Question about the cost implications of running a domestic thermal store

[Terry Fields]

I'd be very grateful for any comments the group may have on the topic of the Subject line.

SWMBO and are away from home doing some house-hunting. A really super property within our price band unexpectedly presented itself.

When the agent who was showing us around opened the airing cupboard door, there seemed to be a wave of escaping heat, and the large rectangular metal box living there bore a label saying 'Gledhill Thermal Store' (it might have had the number 2000 associated with it).

It would appear that the gas boiler in the kitchen 'runs' 24/7, keeping the heat store up to temperature by burning as necessary.

I understood this was an efficient way of heating DHW and CH, with the added advantages of mains-pressure hot water, and no live or stored water in the loft.

Is there a cost burden to running things in this way, e.g. in summer when there's no CH demand, or does the improved efficiency of the system mitigate it in some way?

I've been interested in thermal stores, but never thought I'd have to make a sudden choice involving one - we have to put an offer on the house tomorrow, so any guidance or advice will be greatly appreciated.

TIA

TF

[Vortex5]

I'm guessing it's one of these:

is basically a large thermal store that holds the boiler "primary" water. There are 3 circulation pumps; one for the boiler "flow"; one for the heating circuit and one for hot water via a plate heat exchanger.

Looking at

cylinder products link there is a current Boilermate product but this looks much simpler and does DHW only, no heating.

A very similar modern product which I installed at home last year is the XCel from

The key difference with the XCel is the clever use of thermostatic blending valves to control both radiator and DHW water temperature as well as boiler "return" whilst permitting the store to run at very high temperatures. Up to 75C in fact.

This technology would not worry me at all. It's just an insulated tank with inputs and outputs.

As regards efficiency in the summer I am far from convinced. Ask to see the quarterly gas meter readings back for a year or 2. You will need to get your calculator out......if summer gas usage is any more than 20kWh per day (and that would be for a family) then it ain't specially efficient.

Having said that with Gas at 3-4p per kWh it's not exactly a dealbreaker.

David

[John Rumm]

Even if the controls are not there now, there is nothing to stop you having timed operation of these things if you want.

Chances are however it should be very good in the winter, and ok in the summer. It certainly would not dissuade me from buying the place.

So if you like the place, go for it.

[David Hansen]

On Wed, 23 Jun 2010 18:18:33 +0100 someone who may be Terry Fields wrote this:-

Then it sounds like the insulation is not up to much. Also how well done is the insulation on the primary circuit from the boiler, how long is this circuit?

Provided it has been designed and installed properly a thermal store will reduce gas bills compared to most cylinder systems. The store can accept the full boiler output, most cylinders can not and the boiler cycles on and off, limited by the heat the coil can put into the water in the cylinder, while warming the cylinder.

A thermal store providing domestic hot water probably doesn't need to be operated at more than 55C, though the efficiency of this depends on the boiler.

Do you use a lot of hot water at 2am? There is no difficulty in installing a time clock to control when the boiler heats up the store.

If the store also has solar than it might be set to bring the boiler on if necessary at 6pm. If the store has been heated by solar then the boiler doesn't come on, otherwise the boiler provides some more heat. In the morning the store should still be warm enough to provide water for washing hands and so on.

There is no particular cost burden if the thing has been designed and installed properly.

[Terry Fields]

They look like Grundfoss pumps; the one on my current back-boiler system is 30 years old and has never given any trouble.

The property owners are away on holiday, so I doubt that if will be available; it looks like we'll need to just accept the system, as the house itself is just what we want and is immaculate inside and has had a shed-load of work done in the garden. It's possible that any extra cost of running the heat store wouldn't be recouped by replacing the system, so we'll probably have to accept it.

Quite so! Thanks for your comments.

TF

[Terry Fields]

LOL. With SWMBO twisting my arm it looks like that's on the cards!

Many thanks for your comments.

