ecoTec/uniStor mystery

14 Jan 2010
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United Kingdom
I've seen a number of posts on this forum in relation to the Vaillant ecoTec S.53 problem where the boiler cuts out (water shortage or too great a difference between flow/return temperatures). Most of those posts relate to the problem occurring when running only the CH, and the system working fine with demand from both the hot water cylinder and the central heating system.

Now, my problem is exactly the other way round. I have an ecoTec 637 boiler and a uniStor cylinder, both fitted in the loft close together. Given that the runs to the furthest radiators are rather long, and we had problems with heating in previous years, we had an additional pump (Grundfos UPS 15-60) installed into the CH flow, almost immediately after the CH zone valve. (Perhaps worth saying that without the pump or with the pump turned to its lowest setting, the dreaded S.53 problem also arises when the CH system is running on its own and starting from cold, but with the pump on CH works just fine).

So I can run CH only (with the additional pump on) no problem, and the heating coil in the uniStor cylinder only no problem, but as soon as I try to run both CH and the hot water cylinder, the boiler cuts out showing S.53.

I thought it might be the integrated pump - but then I would get different status/error codes, and I couldn't run the cylinder on its own. I thought it might also be the Grundfos pump taking too much pressure away from the cylinder coil - but in this case, my problem should be that the cylinder doesn't get hot or takes a very long time to heat up, but not the boiler cutting out.

Anyone out there with a potential explanation of what's going on? I've had a Gas-Safe registered installer on site, but the only recommendation I got was to replace the uniStor cylinder on account of there possibly being a blockage in the heating coil, which doesn't make much sense as the cylinder heating works fine on its own (apart from being rather expensive to try on the off-chance, not least because the cylinder is only a bit over three years old).

Suggestions greatly appreciated ...
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