Thermal store or not?

[SimonH2]

Bear in mind that any properly designed and installed thermal store will have a section dedicated to DHW. Typically the CH tapping is taken off part way down the cylinder which means it cannot use all the hot water at the top. Similarly, you can have two immersion heaters - one which only heats the upper cylinder for peak rate top-up if required, the other at the bottom to heat the whole store on off-peak.
So with a 300l store, you might only be able to use 200l for CH. It takes 4.2kJ to heat one kg of water through 1˚C. 200l cooling from (say) 70˚ to (say) 40˚ will take 4.2 * 200 * 30 = 25,200 kJ, or 7kW hours. So if your CH load is 7kW then you've only got about 1 hours worth of heat. However, if you set things so the CH turns off before the stove goes out, that could be enough to give you a bit of a warmup before you get up.

The same maths says that you should have enough heat left in the upper 100l for a shower. You've nominally about 3 to 3.5 kWHr there. Not enough for a bath, but enough for a 10-15 minute shower I'd have thought (I assume your mains water is a bit warmer that ours). Even after the CH has cooled the lower store to (say) 30 to 40˚, that will still transfer some heat to the DHW coil - so you aren't actually restricted to only the heat in the upper cylinder. Also, the shower won't "just go cold", it will reduce in temperature once the store drops to a critical level and get cooler as the store depletes.

Assumptions used above are that you heat the whole store to about 70˚, and that the CH tapping is 2/3 of the way up the cylinder (so the CH has access to 200 of the 300l capacity), and you don't do anything to de-stratify the store.

Unfortunately, storing a lot of heat is not easy - the bigger the cylinder, the more you can store

If setup correctly, you can reheat "top down" so that after lighting the stove, you will get a fairly quick reheat of the DHW supply with the CH picking up when the hot water gets own that far. Once the stove is going, you'd have "unlimited" DHW. I believe it's good practice to run a WBS fairly hot anyway so it doesn't tar up - outside my knowledge area, but I think there are combination pump/valve units that combine a pump, TMV (so heat only goes out to the connected devices once hot), and flap valve to allow thermosyphoning in the absence of power for the pump.

Just random thoughts ...
One way to ensure you keep a minimum level of heat in the store might be to use multiple stats and multi-channle programmer - or some sort of fancy programmable stat/controller. After a certain time in the afternoon, only run the CH pump if a stat low down on the store is above a certain temp. If the WBS is running then you'll have enough heat to keep the CH on - but as the fire dies down, the CH will turn off to leave you a minimum amount of heat for the morning. Similarly, in the morning have a stat (bit higher up ?) to set a minimum temp on the store depending on your DHW requirements.

You also have the option of using your off-peak lecky to heat the store - so there should be no reason not to have it "fully charged" each morning.

 

[SimonH2]

Oops, double post

 

[oilboffin]

Whoa brill and really well explained Simon and your maths seem spot on only one thing wbs do run hot to stop tarring but if you use properly dried and sort of seasoned timber you don't get the tarring in France they cut the timber early in the season to give it time to dry

 

[DeltaT2]

Bear in mind that any properly designed and installed thermal store will have a section dedicated to DHW. Typically the CH tapping is taken off part way down the cylinder which means it cannot use all the hot water at the top. Similarly, you can have two immersion heaters - one which only heats the upper cylinder for peak rate top-up if required, the other at the bottom to heat the whole store on off-peak.
So with a 300l store, you might only be able to use 200l for CH. It takes 4.2kJ to heat one kg of water through 1˚C. 200l cooling from (say) 70˚ to (say) 40˚ will take 4.2 * 200 * 30 = 25,200 kJ, or 7kW hours. So if your CH load is 7kW then you've only got about 1 hours worth of heat. However, if you set things so the CH turns off before the stove goes out, that could be enough to give you a bit of a warmup before you get up.

