Thermal Store - Advice to save gas? (long)

Stormy

Hi,

Edit: I think this may be posted to the wrong group? Should it be in Other heating? My apollogies.

My house has a Thermal Store Central Heating and Hot water system which is as follows: (taken from http://www.diyfaq.org.uk/plumbing/plumbingpage1.html#thermal)

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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

michaels

I have a similar system but with a pumped heast exchanger (makes for a much lighter and longer lasting tank).

I guess the first thing to do is make sure the tank is insulated as well as possible. In the summer when the air temperatire is higher there will be less loss anyway as a lower gradient.

My tank also has an immersion half way up so in the summer I could chose to only heat half the tank. I could also put in a time control to limit the hours the tank was heated for but I am not sure if this would be more or less efficient as obviously a lot of water needs to be heated. With my system there is a thermal mixing valve which maintains the hot water to the correct temperature so if the store is cooler I just get less how water flow rather than cold water.
Sorry these are all thoughts rather than answers - I think the most efficient with these systems may be to have them hot all the time and minimise losses but I am not certain. I would suggest posting your question in the 'in my home thread' where these type of questions are reguarly answered.

Stormy

michaels wrote:

I guess the first thing to do is make sure the tank is insulated as well as possible. In the summer when the air temperatire is higher there will be less loss anyway as a lower gradient.

Yup, my system is about 12 years old I think. The tank has a foam pre-installed/sprayed coat on it which is about 25mm thick. Not sure it would be worth adding a jacket to it.

I have lagged all the copper piping which is warm that goes to and from the tank in the airing cupboard already.

My tank also has an immersion half way up so in the summer I could chose to only heat half the tank.

Yeh, I have an immersion heater too in mine but have not looked at weather its better to switch the boiler off and use the immersion WHEN I need hot water? What do you think?

I could also put in a time control to limit the hours the tank was heated for but I am not sure if this would be more or less efficient as obviously a lot of water needs to be heated.

I thought about a timer control too, as that would reduce the need for the water to be maintained at 75c all the time.

I also thought that it might be worth in the summer, turning the heat on the tank down to 55c or so, as that may be enough for the hot water when it doesnt have to worry about the heating.

With my system there is a thermal mixing valve which maintains the hot water to the correct temperature so if the store is cooler I just get less how water flow rather than cold water.

Mine too although I dont think its working correctly as it seems to be stuck and the water out of my taps for the hot is always litrally BOILING, I could do to replace it but as its just me in the house with no kids or elderly people I know about it.

Cheers,

Mike

Andy_Davies

We've got one in our house and it's on a timer rather than being on all the time - generally goes on for a couple of hours in the morning and a couple in the evening.

Not sure what the temperature is set to on ours but if we could get at the panel easily (it's in a awkward position) we'd probably turn it down a bit as the water's pretty hot even in winter.