Replacement thermal store - advice needed

[cushlow]

Hello! First post here and advice needed! I installed a 210 litre RM Cylinders thermal store into my house 9 months ago with a 8kW solid fuel stove and underfloor heating. Also connections for a future GS heat pump and solar panels.

Last Thursday I drained it down and replaced the stove with a 20kW stove then refilled the system and fired it up about 6pm, everything was fine. At midnight I checked it and noticed a drip, put a bucket under it and thought I'd sort it in the morning - obviously! At 5am I was woken by running water and it was pishing out. Water was welling up near the bottom solar connection from under the insulation implying a large leak at the bottom seam of the cylinder.

On contacting the wholesaler they were helpful and its likely it will be replaced without too much argument. But it will take at least 2 weeks without heating or hot water so fan heaters and round to friends houses for showers...

My questions:
- I'm doubting the quality of this cylinder and wondering if I should get a stainless/glass pressure rated cylinder as I don't want to repeat this replacement in 10 years time
- should I run the underfloor heating through a coil rather than directly off the thermal store to avoid the risk of heating system sludge accumulating in my UF heating tubes in the floor?

Any advice greatly appreciated

 

[Ermintrude]

Fit the best and then forget about it ACV.

 

[Reg Man]

May be RM need to comment, we have had a few negative comments about RM hot water vessel's on this site. But It would be unfair to make any judgement as I believe they are reasonable company.

 

[Roger Welton]

I'm not the best person to say, but 210l sounds a little small for a 20kW stove

 

[Ermintrude]

SLME 600l ?

 

[cushlow]

Thanks guys for your comments. I know ACV and they're good, but will give RM the benefit of the doubt. When all is working correctly size will not be an issue, but I take your point Roger. Thanks again...

 

[cushlow]

Ermintrude - 600 litres?!!

 

[Roger Welton]

Unvented on a woodburner? IIRC (and as this isn't something I do its quite likely I'm wrong!) its 45l per kW of output so looking for 900l thermal store then run a sperate HW cylinder and heating off this.

 

[Ermintrude]

There is a vented variant for this application. Think it goes up to 800l but you can cascade them.

 

[Roger Welton]

Sounds cheap

 

[Ermintrude]

£5k fitted

 

[cushlow]

No - its vented - through the thermal store. The stove gravitates to the cylinder, F&R rising all the way, and the water in the cylinder is the same water as that passing through the stove (just to clarify). There is an open vent out the top of the cylinder to F&E tank and cold feed back. Mains cold passes into a coil in the top of the cylinder and hot comes out: this is mixed with cold at the cylinder (to prevent scalding) then tempered hot goes off to the house. There is also a solar coil in the bottom of the cylinder. From separate tappings on the cylinder circuits go off to underfloor heating (pump operated by cylinder stat) and heating elsewhere.
As for capacity. Originally I had an 8kW (output to water) solid fuel stove. This was just not man enough to ever get the UF htg up to temp. There are also other factors in selecting a stove that may be overlooked: the stove will only ever be run at full output at heat up: once the house is warm only a small amount of fuel would be added to keep it ticking over or to keep the fire in overnight. Also the firebox on the stove is small - there is considerable extra work cutting logs for a small stove. For stove, cylinder (thermal store) and associated pipework it cost me about £3k and I installed it myself.
One other factor - this is a slow responding system with time taken to get the thermal store up to temperature and time taken to get the floor slabs warmed up. So I really needed a higher output stove. With hindsight I could have used a more lightweight for of construction and radiators, would certainly have heated quicker but I still believe the thermal store and UF htg offer some great advantages, and I was willing to put my money where my mouth is and try it all out.
BTW I tried some other slightly innovative things - airtight membrane, insulated foundations, old stone walls internally plastered with lime (breathable) and RW harvesting.
I think now if RM will replace my thermal store FOC then I'll just go with it - it is pretty miserable without htg and HW in Jan!

 

[Roger Welton]

I'll take 2

 

[Ermintrude]

Your thermal store is too small for ufh. Needs buffer to keep slabs warm!

 

[Worcester]

Thermal store way to small for a 20kW stove, as you say you 'batch' fire it and the store should be sized to take a full load, the recommended store / buffer size is 50ltrs/kW. 20kW store recommended size = 20 x 50l = 1000L the poor performance is almost certainly the result of an undersized store. You'll also need to put a lot of heat in initially to build up the thermal store capability of the ufh / floor slabs.

With UFH, where is the sludge going to come from? it usually comes from the radiators more than anywhere else.

 

[cushlow]

Thanks you guys I really need to think this one through. It never occurred to me the thermal store may be too small. And if it was 1,000 litres it would certainly take a bit longer to heat up not to mention space and cost.

UFH does not "need" a thermal store: most UFH systems do not have one. Similarly stoves do not "need" a thermal store, again, most stove systems do not have one (beyond the capacity of the hot water cylinder.) If the system gets too hot then it boils until the heat is dissipated.

I piped it such that if it boils it vents up the gravity flow from the stove into the thermal store then through the vent from the top of the store and discharges over the F&E tank. I could rearrange it so the vent comes directly off the flow from the stove. Boiling it after all must put pressure shocks into what is a weak cylinder. Perhaps this is what caused the failure of the cylinder at a seam at the bottom. Perhaps the stove should be piped through a coil in the cylinder. I can easily change the vent: but changing the cylinder to one much bigger would be major surgery.

Another factor is that the 20kW stove rating is likely for burning coal. In practice this stove will be burning timber, (not completely dry) so more like 12kW when blazing strongly. And a lot less when banked up overnight.

When the UFH pump comes on at 60C (stat half way up the cylinder) there is no chance of boiling: within about 10 minutes the pump has reduced the temp by about 5C and the pump switches off. It then took the original 8kW output stove at least 30 minutes to raise the temp such that the UFH pump ran again. Timings approx! And the system only ever succeeded in warming up 2 UFH zones out of 6 with the stove running continuously for weeks on coal. But the floor slabs are 4" thick conc, about 16m2 area each, and clearly have a huge thermal capacity.

I liked the idea of a thermal store but the reasons I chose this system are more for pressurised hot water and the convenience of a neutral point for connecting different heat inputs - stove, future solar and ground source heat pump.

Sludge - the stove is the source. Otherwise the system is copper, plastic and pex pipe.

A final thought, I've always been a big fan of thermal storage and used it in the structure of the house. It is a 100+ yr old cottage with timber frame extension with block partitions and conc floors, so lots of thermal storage there. And highly insulated. It works well, even if the fire goes out overnight it is hardly any cooler in the morning. But this is energy expensive - a highly insulated portacabin would use less energy (but be cold in the morn!)