The same maths says that you should have enough heat left in the upper 100l for a shower. You've nominally about 3 to 3.5 kWHr there. Not enough for a bath, but enough for a 10-15 minute shower I'd have thought (I assume your mains water is a bit warmer that ours). Even after the CH has cooled the lower store to (say) 30 to 40˚, that will still transfer some heat to the DHW coil - so you aren't actually restricted to only the heat in the upper cylinder. Also, the shower won't "just go cold", it will reduce in temperature once the store drops to a critical level and get cooler as the store depletes.

Assumptions used above are that you heat the whole store to about 70˚, and that the CH tapping is 2/3 of the way up the cylinder (so the CH has access to 200 of the 300l capacity), and you don't do anything to de-stratify the store.

Unfortunately, storing a lot of heat is not easy - the bigger the cylinder, the more you can store

If setup correctly, you can reheat "top down" so that after lighting the stove, you will get a fairly quick reheat of the DHW supply with the CH picking up when the hot water gets own that far. Once the stove is going, you'd have "unlimited" DHW. I believe it's good practice to run a WBS fairly hot anyway so it doesn't tar up - outside my knowledge area, but I think there are combination pump/valve units that combine a pump, TMV (so heat only goes out to the connected devices once hot), and flap valve to allow thermosyphoning in the absence of power for the pump.

Just random thoughts ...
One way to ensure you keep a minimum level of heat in the store might be to use multiple stats and multi-channle programmer - or some sort of fancy programmable stat/controller. After a certain time in the afternoon, only run the CH pump if a stat low down on the store is above a certain temp. If the WBS is running then you'll have enough heat to keep the CH on - but as the fire dies down, the CH will turn off to leave you a minimum amount of heat for the morning. Similarly, in the morning have a stat (bit higher up ?) to set a minimum temp on the store depending on your DHW requirements.

You also have the option of using your off-peak lecky to heat the store - so there should be no reason not to have it "fully charged" each morning.

Grossly over-engineered as always Simon, this Villager stove has only a peak output of 14KW, that's twice a day; once when heating up & once when cooling down. Like all stoves they rarely reach their peak output & you'll find 14KW is only on some grades of house coal.

OP use KISS when installing your system, ignore the over-engineers. Keep everything open vented & ensure you have a bloody good supply of seasoned wood - you'll need it!!! Most of these small stoves, the heat goes up the chimney!!

 

[Agile]

Assumptions used above are that you heat the whole store to about 70˚, and that the CH tapping is 2/3 of the way up the cylinder (so the CH has access to 200 of the 300l capacity), and you don't do anything to de-stratify the store.

As this is not a condensing gas boiler, you can with the wood stove, heat a store to 80 C or even 90 C.

However, adjustment of the heat output is usually a matter of manual hit or miss and not always compatible with CH needs.

Here, we used to have a complicated stove which used electric controls to adjust the burn rate to achieve a constant temperature. But it gave very little direct heat output so it was replaced with a simple model which gives out about 6 kW to the room and about 6-8 kW to the rads. But it does use a lot of wood!

Tony

Untitled

• Agile
• 4 Apr 2009
• 29

 

[SimonH2]

Grossly over-engineered as always Simon, this Villager stove has only a peak output of 14KW, that's twice a day; once when heating up & once when cooling down. Like all stoves they rarely reach their peak output & you'll find 14KW is only on some grades of house coal.

Hmm, open vented tank of water with a coil in it is "over engineered"

It can be made as simple or complex as required. If the OP wants something that's at least semi automatic then some controls will be needed - but if he's happy to drive it manually (and take the consequences for not getting it right) then he can do so. That's his choice to make, I was merely pointing out the basics and some options - many will be happy driving the system manually, for many, automation is required for a reasonable WAF.

From the way the question was asked, the OP has just been told "won't work" without having been given any useful information to see why - or even if the statement is true. I was just trying to give him some information so he can look at his situation and figure out what might, or might no, work for him.

As this is not a condensing gas boiler, you can with the wood stove, heat a store to 80 C or even 90 C.

However, adjustment of the heat output is usually a matter of manual hit or miss and not always compatible with CH needs.

Yes, I nearly wrote that, but didn't want to put too much in at one go.

