Solar thermal March 2011	back to homepage

In our last solar thermal update we talked about problems we were experiencing with our Gledhill thermal store, related to the level of insulation supplied with the store and resulting heat losses (see September 2010 update for details). The store was supplied with only 50mm of some kind of fibre insulation inside a zipped plastic jacket, very different from the 100mm of foam insulation inside a hard casing which we understood to be the norm with thermal stores. We contacted Gledhill about this and received a rather bizarre response - the technical department told us that our problem was caused by our only having solar thermal and a log burner to heat the store. The reasoning explained to us was that if we had an oil or gas boiler to heat the store then heat losses from the store would be automatically replaced by the boiler firing up to heat the store. Not the most green thing to do - burning oil or gas to heat a store which loses heat to the atmosphere!
Further questions eventually resulted in confirmation from Gledhill that their Torrent thermal stores are designed to meet building regulations for heat loss from thermal stores only if they are installed in an area with an ambient temperature of at least 20 degrees, typically an airing cupboard. Our store is installed in the loft, which is unheated and uninsulated, so ambient temperatures in cold weather can fall close to freezing and this results in very high losses from the store - as much as 15 degrees C overnight. We have now constructed an insulated room around the store, which has reduced losses to a more acceptable level of 3-5 degrees. However, it is still not ideal as losses from a store in an airing cupboard tend to warm the house, where most of our losses go into the atomosphere. We have more work to do on the insulation, so hope to reduce these losses further. This has resulted in significant additional cost to us, and we have been in discussions with Solar Technologies (part of British Gas) about this as they specified the store. We have now reached an agreement with Solar Technologies whereby they have agreed to compensate us for the aditional cost incurred in creating the insulated room for the store, so we are happy that they have accepted some responsibility for the survey and specification of the store. We will provide further updates as things develop.
The lesson of all of this would seem to be to look very closely at the insulation specifications and performance of any thermal store you are considering, although we haven't come across any other make of thermal store with the same low level of insulation as the Gledhill Torrent RE Solar. We were also disappointed with the documentation, performance data and installation instructions supplied by Gledhill with the store. For example, there is a download available on the Greenspec site of the Hot Water Association's "Performance Specification for Thermal Stores" which specifies that the store should have a data badge and clear labelling of tappings, neither of which we managed to find on our store. The Greenspec site also advises buyers to "ensure that there is adequate insulation to the store (100mm + PU foam)".
Notwithstanding the above problems, the house has been warm through the hardest winter for a long time. We have had to burn more logs than we anticipated, and the losses from the store have meant for example that there is less capacity to run the underfloor heating in the morning before we have to get the log burner going. At times when we have had we have had visitors staying we have had to resort to the immersion heater to top up the hot water for showers, although this is slow and expensive. Overall our heating costs have been around £800 for October-February, which is significantly higher than we had expected but probably much lower than the house would have cost with the old oil-fired heating. But there is a lifestyle issue to consider here - getting up early to get the log burner going can end up as something of a chore. We are looking again at some sort of automatic heat source to back up the solar and biomass, possibly an air-source heat pump although we're far from convinced about this. We don't want to revert to an oil-fired boiler, but it has reluctantly become a consideration again.
Regarding the problems with the domestic hot water - because the DHW is provided by passing cold mains water through a heat exchanger coil in the thermal store, the hot water tends to come out around 10 degrees C cooler than the temperature at the top of the thermal store. If this is much less than 55 deg C we get water at 40-45 deg C at the showers and the thermostatic mixers don't perform well. To deal with this we are considering an instant in-line water heater to boost the temperature by a few degrees when it is needed. Because the boost is very small for a short time, and only the water needed is heated, the electricity required should be very small. The problem is that there are almost no instant heaters available which can accept pre-heated water - and these tend to require 3-phase electricty. From the web we have picked up that this is quite a common problem with relying on solar heated water - we think we may have found a solution at this year's Ecobuild show, watch this space for details.