thermal store with mains fed hot water?

7 Aug 2009
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United Kingdom
I've got a problem with a thermal storage set up

basically the system is a thermal store heated by a gas boiler also running the central heating. The thermal store is fitted with a mains fed heat exchanger coil which on exiting the thermal store goes through a mixing valve to a preset temperature of about 50°C

The boiler is heating the thermal store to 80°C and shutting off, but when you open the hot tap the water starts coming out fairly fast and hot but after a sink full it slows down to a trickle and goes cold.
The temp gauge on the thermal store does drop to about 76°C.

I've tried adjusting the mixing valve and it started leaking, so i've renewed the sealing O ring inside but there still seems to be very little hot water available at the taps.

The system would happily fill a bath and do a sink full of washing up on a single heating of the thermal store when it was installed about 4 years ago.

Anyone have any idea. Is it worth changing the mixing valve, do these often go and limit the flow?

The thermal store is made my a company called McDonald Engineers
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I would suspect the mixing valve is the problem, although you can dismantle them they are not very repair-friendly.
I thought it might be the mixing valve as the store was holding temperature.
There is not much room for pipe alterations as the valve is very close to the cylinder and expansion pipe so i can't shop around for a different one i'll have to go with the manufactures one at £100 :(

The only other thing i thought it might be is the heat exchanger coil is getting clogged with limescale (very hard water area), i guess i can only find this out once the mixing valve has been changed and ruled out?
Hot pipe into TMV is hot and blended is cold, replace TMV.

Hot pipe into TMV is cold, descale heat exchanger coil. You're in a hard water area and you could expect limescale deposits to form on the DHW/secondary side of the heat exchanger. This will act as an insulator and reduce the rate of heat tgransfer from primary to secondary water. You'll get hot watre, but at a reduced flow rate. The water stored in the coil will heat up again once you stop drawing it off.
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Removed the mixing valve the coil was completely clogged with little nuggets of limescale.

I rigged up a old centeral heating pump with a coulple of bits of hose and a bucket. and Pumped 2 tubs of Fernox DS-3 desclaer mixed with hot water through the coil and it's now clear and heating and flowing like new.

Turns out the way these heat exchanger coils work by heating a small volume of water rapidly as it passes throught the thermal store is far more prone to creating limescale deposits which form these little green nuggets of limesclae. This is what i had coming out of the hot taps soon after the system was first put in but i assumed was just a bit of debris left in the pipe work from the instalation.

Now looks like i'll have to get a water softener fitted to stop it from happening again !

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