what is it about the opening suggestion (fitting a pressure reducing valve) that would be asking for trouble? It's not as simple as ensuring that the incoming feed from hot and cold are both 3 bar?

 

[terryplumb]

Diyers do things in ignorance and some cowboys are no better. I for one am quite happy about both as they give me plenty of work putting things right ! The hot water cylinder should have a special flange fitted that is the take off point for the shower pump. Do you really need to pump hot water to bath and basins ?

 

[terryplumb]

Also, I'm not trying to sound churlish, but what is it about the opening suggestion (fitting a pressure reducing valve) that would be asking for trouble? It's not as simple as ensuring that the incoming feed from hot and cold are both 3 bar?

You dont know what the mains cold pressure actually is . lets say its two bar ,what do you think the prv would do ? Mains pressure is not a constant , everyone else is tapping into the supply to varying levels at any given time. Taking the hot and cold from the same storage tanks gaurantees the same head.

 

[terryplumb]

Dont get me wrong ,i have seen set ups that work reasonably well with pumped hot and mains cold ,but have seen a lot more that dont. Those that do is down to good luck rather than correct plumbing design.

 

[Tykey]

You dont know what the mains cold pressure actually is . lets say its two bar ,what do you think the prv would do ?

Thanks for the clarification, that makes sense. I guess if the mains pressure is 2 bar then the PRV wouldn't be doing anything and we'd still have the unequal pressure.

Do you really need to pump hot water to bath and basins ?

The current pump has died, and although we've been "making do" with the unpumped hot for the past few months, it takes an age to fill a washing-up bowl, let alone a bath. We've been waiting to replace the pump until we got around to re-doing the en suite, in the hope that we would be able to find a solution that serves both purposes - decent flow rate for the hot water around the house, and a good new shower.

 

[terryplumb]

You must have a pretty poor plumbing set up !! Why dont you get a professional plumber in to advise you. A pumped shower delivering say 15 litres per minute would use 150 litres in ten minutes , you certainly wouldnt be able to run a bath at the same time or shortly after , for instance, as you wouldnt have enough stored hot water.

 

[Hot&Cold]

Will a pressure reducing valve - such as this Honeywell one

doubt that is the intended Uk use for that valve but under circumstances with other safe guards it can work to an extent,why not as mentioned by terryplumb. Fit a cold tank supply to a twin impeller pump.

Or if your hot water cylinder is getting on in years and you have decent mains water pressure & flow to your property have an unvented cylinder fitted

its not worth skimping on your property's plumbing