I'd like to follow up my earlier threads with a request for further advice.

I am looking at a renovation of a 200 sqm house which has a relatively modern oil boiler (fitted 2004) but needs a re-plumb (not updated since the 1960/70s). There is no gas or prospect of it and I am unlikely to be able to afford solar, heat pumps or wood burners anytime soon.

So, if I were to take the view that all I've got is oil and electric, does a thermal store still make sense?

The advantages seem to be that it is safer (not pressurised) and allows for alternative / additional heat inputs in the future, whilst providing an as-good-as solution as a pressurised system.

Is this the case?

I also remain uncertain as to whether the CH should be run from a thermal store as well.

And what about UFH? The implication is that because this runs at a lower temperature then it is is suited to take-off from the lower strats of a thermal store. But does this argument still hold [hot] water when there is no solar/heat-pump input?

It will located on the ground floor in a dedicated boiler room. The house itself is to have a full EWI retrofit, with all new windows/doors so should be fairly well insulated when done.

But it is your comment about big heat losses that interests/bothers me - because these are the two sides of the argument I keep coming across that cast doubt in my mind. Are they a practical, proven technology or just a clever idea that is not quite there yet?

The thing is, it is very easy when researching to find that thermal stores are widely used across Europe and have been for some time, plus they are supposedly becoming more common in commercial new builds in the UK - which sounds great, but I have never actually come across anyone who has one outside of forums like these.

That's interesting Pile-o-Stone. This is where it would be useful to understand the heat/energy calculations, because what you are saying is that from experience you are having to put more energy into a thermal store system than you would a conventional cylinder.

The argument I had seen was that even though with a TS you needed to heat a larger body of water, provided the tank is well insulated the overall energy in and out was still similar to a conventional cylinder (assuming that you are using the hot water on a regular basis).

Every time you have a thermal store the have to heat losses associated with it. Ideally these should be losses into the heated envelope but there will be pipe losses too and losses at times that you don't want them all adding to your cost.

Just to point out we have the opposite experience to Pile-o-Stone.

I just can't get it when people talk about "heating up" their thermal store. This implies that users will continually be letting their stores cool down substantially and then heat up again. But that's a straw-man argument because that's not how they are used. When it's in-use the boiler keeps it at a constant temperature, within the buffer margin controlled by twin stats. The store controls also ensure that the boiler is always kept in condensing mode (low return temp) and running for longish burns, which I believe has a measureable beneficial effect on fuel consumption. As we have a gas boiler and solar coil (no woodburner etc), our store is only the size of the hot water tank we'd have anyway. If it is a little larger, as the store runs at a lower temperature than the equivalant hot water tank would (see previous threads regarding legionella risk and outdated beliefs that stores should run really hot), so I think we're losing less heat than an equivalent non-store system.

I can see it being a very minor problem in terms of delay in getting heat out of radiators when you have been away for a week or two and let the store cool down but you're talking minutes, and in normal operation the issue just doesn't arise. The time-lag issue would apply to hot water availability in a conventional system too, because of the top-loaded design of a direct-fed store, you get hot water out quicker (as per Mark's last para).

There are some points that ought to be re-stated:
1. Yes clearly the store should be well insulated, solution: don't buy one that isn't
2. The whole system needs to be designed properly, including the size of your store, I completely agree with Mark's point about thermosyphoning. Once you're looking at integrating solid fuel sources the design issue is doubly important
3. If your store is within your thermal envelope and is well insulated anyway, then I don't get Tony's point
4. Yes stores are great when integrating multiple sources but they are also useful in simpler setups

(Losses from thermal store into the house in summer are not necessarily helpful...)

No, you don't heat the whole store to 80C (in fact you shouldn't heat any of it that hot, as gas boilers are more efficient at lower temps too). So you heat the top part to 60C, and that leaves plenty of lower store to accept solar input around say 45C. The overall energy balance is actually rather complicated to calculate, which is unfortunate, but the point is that it's easy, and expected, to supply solar heat at a lower temp to boiler heat into the same store at more-or-less the same time. Optimising the control and plumbing for this is tricky.

Hi Joe

The way we operate things differs between the seasons and to a small extent with the weather within seasons. In fact that's one of the main difficulties: they do best (i.e. save most energy) when tinkered with a bit.