If I were doing this for myself, I'd do it as :

CH tapping from somewhere part way down store, TMV (mixing CH return with store water) to control CH flow temp, TRV on every rad (and no room stat), and modulating pump (eg Grundfoss Alpha 2L) - and run the pump on a time switch. That's actually what I have in the flat.

WBS on loop with TRV so that water circulates in the stove until it's a certain temp and then diverts to top of store. This would give "top down" reheat (ie give you good DHW fairly quickly). Once the store is nearly fully heated, then you'll end up with full circulation round the WBS and store and heat the entire store to a higher temp. As I say, it's not my area but I believe there are units designed specifically for this, and including safety features like check valves that will allow thermo syphoning if the pump isn't running. Obviously, some form of heat dump is needed, I'm not qualified to say what that needs to be or how it's controlled - but none of this is "new and bleeding edge" technology.

EDIT: Depending on setup and store design, and WBS tapping positions, it may "just work" if setup as a thermosyphon.

Any increase in temp over the "base" target is more energy stored - as long as you don't go too high and start boiling the water in the stove.

The heat output of the stove isn't (or shouldn't be) all that critical as long as it's sufficiently more than your usage - the less the difference, the longer you'll have to keep it burning to heat the store. If you are out all day then that's going to restrict your scope for long running times - so a small output will restrict your ability to heat the store. Only the OP knows how big the house is, how hot he's likely to want it, and how much heat it's likely to need.
I'm assuming that running a boiler off the back of the stove is going to reduce the heat given out to the room ? Or is it normally a matter of having to feed in more fuel to get the extra heat out ?

At the other end of the equation, the OP can trade required temp vs radiator size. Bigger rads will heat with cooler water and so allow more useful heat to be extracted - but then it will take longer to reheat when the stove is lit. Combining bigger rads with higher store temperatures would mean more useful heat stored. Eg, dropping the store from (say) 90˚ to 30˚ would give you double the amount of heat extracted compared to the figured I gave (as an indication) above. But then you have twice as much to put back.

 

[DeltaT2]

Grossly over-engineered as always Simon, this Villager stove has only a peak output of 14KW, that's twice a day; once when heating up & once when cooling down. Like all stoves they rarely reach their peak output & you'll find 14KW is only on some grades of house coal.

Hmm, open vented tank of water with a coil in it is "over engineered"

It can be made as simple or complex as required. If the OP wants something that's at least semi automatic then some controls will be needed - but if he's happy to drive it manually (and take the consequences for not getting it right) then he can do so. That's his choice to make, I was merely pointing out the basics and some options - many will be happy driving the system manually, for many, automation is required for a reasonable WAF.

From the way the question was asked, the OP has just been told "won't work" without having been given any useful information to see why - or even if the statement is true. I was just trying to give him some information so he can look at his situation and figure out what might, or might no, work for him.

As this is not a condensing gas boiler, you can with the wood stove, heat a store to 80 C or even 90 C.

However, adjustment of the heat output is usually a matter of manual hit or miss and not always compatible with CH needs.

Yes, I nearly wrote that, but didn't want to put too much in at one go.

If I were doing this for myself, I'd do it as :

CH tapping from somewhere part way down store, TMV (mixing CH return with store water) to control CH flow temp, TRV on every rad (and no room stat), and modulating pump (eg Grundfoss Alpha 2L) - and run the pump on a time switch. That's actually what I have in the flat.

WBS on loop with TRV so that water circulates in the stove until it's a certain temp and then diverts to top of store. This would give "top down" reheat (ie give you good DHW fairly quickly). Once the store is nearly fully heated, then you'll end up with full circulation round the WBS and store and heat the entire store to a higher temp. As I say, it's not my area but I believe there are units designed specifically for this, and including safety features like check valves that will allow thermo syphoning if the pump isn't running. Obviously, some form of heat dump is needed, I'm not qualified to say what that needs to be or how it's controlled - but none of this is "new and bleeding edge" technology.

