ideas to tackle poor pressure, combi boiler

[Andrew]

Hi all,

I have a maisonette flat, 3rd and 4th floor of 4 storey block. Pressure is very low, sometimes not enough to produce any hot water at all in the bathroom at peak shower time 7:30am. The shower has become unusable.

I have a combi boiler downstairs, a 7.5Kw electric shower off the mains upstairs (selected for its low min pressure requirement) a cold water storage tank behind the shower (base of tank just below that of the electric shower head) supplying bath and toilet taps + cistern.

Several people have experienced similar issues in the neighbourhood but still no action from Thames Water. Apart from chasing TW and the council to add a pump to the block (they are the landlord) I am running out of ideas on how to solve this from within...

The electric shower keeps cutting out now on the thermostat as pressure is low. The shower attachment on the hot and cold mixer of the bath gives low flow if hot open only. As soon as the cold is opened too, the flow drops right down.

I understand it's not possible to pump from mains. There is no hot water storage tank. Some ideas I had:

1- add a new hot water tank upstairs and an electric thermostat on a separate timer to have a shower hot water storage, add pump and therm shower 2- pump from the cold water tank into the combi boiler instead of feeding combi boiler from the mains, use existing shower attachment to test. Add therm shower later if successful. If necessary, add another cold tank under the existing one (there's some room where old hot cylinder used to be) 3- replace combi with standard boiler and reinstate hot water tank upstairs, add pump and thermostatic shower (worst case!)

With any of these pumped solutions the worry is that the tank runs dry. The mains can be so low in pressure that it could take 1 hour to refill the tank. I need to be able to provide a shower for up to 4 people.

I'd be grateful for any thoughts on these, or any other suggestions. #2 is the favourite at the moment, if possible. It seems like this is an increasingly prevalent problem in London.

thanks, Andrew

[Ed Sirett]

Indeed it is.

#4 If the pressure is good at most times of the day but not when you need it and you have the space then an accumulator (giant potable expansion vessel) is T'd to the incoming main and a non-return valve is added to the main supply upstream of the accumulator. Size the accumulator to be at least twice the water usage during the peak period, set it's charge presssure to the lowest pressure that still works the DHW.

There is an increasing use of potable grade negative head boost pumps, usually coupled with a potable grade storage cistern, usually installed at low altitude (so they have a good refill rate).

A reasonable pump will probably set you back £400+

[Andrew]

I'm a bit out of my depth with a couple of the terms you mentioned - negative head, potable grade (it's not for drinking water, just for shower so have I misunderstood?)

I will not be doing the work myself but want to understand some possible solutions first. My experience of plumbers around London is that they are not interested in problem solving/investigation jobs.

I have chatted through the problem with a plumber already but no mention of accumulators. Should I be requesting one? Or find another plumber that suggests one himself...?

thanks a lot, Andrew

[Ed Sirett]

here:

It's connected to the mains, so it's potable end of story. Also the potable grade units are corrosion proof.

A negative head pump is one that attempts to maintain a certain outlet pressure (it does by having a small expansion vessel, a non return valve and a pressure sensitive switch). Most boost pumps are positive head (because they are cheaper) and switch on when a flow of water is detected.

Well what was the alternative solution to the problem?

[Andrew]

discussed on the phone was just to pump from the existing cold water storage tank upstairs into the boiler, presumably using what I now think to be a positive head pump. I understood the mains input to the boiler would be removed - is that actually possible?

I have a site visit lined up for Thursday so maybe this will be elaborated. Having spoken to a plumbers merchant, they advise not contemplating pumping into a combi unless the tank is a minimum of 50 gallons. I believe mine is 50. I'm still concerned about that running out, burning out the pump and pumping air into the system. (3 adults wanting showers and pitiful refill rate when the pressure is right down)