As a fan of good old Isambard I am biased, A thermal store is simply a method of storing heat in water so if you have solar and/or a wood burning stove you dont have to have a shower when the sun is shining or the stove is lit. I am also a fan of non complicated electronics/mechanics (KISS and less to replace when it wears out or goes wrong). Other more clever people on this forum will tell you what size store you would need but it seems sensible to me to use what you have and store it for later use. Some also talk about losses but if the store is within the house its not a loss because it helps heat the house.

Thermal stores are a way to maximise your heat losses in my book. there can be reasons why this is not that bad but in an "ordinary2 house it could be better to travel a more conventional route.

The most useful thing about a thermal store is that you can combine different source of heat for DHW production without worrying about the temperature at the bottom of the tank or running regular immersion heater cycles to kill legonella. So you can leave the bottom of the tank at a relatively low temperature to maximise the solar thermal contribution.

You can also use it to buffer heat for the space heating system. This is useful for batch burning boilers, e.g. biomass or log burning boilers, to reduce the number of burn cycles. However, you need to ensure the heating system doesn't stir the tank causing destratification & wasting high quality hot water.

It may take some trial & error to get this aspect right & it may not be worth the bother depending upon what you're using as a heat source. For example, if you're using a gas fired boiler there's little or no benefit to using it as a space heating buffer, but you may still choose to use one to combine the gas boiler & solar thermal for DHW production.

David

Do you know what your space and DHW loads are (or will be). You really can't make a decision until you do.

Posted By: wookeyYou sure that a buffer isn't still useful if you have a well-insulated building and typical gas boiler (which means it probably won't actually modulate down to the heating load, so some buffering to reduce cycling is useful)

There may be a case depending upon the boiler capacity, the heating load & the amount of heat capacity (water) in the system.

David

If your tank was at 65C and you had a shower at 35C (numbers not important, concept is), your incoming waster was at 10C, then you need to add twice as much cold water as there is hot, so you will use 5lt/min of hot and 10lt/min of cold.
Why knowing what you actually need is important before you can size anything.

So to run a 300lt tank down to the minimum showering temperature of 35C is going to take about an hour (I think).

seems to that you may as well use the oil tank as the thermal store, ie only heat water when you want it hot (tank full) and minimise heat losses

As some have pointed out on here, Gas competes favorably in total CO2 calculations with the renewable technologies (especially taking into account structural costs). It is not my personal opinion as I think the whole issue is more complex (e.g. personal investment as well as financial and associated lifestyle changes). It is very convenient though!

I personally favour the thermal store and am going down this route myself. I am, however, taking part in a retrofit from which the gains are likely to be more personal than financial and the 'payback' in £ is unlikely to occur in my lifetime. The thermal store offers very good future options as you can connect almost anything to it. As far as the oil boiler goes, Tony, I think that the reduced cycling increases efficiency and if the TS is internal to the insulation envelope then there are no significant losses (except perhaps on very hot summers days but ST can solve this). It is, however, a large financial investment.

I think this has to do with ST not being a solution in itself. Gas is relatively low CO2 total (as far as I am aware) whereas other systems require significant structures, pumps, thermal stores, alternative heating for cold days or winter etc etc. They also cost significantly more than a gas combi. If you combine this and for instance spend all the money saved by using gas on insulation, planting trees etc there may be something in it. But as I said, this is not my opinion, just something that I have noted others alluding to on other threads. I personally favour the 'green' tech as a) I like gadgets, b)it makes me aware and thus careful of use, c)I like self sufficiency, d)it makes me feel smug!

Oil boilers can't modulate down, the burner is either fully on or fully off. A thermal store is worth considering if you have a lot of seperate zones/UFH or TRV. Typically one or two zones will call for heat at a time rather than all at once. This can mean the boiler is called to deliver low power continuously which it can't do by throttling down so it cycles the burner on and off. A thermal store can help reduce the cycling from perhaps once a min down to once every 20mins or longer.

Our 300L store copes with three showers an hour but has quite a big boiler to recharge it so hard to judge.

people confuse thermal stores with buffer vessels...

thermal stores store heat at a high temperatures to make a heat exchanger work, i.e. they are inherently wasteful.. they generally supply domestic hot water.

Buffer vessels on the other had store heat at any temperature when it is being produced but not consumed...they are combined with other means of storing hot water.

Britain loves its thermal stores... hence more than a few trashed gas condeninsng boilers....operating way out of their comfort zones!

<blockquote><cite>Posted By: Phil.Chaddah-Duke</cite>Alec, could you expand on why the gas condensing boilers get trashed with thermal stores, we are proposing this with ST. Surely there is way of controlling the boiler to only heat the upper part of the thermal store?</blockquote>

I suspect it's to do with condensing and potentially high return temperatures from a thermal store. Condensing boilers don't condense if the return drops below about 55 deg C. As thermal stores are best run at high temperatures to maximise heat storage capacity they are probably not best run from a low'ish temperature source like a condensing boiler.

