Originally Posted by Doctor Evil

Gledhill have changed the specification over the years of the BoilerMate. Yours will have an internal DHW take-off coil. The new models have plate heat exchangers. Is that so? It appears the thermal store is struggling to cope after house extensions. All is not lost.

Q1. Does the store gives adequate showers in summer when the heating is not extracting heat from the store?

Q2. How did it perform, in heating and DHW, before the hosue extensions?

Q3. What percentage has the house increased in size?

Q4. Does the boiler heat the store via an internal coile, or does it heat the store direct (the same water in thye store and boiler)?

Q5. Is the CH taken off the store. Most Boilermates this is the case.

Firstly:

a) I would replace the boiler with a larger boiler of approx 30kW (twice the size of the extisting) with 28mm flow and return to the thermal store. This will give a far higher recovery rate and be heating up the store as water is being drawn-off.

b) Fit a blending valve on the boilers flow and return to ensure only 75-80C water enters the store.

c) De-scale the DHW coil, as this may degrade performance if scaled up. Ring Gledhill in Blackpool, they will recommend a solution to de-scale.

d) Fit a flow switch in the cold DHW feed to switch off the CH pump when DHW is called to prevent heat being extracted from the store to the rads when DHW is being called. The flow switch can energise a relay to cut out the pump. Simple to do.

If still not up to scratch, then do:

e) Have the flow switch also override the thermal store cylinder stat and bring the boiler in immedately DHW is called. You could put this on a 20.30,40 second time delay to prevent nuisance boiler cycling. This will pump heat into the store imediately, whereas if you rely on the cylinder stat you would have lost 1/4 to 1/3 of you hot water before the boiler starts to pump heat into the store.

Installing the above two points is not difficult or expensive and saves a wedge on a new store. This should cure the problem.

Further:
If the store is being heated adequately and the problem is that the internal DHW coil is not man enough produce the hot water, which I doubt from what you say, You can always retrofit a plate heat exchanger and pump for DHW take-off and use the existing DHW as a pre-heat, in essence turning it into a hybrid heat bank/thermal store.