EDIT: Depending on setup and store design, and WBS tapping positions, it may "just work" if setup as a thermosyphon.

Any increase in temp over the "base" target is more energy stored - as long as you don't go too high and start boiling the water in the stove.

The heat output of the stove isn't (or shouldn't be) all that critical as long as it's sufficiently more than your usage - the less the difference, the longer you'll have to keep it burning to heat the store. If you are out all day then that's going to restrict your scope for long running times - so a small output will restrict your ability to heat the store. Only the OP knows how big the house is, how hot he's likely to want it, and how much heat it's likely to need.
I'm assuming that running a boiler off the back of the stove is going to reduce the heat given out to the room ? Or is it normally a matter of having to feed in more fuel to get the extra heat out ?

At the other end of the equation, the OP can trade required temp vs radiator size. Bigger rads will heat with cooler water and so allow more useful heat to be extracted - but then it will take longer to reheat when the stove is lit. Combining bigger rads with higher store temperatures would mean more useful heat stored. Eg, dropping the store from (say) 90˚ to 30˚ would give you double the amount of heat extracted compared to the figured I gave (as an indication) above. But then you have twice as much to put back.

Again grossly over-engineered & will have a large effect on the way this appliance controls itself, you clearly have no experience with this type of installation.

 

[SimonH2]

Two posts from DeltaT2, both of them offering nothing useful, both criticising the offering of information intended to allow the OP to decide for himself what may or may not work - or at least be able to quiz those you are trying to sell him one thing or another and find out if they actually know anything, or are just being like certain plumbers here who will slag off anything they don't understand while pretending that the reasons is anything but lack of knowledge.

We don't know enough about the OPs situation. It seems that DeltaT2 doesn't like any sort of controls. The OP may have "another half" who has a low acceptance level of systems that have to be carefully handled for them to work - ie anything without at least some automation may well not be acceptable. "Sorry, no shower this morning dear, you didn't go through the right evening process" might not cut it Only the OP can assess these questions, and it makes sense if he can do so with some ideas of what's practical and what's not.

It might be that all the OP needs is a rad loop of the back of the stove and a coil in the DHW cylinder - but even that will need some controls to work properly, and at the very least a pump (or is using a pump at all grossly over engineering it ?).

 

[LondonChap]

The OP did say what he wanted in answering me. He wanted to keep the electric unvented cylinder and run the rads from the solid fuel stove. All he approaches in France are saying that he needs a cylinder to run the stove. And that appears what all area saying here as well.

So, he is best using a thermal store cylinder, as he is in a remote location, that can accept multi-fuels and solar ready for the future. Also the hot water will be heated by cheaper solid fuel (wood, etc) with electric backup for the hot water and even the rads if he wants that as well.

Can the stove heat rads directly? Why not? If the rads are pumped, the system vented and proper overheat provision installed what is the problem?

 

[DeltaT2]

Two posts from DeltaT2, both of them offering nothing useful, both criticising the offering of information intended to allow the OP to decide for himself what may or may not work - or at least be able to quiz those you are trying to sell him one thing or another and find out if they actually know anything, or are just being like certain plumbers here who will slag off anything they don't understand while pretending that the reasons is anything but lack of knowledge.

We don't know enough about the OPs situation. It seems that DeltaT2 doesn't like any sort of controls. The OP may have "another half" who has a low acceptance level of systems that have to be carefully handled for them to work - ie anything without at least some automation may well not be acceptable. "Sorry, no shower this morning dear, you didn't go through the right evening process" might not cut it Only the OP can assess these questions, and it makes sense if he can do so with some ideas of what's practical and what's not.

It might be that all the OP needs is a rad loop of the back of the stove and a coil in the DHW cylinder - but even that will need some controls to work properly, and at the very least a pump (or is using a pump at all grossly over engineering it ?).

But your extensive drivel is all about electrical controls & a multifuel stove. With the number of Power Cuts in rural France, the system should be 'gravity' based as much as possible. So I still stand by my educated opinion; that you don't know what you're are talking about.

