cold mains accumulator - any experiences?

Have the plumbers in on Monday do do a system overhaul (including plinth heaters) so doing some long overdue testing.

Combi boiler is Worcester Bosch Greenstar CDi Classic 38CDi. The manual says it will manage a 30C rise in water temperature at 19.1l/min maximum flow rate. [Note to self - measure incoming water temperature! Interesting - the test bucket I just filled from the outdoor tap is showing 16.1C!] So depending on incoming water temperature this should provide a good flow of hot water to one shower, and perhaps adequate to two showers.

Cold water flow:

(1) Outside tap (near mains stop tap but with pressure limiter closer to mains)

(2) Downstairs mixer shower cold as possible (scale only goes down to 20C) fills 12l bucket in approx 1 min 10 seconds or 10.23 l/min.

Hot as possible (scale shows 48C as highest, but control turns past that) fills the 12l bucket in 1 min 39 seconds or 7.28 l/min.

So some flow loss there on the route through the combi. [Or in the mixer thermostat.] Temperature (including initial cold flow) is 43C.

(3) Upstairs bath mixer tap and shower diverter cold as possible (below

(4) Now the crucial one - upstairs bath/shower mixer hot only. mixer goes beyond 55C.

Well, there is obviously a problem here compared to the rest of the system. This might be due to a dodgy isolator valve (long suspected, never got the energy to drain the system then check and replace.

However with a ground floor cold flow of just under 30l/min I would expect a better performance from both shower mixers. I would like to be able to run the combi flat out heating 19.1l/min water in the summer.

I will dig up the details of the shower mixers, but they should be full mains (high) pressure mixers so I wouldn't expect them to constrain the flow that much. [Again, they may have been sized for a maximum flow of around 10l/min so the boiler could service both at once. So long ago!]

There is a water softener so I have to factor in the maximum flow rate through that (another possible bottleneck) and possibly test with it bypassed but I do seem to be losing an awful lot of water flow from

Summary: I have one hard fault in the upstairs hot water flow to the bath. I also have doubts about the flow rates to the other shower. However in a previous property we had the same model of bath/shower mixer with a smaller WB combi and that filled the bath reasonably quickly with hot water which is why we went with it again.

We shall see what we shall see on Monday when the plumbers come.

This is being driven because we want to upgrade the upstairs shower because climbing in and out of the bath is a pain and also more difficult as we get older. Also we never have a bath because it takes too long to fill!

Cheers

Dave R

An accumulator works when you have adequate mains pressure, but poor flow rate (due to narrow / restricted pipes or excessive length of supply pipe). Normally you would get a decent static pressure reading, but then the pressure rapidly drops when the water is flowing. So you get an inadequate rate of delivery.

An accumulator is a tank with s (air) spring. This fills from the mains at whatever rate it can - stopping either when full on when the stored pressure matches that incoming mains pressure.

When you open a tap, water will be pushed by the incoming mains pressure

- but also the "spring" pressure from the gas spring in the accumulator. You could draw water from the accumulator even with the mains feed to it turned off, up until the point there is no more energy stored in the spring

You will get some additional "head" due to the height. (and also longer recharge times for the same reason)

and that.

I believe there are some accumulators that do feature pumps however I have not looked into the detail, and in what situations it would be considered ok to pump from the main.