Thermal store question

[GoferBlue1947]

I have a 25kW wood pellet boiler that provides DHW and runs the C/H system. The C/H system is open vented i.e. has the traditional F&E tank in the attic. A 400 litre thermal store forms part of the C/H system. IMO the system was badly designed in as much as the 400L. thermal store is a waste of space! I understand the concept of having a TS acting as a buffer tank so that the wood pellet boiler is not constantly cycling on/off; however 400L is far too small for this purpose and one should be looking at 2000L or so. I have neither the space nor the cash for a 2000L TS!

My question is: should I just get rid of the TS altogether and would it really have any detrimental effect on the boiler if I did?

You are probably thinking – why get rid of the TS? Well several reasons:

1. I would like to change to a pressurised system so I can get rid of the F&E tank and all the issues that come with having that. The TS cannot be pressurised.
2. It costs a small fortune in cleanser and corrosion inhibitor because of the large volume of the TS (it is a direct set-up BTW), which is probably more than the volume of the rest of the system put together.
3. I cannot pressure flush the system because of the TS.
4. There is currently a problem with air/gas in the C/H system, so hopefully moving to a pressurised system would help with that too (after giving the system a good clean out).

Any thoughts most welcome. Thanks.

 

[endecotp]

400L is far too small for this purpose and one should be looking at 2000L or so.

Why do you say that?

If the temperature difference were 20 C:

400 kg x 4 kJ/kgC x 20 C = 32 MJ
32 MJ / 24 kW = 1333 seconds = 22 minutes.

That sounds like a useful amount to me.
What do the boiler manufacturers recommend for efficient operation?

 

[Dan Robinson]

Why do you say that?

If the temperature difference were 20 C:

400 kg x 4 kJ/kgC x 20 C = 32 MJ
32 MJ / 24 kW = 1333 seconds = 22 minutes.

That sounds like a useful amount to me.
What do the boiler manufacturers recommend for efficient operation?

22 minutes is not much for a pellet burner.

 

[endecotp]

But better than nothing?

 

[Dan Robinson]

Indeed. You would be insane to get rid of the store completely. One wonders of the OP understands the operation as much as he claims.

 

[oilboffin]

My thermal store is home made by me it's made out of 8mm copper and I made it square it is 1.5mtrs square by2.5mtrs tall and it's insulated with 2 layers of kingspan it's a beast it takes the solar panels which are in a green house to keep the wind of them and a waste oil burner.it's a beast and works a treat.Bob

 

[GoferBlue1947]

Indeed. You would be insane to get rid of the store completely. One wonders of the OP understands the operation as much as he claims.

Please give reasons why I would be insane?

 

[endecotp]

Without any thermal store at all, your boiler will cycle on/off more frequently than is good for either efficiency or lifespan.

 

[GoferBlue1947]

But better than nothing?

What would be wrong with nothing? What advantage does "22 minutes worth" of heat bestow on my C/H system?

 

[GoferBlue1947]

Without any thermal store at all, your boiler will cycle on/off more frequently than is good for either efficiency or lifespan.

Sorry my post has crossed with yours in the ether! My pellet boiler can modulate from 25 kW down to 5 kW as/when necessary. Would it necessarily cycle on/off or might it just tick-over at 5 kW?

 

[Dan Robinson]

It's got nothing to do with the heating system and everything to do with the pellet boiler. It's not a gas burner, it is not designed to turn on and off.

Yes 2000 litres in not really sufficient, but as said, it is better than nothing for reasons that should be patently obvious.

 

[Roger928]

What make is your pellet boiler? The manufacturers usually advise on buffer storage capacity.
Some say multiply the manufacturer’s declared minimum heat output by 55l as per the biomass boiler calculation.

 

[GoferBlue1947]

What make is your pellet boiler? The manufacturers usually advise on buffer storage capacity.
Some say multiply the manufacturer’s declared minimum heat output by 55l as per the biomass boiler calculation.

It's an EkoHeat 2500 (25 kW). I'd have to ask them the question as there's nothing in the manual. The minimum heat output is 5 kW so that using your formula would equate to a 275 litre TS

 

[endecotp]

I know next to nothing about these boilers so my advice is to do what the manufacturers (or installers, or other experts) advise.

But since you've asked - yes modulating down obviously helps. But does it start at a high output and then modulate down after a few minutes, as some gas boilers do? If it does then you may find that it doesn't have the opportunity to modulate down before its internal thermostat turns it off. And it's also quite possible that your heat demand will sometimes be below 5 kW, if you have TRVs and most rooms are up to temperature.

 

[GoferBlue1947]

It's got nothing to do with the heating system and everything to do with the pellet boiler. It's not a gas burner, it is not designed to turn on and off.

Yes 2000 litres in not really sufficient, but as said, it is better than nothing for reasons that should be patently obvious.

I understand that it's not designed for on/off operation - far from it, as it takes an age to get up to full steam again once it has restarted. I'm asking would the boiler necessarily be cycling on/off if it runs at minimum power (5kW) - would it not simply "tick-over" until the room stat is satisfied and then turn off? Is 22 minutes of heat reserve really a significant advantage?