HI,
Doing this all from memory so may be a bit off,
I did a lot of theoretical and experimental work with running oil boilers into thermal stores (search back around 2007-2009). The key to operating the store as an interface between boiler and heat load was to balance the relative flow rates and temp differentials. This fell into two operating scenarios A) boiler on whilst discharging the store, or B) boiler off whilst discharging store (a straight charge then discharge cycle). The objective was to retain the oil boiler in condensing mode - always. This is hard as there is no modulation as with gas, and the dew point is lower at 47 deg, therefore as the delivery temp is fixed by the temp rise through the boiler then there is a limit to the top temp of the store. A typical temp rise is 20 deg, you will struggle to get more than a 10 – 12 deg drop balanced through radiators, thus the return temp to the boiler rises with each pass, and so forth until the boiler thermostat knocks the flames off but keeps the pump running etc etc etc. This can be alleviated by putting the store in between as we know. Stratification is either achieved by limiting each recharge and discharge to a single pass, or not bothering at all as the pumps will be horsing it through on many passes. The store needs to have a volume with sufficient energy to supply the heat load for a reasonable time before a recharge is required. The problem is that water returning to the store is too warm – it still has lots of useful energy in it. You can temper down the heat load supply water by introducing some of the returning cooler water but you can’t temper down the heat load returning water any further, other than by removing more heat. So you could feed the radiator return in to a UFH system (only if you do the calcs) then it might go back to the store at a reasonably lowish temp.
By the way, because I've looked at this from the boilers perspective I see the heat load as a cooler thus if the water returns too hot then it’s a problem.
So my objective was that water to the heat load went out hot (quick response) but came back very cool – lots of energy removed, suitable for ST at the lower end etc and all in one pass.
And
Water to the boiler hot enough such that when running though a loading unit (preferred ACASO) a reason able exit temp to the store could be achieved. Say running 45-75 deg, this is about the max temp rise you’ll extract from conventional boiler designs. Most of the manufacturers wont condone anything over 20 deg. Failing that you could just run the boiler 40-60 deg but with out the store you wont get rid of the temperature via the radiators quick enough so it quickly peaks and short cycle. Simply in put and output rates do not balance. This is where bigger radiators can help.
Control of boiler is from the store via upper and lower stats to trigger off at lowest position (set to higher temp), on at top position (set to lower temp).
This is for buffer / accumulator stores – I’m not talking about the smaller unit whose main function is DHW production.

The store on the oil combi is only for the heat slave types and most don’t feature this aspect. This does two things firstly it give a rapid response as when a tap is turned on the cold mains flow switch initiates the start up procedure so you will get a fair volume of cold water through the heatX before the flames come on. Secondly it does offer a small buffer, but the significance is that with both DHW & Heating controls to OFF you will get enough DHW for say hand washing throughout a day. The pump via the diverter will send that stored primary hot water through the heatX but not replenish it.

BTW – as to the above running the oil combi as DHW only – how you cover the pump over run when the tap is closed. The residual heat is sent to the rad circuit by the diverter valve and the pump (but no burner fire signal) this will override the time control.

Don’t get the opportunity to post much these days, not as keen on combustion as I used to be anyway. We have done a lot of discussion threads on conventional oil / gas boilers running into fixed load store scenarios, but was quite a while ago.

Oh and yes I do like thermal stores or whatever you like to call them. A big tank of hot water that does all we ask of it is actually quite hard to design / model / analyse.

Cheers, Mike up North

Mike, the oil combi I have is fairly agricultural, non-condensing, and both the pump and burner stop as soon as both a) the tap is closed and b) the TS reaches setpoint temp. The boiler is intended to run like this in DHW-only mode during summer anyway. I agree it would be nice for the pump to run on a little after the burner stops, to recover residual heat from boiler into the TS or rads, but this one is just not that clever.

Agree that this is not the kind of thermal store the OP referred to (sorry for diversion) but illustrates your point that TS only needs to be big enough to match the 'supply' and 'demand' characteristics (time/flowrate/T/ deltaT) - in this case 80ish litres is plenty, as the supply can deliver a lot of heat at high T and fairly short notice, and then shut down again.

Its more to do with incorrect application of a gas boiler in my opinion.. If you have a gas boiler on site why aren't you using it for the application it was designed for that is heating and hot water....

The high efficiencies achieved by gas boilers are at the expense of robustness, and I can't think of a boiler that doesn't take compensation controls to lower the flow temps to increase efficiency.

The other major issue you have with condensing boilers is the high flow rate that has to go through the heat exchanger upsets the stratification in the store making it pretty difficult to design a system that keeps the return temperature down...especially as the boiler will be sized for the whole building presumably

throttle the flow rate down and the boiler won't like it.....