 

[SimonH2]

But your extensive drivel

That is offensive, please moderate your comments a bit.

is all about electrical controls & a multifuel stove.

I never mentioned multifuel - not once. You appear to be reading things you want me to have written, not what I actually wrote.

With the number of Power Cuts in rural France, the system should be 'gravity' based as much as possible.

Did I not also mention gravity ?

What the OP needs depends on his arrangement - something I keep pointing out.
IF his setup is that he can run the stove and store on a thermo syphon then that's fine. If his arrangement is such that it's not practical then he'll need a pumped system. If you actually read what I wrote, you will also see that in talking about a pumped primary, I did mention that there are control blocks designed specifically for this sort of thing and which include the requisite ability to thermosyphon when there isn't power for the pump.

And I'll throw a question back at you. If having anything electrical is so bad, could you please explain how to run a radiator system with no pump - and no you cannot design the layout of the house to make a thermosyphon work, you have to work with the constraints of a "normal" layout where it's not going to syphon efficiently on it's own.

Now, are you actually going to add anything constructive to the thread, or just keeping adding useless "you're a ****ing idiot" posts that add nothing useful ? Your count is now 3 posts, 0 of them containing anything useful to the OP.

 

[DeltaT2]

But your extensive drivel

That is offensive, please moderate your comments a bit.

is all about electrical controls & a multifuel stove.

I never mentioned multifuel - not once. You appear to be reading things you want me to have written, not what I actually wrote.

With the number of Power Cuts in rural France, the system should be 'gravity' based as much as possible.

Did I not also mention gravity ?

What the OP needs depends on his arrangement - something I keep pointing out.
IF his setup is that he can run the stove and store on a thermo syphon then that's fine. If his arrangement is such that it's not practical then he'll need a pumped system. If you actually read what I wrote, you will also see that in talking about a pumped primary, I did mention that there are control blocks designed specifically for this sort of thing and which include the requisite ability to thermosyphon when there isn't power for the pump.

And I'll throw a question back at you. If having anything electrical is so bad, could you please explain how to run a radiator system with no pump - and no you cannot design the layout of the house to make a thermosyphon work, you have to work with the constraints of a "normal" layout where it's not going to syphon efficiently on it's own.
.

Of course you can design & install any system to operate on a complete gravity system, both HW & Radiators; again you're showing your complete lack of practical experience. Oh you can write well, but it's all over-engineered drivel. How do you think systems were installed before circulation pumps were in common use??.............

 

[LondonChap]

Of course you can design & install any system to operate on a complete gravity system, both HW & Radiators;

That is not 100% true. The heat source has to be below the radiators and cylinder and horizontal pipe runs not very long at all, otherwise it does not work. That I do know.

 

[SimonH2]

Now, if you want to show yourself up then carry on responding to what I didn't write. If you want to be useful, then try writing something positive instead of looking for ways to slag people off.

I didn't say you can't have a gravity system IF your building layout is suitable. In fact I specifically have allowed for that if you care to read what I wrote.

But I strongly suspect that the OPs building wasn't conveniently designed to make a fully gravity system practical. I don't know, you don't know (we haven't been given any specifics about the building), and the OP won't know either based on your posting nothing but complaints about what other people are writing.
Note that word, practical - it may well be possible to design a system that works with next to no height in which to create the pressure difference that will drive the syphon; but if that then requires pipes of a size and/or layout which can't reasonably be fitted in the property then it's not a practical system.

We don't even know that the only location for the store is above the stove. We can guess that it probably is, but we don't know. Now, can you explain how to make a system that will thermosyphon to a cylinder that's below the heat source ?


As an aside, non-pumped systems (and the difficulty of finding a competent plumber) came up in conversation at home only the other day.

 

[DeltaT2]

Of course you can design & install any system to operate on a complete gravity system, both HW & Radiators;

That is not 100% true. The heat source has to be below the radiators and cylinder and horizontal pipe runs not very long at all, otherwise it does not work. That I do know.

B.llox mate, you don't know what you're talking about either.