A small 2-storey house has a typical gravity-fed shower with the H & C header tanks not very far above the shower head - so fairly poor pressure/flow. I'm wondering whether to fit something like an ST Monsoon twin pump to boost both H & C to the shower and bath, or whether to convert everywhere to mains C and only pump the H to the shower and bath. What do the local experts advise?
Check to see what pressure differential will work with the mixer as some don't like imbalance.
Then fit the pump to both.
It's a simple non-thermostatic mixer so providing the pressures are in the same region it will "work". My guess (not yet measured) is that the mains C is 2-3 bar and pumped DHW would be around 2 bar so is that not balanced enough? Why do you recommend pumping H&C? In my own place I've changed to mains H&C everywhere and it's good to have non-stored water available in the bathrooms, hence the reason to consider mains C and pumped H in the other place (changing to mains H would be too much of a pain).
It will be far easier to get a decent balance if the hot and cold supplies start at the same pressure, and are boosted by equal amounts. I would fit a twin pump to the existing feeds.
Mains pressure cold + gravity Hot water + venturi mixer = good shower
+1
Because you then have control over what's going on. If you provide an equal boost to both sides of a balanced gravity supply, you'll still have a balanced supply.
With mains cold and pumped hot, there's no guarantee that they *will* be balanced. Having equal(ish) static pressures is by no means the whole story. A lot depends on how the pressure of each side changes with flow rate. If the cold water comes through a long thin pipe, you may have nothing like the measured static pressure once water starts to flow. It will also be dependent on what's happening elsewhere in the house. If someone opens another tap or flushes a toilet, the cold flow to the shower may reduce drastically - causing the mixture to get uncomfortably hot.
That sounds interesting - tell me more.
Nice in theory but few & far between and IME can just not work and no way of finding out why not.
Don't. Don't even think of it. BTDTGTTS :-(
Apart from what others have said, with only the hot side pumped when you turn the shower down cool the flow switch on the pump can drop out so you suddenly have un-pumped hot. And a negative head pump (which is expensive anyway) doesn't get you out of that jam: when the flow's too low to keep the pump running and you're relying on the pressure switch to keep the pump running the hysteresis on the switch does no favours for your unfortunate shower victim. (Last time I had to do one with mains C and pumped H I ended up putting a flow switch on the cold side to bring the pump on, but that's a kids don't try this at home, warranty void hack.)
Others have indicated why this is probably not a good idea (for the shower anyway), I'd agree with them.
No reason why you can have both mains and stored CW supplies in the bathroom.
In the current bathroom refurb, I'm going to pump HW and stored CW to the shower and bath (and maybe hot to hand basin for a bit less plumbing). The handbasin will have mains CW. Of course much depends on the plumbing arrangements. In my case the mains comes up through the bathroom. (the hard bit was getting the new cold feed for the pump from the cold cistern as the run is not very easy is this old house). In a typical house though the bathroom is under the loft, and the mians is up there to feed the CW cistern.
The basin should have mains cw, where are you going to brush your teeth if not?
I bow to your greater experience John!
I think they rely on a relatively high temperature hot feed that therefore needs a fair degree of cooling and so a good flow of cold water to work the venturi. I would imagine they don't cope too well with air locks on the hot side. I've not had one in my domestic plumbing experience but was relaying positive comments I had read elsewhere on another forum from people that I know personally. I've just changed from gravity DHW to mains pressure DHW but still retained heat storage at gravitational pressures. Saves all the regulatory issues and works well. Possibly more than the OP want to do though.
Yes, I need to keep this shower improvement quick and simple. In my own place I nearly went for a thermal store but eventually decided on a megaflo - very pleased with it.
Thanks John - that's a very useful reply and is enough to put me off the idea.
All the houses I have lived in (in NZ) do not have header tanks. The hot wa ter cylinder is around the same level as the shower. There's a pressure red uction (Ajax) valve to reduce the pressure to the tank, which is copper. Th ere's another Ajax valve at the top of the tank and a vent pipe outside to allow for the expansion of water, or there's a pipe going up a few metres t hrough the roof. This system gives adequate shower pressure and is very cheap and reliable. Why would anyone have anything different?
History.
The same reason emerging economies have moved straight to mobile phones and high speed wifi networks rather than roll out a copper network. Much of the UK still has the equivalent of "dial-up" speed water delivery and there isn't the capacity to provide everyone on the pipe network with high flow, high pressure water.
In such a system, storage tanks make sense.
Tim
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