How to get a thermostatic shower from a gravity copper tank?

bobfredbob

Can someone explain the options?  I was brought up on a simple electric shower.

My house has a system boiler with an old copper hot water tank (approx 120 litres), and cold water storage tank is probably around 220 litres.  The upstairs bathroom has no shower at all.  The downstairs shower was an 8kw electric shower, but that is on its way out.  We'd like to refit upstairs to a be a shower only and get a downstairs thermostatic shower.  We'd probably only use one shower at a time.

Downstairs, we've been using the non-thermostatic bath mixer tap for the shower.  But we have problems getting a stable temperature and we have young children and soon to have an elderly relative staying.  It reaches a stable temperature for several minutes, but then it will suddenly get scalding hot even with no other hot water drawn.  Perhaps water mixing in the tank after a couple of minutes means hotter water is flowing?

So, I thought a thermostatic mixer.  But I have trouble finding a good low pressure thermostatic bath mixer taps.  Most have very poor reviews or refuse to give the litres per minute.  I saw some shower-only mixer bars that deliver 3lpm, which is even worse than my electric shower.

Do all low pressure thermostatic mixers have poor flow rates due to their mixing?

For a little more, there are a number of power showers.  But, I think I'd need dedicated pipework from the water tank to both of the bathrooms?  So the plumber couldn't just hook in to the existing bath plumbing?

I had two people in to quote for an unvented cylinder.  Both said it was difficult to do the discharge piping.  Neither supplied a quote.  We get around 25lpm I think.

If we had the cylinder replaced with an unvented cylinder under the stairs (where it might be easy to run pipework under the floorboards), would we hear it filling up or other noises?

However, we're not sure we like the idea of having to get it regularly serviced.

In the past, I'd be tempted to just fit a combi and get high pressure everywhere.  But, with the government all confused over gas and on-demand combi-boilers/electric showers and wanting to "encourage" a reduction in gas, it seems we should perhaps stick with a tank instead of fitting an electric shower upstairs and downstairs.  But, I'm open to ideas and any experiences.

Leon_W

I had a shower upstairs fed from a gravity tank. It was useless so fitted one of these shower pumps...... https://www.screwfix.com/p/salamander-pumps-ct50-xtra-regenerative-twin-shower-pump-1-5bar/20423

Really good now !

grumbler

bobfredbob said:

Do all low pressure thermostatic mixers have poor flow rates due to their mixing?

In my bathroom I have an external thermostatic mixer valve that feeds a bidet shower and the hot water pipe of my basin mixer tap. So, in the tap I can easily make water colder, but not hotter than the set temperature. The flow is the same if I open hot water only (through the thermostatic valve) or cold water only (no valve). And it's the same gravity system as yours.

For a shower you may need higher flow than for a basin - there are bigger (22mm) external thermostatic mixer valves for this capable of serving, e.g. a bath.

FreeBear

I fitted a Triton T80 thermostatic shower - Not the cheapest on the market, but I find it excellent for temperature control. Worth bearing in mind if you decide to go for another electric shower.

With a gravity fed HW system, a pump is essential if you want a decent shower. Couple it with a thermostatic mixer, and you should be good to go.

grumbler

FreeBear said:

With a gravity fed HW system, a pump is essential if you want a decent shower.

Possibly not that essential for a downstairs shower.

However, the cold water has to be from the loft tank, not mains, to avoid problems because of the pressure difference.

BUFF

I imagine that your bath probably has 22mm pipe work to it?
I believe that what grumbler is suggesting is something like
 https://www.pegleryorkshire.co.uk/EN/Brands/Pegler/Thermostatic_Mixing_Valves/Thermostatic_Mixing_Valves/product/PEG402_Thermostatic_Mixing_Valve (currently available on ebay for under £20 delivered). Flows ~12lpm at 0.5bar/~16ft head).

nofoollikeold

1. With a tank based system the pressure at any outlet is given as the vertical distance between the base of the cold water storage cistern and the outlet. 1 metre of height is roughly 0.1 bar of pressure.  
2. Thus for the average two storey property with a CWSC in the loft, the pressure at a ground floor bathroom shower head is around (0.02 + 2.5 + .02 + 0.25) = 0.28 (bar).  Where .02 is the thickness of the ceilings, 2.5 an average room height and 0.25 the distance below the ceiling of a shower head.  
3. There are very few (if any) thermostatic showers capable of giving a decent shower at such low pressures.
4. An alternative you might consider is a shower pump situated in the airing cupboard by the foot of the hot water cylinder (HWC).  This would need:
4.1 A dedicated feed from the CWSC.
4.2 A dedicated feed from the HWC.
4.3 Possibly dedicated hot and cold feeds from the pump to the shower(s).  However, depending on your configuration you might be able to use a "universal" pump to pressurise all the existing hot and cold tank fed outlets.  

bobfredbob

Thanks everyone.

I've looked at the Salamander pump specs and took another look at our hot water tank.

We have what I guess is an Essex flange and some other pipe from the loft with a tap draw-off on it and then that pipe continues to who knows where under the floorboards!  Both taps connect to nothing.  So, perhaps there was a pump there before.  Possibly a whole house pump.  Maybe that pipe is the sole draw-off of cold water for the house so a pump is easy to fit in the airing cupboard.

A whole house pump seems a little expensive to buy.  I guess you just get used to the pump noise.  The bathroom-only pump is cheaper, and I guess you don't hear the noise over the shower noise.

The Triton T80 looks good for the downstairs bathroom and should hopefully fit in place of our existing Triton.

I'm assuming it's not cost effective to replace the copper tank with another vented tank, even though new tanks are more insulated.

The external thermostatic mixer valve looks interesting.  I guess for downstairs it would be around 4m head/0.4bar, so that's around 12lpm with that valve, upstairs would be around 0.1bar 5lpm.

Since washing machines are cold-fill only nowadays, I wonder if any money would be saved if it's fitted to supply warmer water to the machine?

grumbler

nofoollikeold said:

2. Thus for the average two storey property with a CWSC in the loft, the pressure at a ground floor bathroom shower head is around (0.02 + 2.5 + .02 + 0.25) = 0.28 (bar).  Where .02 is the thickness of the ceilings, 2.5 an average room height and 0.25 the distance below the ceiling of a shower head.  

As you add 0.02 (!) for the thickness, you have to add at least 0.1 for the loft joists and at least 0.15 for the room joists. Also in my loft the tank is about 0.5m high from the loft floor and has at least 0.5m of water in it, that makes extra 1.0.

And my shower head is 40cm below the ceiling. I know, this is nitpicking, but only because of your 0.02.

chrisw

So called digital showers such as those from Mira have a built in pump for non-combi systems. They are really good but a bit pricey, around £500.

grumbler

bobfredbob said:

Since washing machines are cold-fill only nowadays, I wonder if any money would be saved if it's fitted to supply warmer water to the machine?

What fitted? Warm water instead of cold? You are unlikely to save anything because most water is used for rinsing.