Thermal Store - Advice to save gas? (long)

Stormy

Hi,

My house has a Thermal Store Central Heating and Hot water system which is as follows: (taken from http://www.diyfaq.org.uk/plumbing/p...e1.html#thermal)

Reposted here.

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THERMAL STORES
By Rick Hughes 25/9/1997

These are open vented ( so OK for DIY install ) hot water cylinders, not unlike a domestic hot water cylinder. The big difference is the internal heat transfer tubing, and the way the hot water take off occurs.

In a conventional DHW tank, the transfer coil is connected to the boiler, and the boiler is also connected to the central heating pipe feed and return. The hot water is run off from the DHW cylinder, being replaced by the cold water cistern, and this is heated by the transfer coil. The coil has to transfer all the boiler heat and this is the weak point, resulting in long warm up times and long recovery periods.

The Thermal Store differs in that the boiler is connected directly to the cylinder, circulating the whole cylinder contents. This direct transfer means faster response (mass of hot water = thermal store).

The transfer coils are typically much larger in surface area than in typical DHW tanks and are 10's of metres long, coiled up and usually finned. There are two of these transfer coils. The one at the lower part of the tank feeds the heating system and is typically set to be around 60 degrees C. The upper transfer loop has one end connected to the incoming mains water supply, and its other end is the domestic hot water feed.

The upper part of the tank is set to run at around 75 degrees, the large store of hot water in the tank allowing you to run the cold water through the coil at 5 or more gallons per minute. Note, however, that this will only be the case if your rising main can supply it! (See section 5.2). Some even include a return loop and water pump to give instantaneous hot water at mains pressure, as in hotels. So you get mains pressure hot water, which is one big benefit of this system.

The other big benefit is that there is a large thermal mass of hot water in the cylinder. This means that the boiler can run for a long efficient burn, rather than the repeated small cycles that are inherent with a traditional coil connection.

Thermal stores can be used with any wet heating system, and are almost mandatory in Under-floor heating systems: the store provides the thermal mass at a LOW temperature that makes it ideal for use with such a system. Otherwise, the small volume of water in the UFH pipes would cause repeated cycling of the boiler. The temperature differential in the taller cylinder with a lower and upper stat also means they can be used effectively with condensing boilers, which otherwise present a problem for under floor heating.

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Now thats over, because my tank is effectively on 24/7 at 75c or being maintained at that for my heating and water. What can I do to conserve gas, as my system seems to me to be wasteful on gas because of all that heat I am generating and not utilising.

In winter its not such a big problem as in the night the heating may come on at a low temp of 14 or so to tick over but in the summer it seems a complete waste of energy.

I suppose I could just turn my boiler on and off which means when I want hot water I turn it back on to heat the tank up again.

What do people think? At some stage in the future I may replace the system but for now money is a bit tight.

Cheers,

Mike

economiser

Provided your heating system is not running when heating is not required and your "thermal store" is well insulated you should not be wasting much heat. You don't say what type of heating system you have - is it conventional radiators? Can you not turn off the heating in summer? Can you give a little more detail? Is your system similar to the ASC II sketch on the link you give?

Stormy

Hello,

Thanks for replying. Yes the system is identical, 2 pumps, one for the boiler circuit to heat the tank and one to the radiators (5 radiators). The central heating element is controlled by a digital programmer (7 day timer which is good for winter but not much use in the summer). The digital programmer basically tells the pump to circulate heat from the tank, then the boiler will come on to heat the store back up. I just manually switch it down to 12c when I want to switch it off (which works).

Basically with a lot of people on the gas forum saying its wastefull to have a conventional water store on 24/7 I thought along the same lines, as in essence my heating tank is just like a convential hot water tank in the fact it holds a large store of water at 75c which during the summer will only be used for my hot water and not the heating.

I do have an electric emmersion heater in the tank too, is it cost effective to maybe switch off the boiler and use the emmersion? I gut feeling tells me not as electricity is more per kwh than gas.

PS: with regard to lagging I have lagged all the copper pipes to/from the cylinder in the airing cupboard and the tank itself has factory installed 25mm foam insulation, I dont suppose its worth adding a jacket over that is it?

Cheers for any advice.

Mike

economiser

I think it is all a question of degree. Yes, it will cost more to run you water storage system 24 hours/day. However if you don’t heat it for some periods the savings will be quite small. The tank is full of hot water and if it is well insulated then it will not lose much heat during the “off” period, when you don’t envisage using hot water, and will therefore not require much “topping up”. Thus your savings by turning it off will be small, but there would be savings. I would think in winter turn it off and on with the heating system. In summer you may find a heating period in the morning and perhaps late afternoon may be enough. It depends on how much hot water you use when and what the storage capacity is.

Is it worth adding a jacket? Well again yes, it will reduce the heat losses but probably not by very much. On the other hand they are not expensive so it will pay back in due course.

The immersion heater could well be a good bet as the boiler is obviously “oversized” for just heating the hot water and will not be operating efficiently in summer for example.. To be really effective though it would be better if you were on a day/night tariff but this may not be worth doing unless you have other electrical loads you can utilise at night. We have a heated greenhouse for example and run our dishwasher at night. Some people I know do run their washing machine at night but one has to consider the neighbours. Living in the country it isn’t a problem for us but it could be in a flat for example.

Of course it depends on the amount of hot water you use and the pattern of usage you have but overall though I don’t think you will make dramatic savings whatever you do.

Sorry I can’t be more specific but there are so many ifs and buts and every case is different.