The boiler always on scenario has happened in the last couple of winters when there's no chance of getting any solar input. Actually I was a bit misleading there too - we do switch the boiler and heating off when we're out of the house during the day even in those conditions. From 9 till 5 the store loses just a few degrees under those circumstances, and when we get in we switch on both: getting instant heat into the radiators from residual heat in the store and also topping the store up overnight. Boiler is then "on" till the next morning, firing up when the store needs heat.

In the summer, it's obviously heating off and store heated by solar. If it's been a dull day then the next morning I can read the temperature of the bottom of the store, and based on experience, I know how long to run the boiler for to load up the top of the store with enough heat for a couple of showers. This has to be done manually, I press the boost button, set a timer and go and make a cup of tea, turning boost off when the timer beeps.

The spring and autumn and warm-ish wintertime are the trickier ones. Basically it involves looking at the weather + weather forecast and thinking ahead to make most use of solar input: when the boiler isn't needed then the store still doen't cool because it's got the solar input. Then if a bit of extra heat is needed it's press boost on the boiler circuit, but the first heat that comes out of the store is effectively from solar.

I'm the first to admit this is not KISS. But I'm really happy with my energy bills nonetheless.

One thing to add: our house is reasonably well insulated. With our old boiler, it would only ever run for about 30 seconds, even when I presume it was modulating down as far as it could, because there was never that much heat demand, except when heating the house from cold. Our old boiler was not a condensing model, but I definitely felt that there was no way it could get into condensing mode with such short burns and hot return temperatures. This is what set me on the thermal store route in the first place. But I also admit that ours is more of a buffer tank (210l) than a "store" of heat.

I think SteamyTea's comment sums all of this up for me - it's all 'a bit complicated'; apologies if I am not getting the sentiment right here ST, but setting up and running a Thermal Store seems to be a complex business when, as you say, all that needs to be done is heat the house.

I can't help thinking that Tony's initial response also follows this sentiment - that unless you really have to, Thermal Stores are overkill in a simple situation. In all of the threads I have read and submitted to, there has never been the sentiment that 'it just works'. I can see, however, that time will change all that.

MVHR is another subject of interest for me - in another thread though.

Does the MVHR main unit have to be high up in the house, such as the loft - or could it be on the ground floor? I am looking to have a ground floor 'boiler room' for boiler, tank etc.

dovecote,

regards positioning of the MVHR, the key thing is the routing of the ducts. Try designing the run for these (say 5 inch diameter), and that may lead you to finding another location. Remember you need to insulate all the ducts, so the shorter the runs the better, plus fewer obsticals to get around, and also you need a drain point to get rid of the condensation.

Nearer the roof means the intake & exhaust will be short (don't really want these at ground level outside), and since these will carry freezing cold air in winter, also good to keep these short, and outside the thermal envelope.

Cheers, GreenPaddy.

Thanks MarkBennet & GreenPaddy - soemthing to think about there.

Can I sanity check the use of MVHR:

- Does MVHR complement UFH and could I do away with [most] radiators?

- How tight does the house have to be sealed for MVHR to be worth it and does this mean no trickle vents?

- I am one of those people that likes to sleep in an ice-box - i.e. bedroom windows wide open at night, even midwinter - but still likes a warm living space. Any design suggestions?

- Just how noisey do these systems get in reality - both the pump & the air vents?

Posted By: dovecoteI think SteamyTea's comment sums all of this up for me - it's all 'a bit complicated'; apologies if I am not getting the sentiment right here ST, but setting up and running a Thermal Store seems to be a complex business when, as you say, all that needs to be done is heat the house.

I can't help thinking that Tony's initial response also follows this sentiment - that unless you really have to, Thermal Stores are overkill in a simple situation. In all of the threads I have read and submitted to, there has never been the sentiment that 'it just works'. I can see, however, that time will change all that.

MVHR is another subject of interest for me - in another thread though.

For me, having to run the boiler for a few minutes when the previous day has been cloudy, while I make my tea in the morning, is an acceptable inconvenience for what is an incredibly efficient (and low running cost) system to heat home and water.

The advantages would stack up more if one needs to integrate solid fuel, UFH etc.

I fundamentally disagree with Tony's point regarding heat loss. As I've said above, any hot water tank loses heat, there's nothing different about a thermal store.

I completely agree with Tony's point that addressing overall insulation and draught proofing should be the top of the list for any renovation.