TF

[Terry Fields]

The day we viewed was a hot day, and if the airing cupboard (all internal walls) has good insulation above it in the loft, then on looking back perhaps it isn't surprising the airing cupboard was on the warm side.

That's good news, and as other have commented, not a deal-breaker.

Many thanks for your comments.

TF

[Vortex7]

Interestingly after I installed my thermal store there was one consequence I did not forsee......that my 4 teenage childrens' behaviour would change.

Now we have practically unlimited mains pressure hot water they have longer showers, using more water and heat than ever before.....and in this weather twice a day. It drives me crazy.

I installed a fairly large solar collector earlier in the year, which delivers 9-10 kWh to the store on a good day....but against my expectations there is no way I can go off-grid even for a day (gas-wise) so long as my children are in the house, even in a heatwave because of our increased usage.

Part of me wants to install more solar (to double the output) but that simply does not stand up to financial scrutiny. When 1kWh of gas is

3p that effectively values 1000kWh of solar heat at £30.00. The kind of panel you need to deliver that (like this: is about £700. 25 year payback.

[Nutkey]

So turn off the gas heating on sunny days, and see if the behaviour moderates back....

[Jim]

What's needed is a shower which automatically restricts flow after a certain amount of water has been delivered.

[Vortex7]

No chance if I want a hot bath in the mornings, unfortunately.

Boiler is off from 2300 to 0500 in any event.

I'm a bit obsessive about logging data so here are the last 72 hours if anybody is interested:

* Yellow is top of store. Each "jink" is a boiler burn. Note this fills the store from the top downwards.

* Pink is bottom of store (each downward step represents hot water use). Solar "fills" the store from bottom upwards pulling the line up. * Dark Blue is solar manifold temp * Pale blue is solar return temp

So yesterday involved 5 "burns" of the boiler; Nice bit of solar energy (about 8kWh) taking the entire store to about 67C

General evening usage plus 2 Showers used around 2315 last night and the usable energy was used.

+++

ps time axis screwed up due to a "layer 8 problem".

[dennis]

You need one of these..

't tell them and reduce it a bit each day.

[Vortex7]

Nice suggestion but with no en-suites in our home but the same facilities are used by all. In fact our water supply is regulated to 2 bar anyhow (it's 10.8 bar in the street)...so there's not so much headroom to play with.

On the thermal store there is a circulation pump dedicated to the HW, controlled by a flow switch. I was thinking of some kind of one shot timer in series with the switch.......that would work.

[Terry Fields]

Many thanks for the comments and replies.

I took the view that the heat store was already there, it is a reputable make, the cost implications appear to lie more in how it's used, and it isn't a deal-breaker.

Result is our offer has been accepted, so in a few weeks we'll be living in Basil Fawlty country in a detached, 4-bed, 1 ensuite, study, lounge, dining room, cloakroom, uber modern kitchen, superb garden with no gardening to do, garage and two parking spaces house with of course, a heat store!

Just a shed load of money to fork out now :-( but SWMBO's happy.

TF

[David Hansen]

On Thu, 24 Jun 2010 09:15:49 +0100 someone who may be Vortex7 wrote this:-

Some families cope with this by discussing the issues and putting a timer in the shower. Not one that controls anything, just one so that people can see how long their showers are.

[David Hansen]

On Thu, 24 Jun 2010 18:06:39 +0100 someone who may be Terry Fields wrote this:-

Let us know how you find the thermal store when you have been in for a while.

[David Hansen]

On Thu, 24 Jun 2010 11:34:23 +0100 someone who may be Vortex7 wrote this:-

You go to bed dirty and wash in the morning. Ugh:-)

[Doctor Drivel]

The 2000 is very well insuated. The pipes inside the unit may need insulating. A simple, cheap task.

A clock can be put on the 2000 to switch in and out DHW. One maybe on the front of the unit.

[Doctor Drivel]

Put flow regulators on the showers. They like the high pressure, so cut back the flow.

[Doctor Drivel]

There is provision on the MCB for an external clock.