Biomass log boiler, accumulator tank problems

a1154 · 14 December 2014 at 6:19PM

Hi, I have a biomass boiler for logs, and an accumulator tank 2000 L running under floor heating and hot water for 2 houses. Its been running for several years now and i do like it, but i have never been conviced it is set up correctly and running efficiently.
I dont really have any issues with it until the weather gets sub zero and then the draw on the tank for heating is unable to cope.
It was sized correctly (apparently) but doesnt work well enough, we have had to bring in additional heating.
One odd problem is that the stratisfying (not sure how to spell that) tank is kind of balanced in favour of hot water (comes out at the top) and not heating (comes out at the middle).
I need 40 degrees to run the heating.
The top is always hot, but the middle gets cold very quickly when the outside temperature is zero.
Then thats it, need to fire the boiler. There is no way of getting hold of the hot top of the tank for heating.
Its not wasted, as you still get the energy on burning (ie its quicker to heat the whole tank) but it means we need to burn more often than we should. I think.
Anybody else got a similar setup?

Silent_Dancer · 14 December 2014 at 9:22PM

You really need to get whoever installed and design the system back to have a good look at it. It could be that the boiler is undersized, the boiler is not running efficiently, there is set up problems with the thermal store or there are problems with the underfloor.

One question though is the underfloor heating directly supplied from the accumulator?

Robwiz · 14 December 2014 at 9:27PM

Sounds like your stratification, correct spelling!

, is set up correctly – you should have a reservoir of heat for your hot water. When the weather gets cold you have to fire up the boiler and get heat into your thermal store because the extra demand from the heating will quickly use up the stored heat.

When I looked into log boilers it was clear that when it's really cold there's a need to burn two or even three times a day. The upside of a log boiler is the simplicity and reliability, the downside is that when it's cold you have to work harder loading it and batch firing.

a1154 · 15 December 2014 at 4:15PM

Hi silent dancer, not sure what you mean by directly supplied - it is via a heat exchange unit. We decided on this system and had to bully the installer a bit, he wasnt really confident. We are in a remote(ish) area and choices are limited.
Hi Rob, im grumbling about having to burn every day when there is still heat left in the tank. It seems inflexible. There is always hot water to be had but i suppose i want to use the energy where i want it. I dont know how you would design it better but it could do with a rethink.
I have always been surprised how quicky the middle tank goes cold when its freezing out. I guess for most of the year it works prtty well though.

Robwiz · 15 December 2014 at 4:52PM

Hi a1154

If you're only burning once per day when it's cold then I'd say your system has been designed pretty well! In my view it's a good thing that you still have hot water for washing when the heating reservoir is used up. All the heat from that middle tank has gone into the two houses.

Smiley_Dan · 15 December 2014 at 5:01PM

a1154 wrote: »

I have always been surprised how quicky the middle tank goes cold when its freezing out. I guess for most of the year it works prtty well though.

Are these types of tank direct (you use the actual water in the tank for DHW) or indirect (DHW comes through a coil)?

I notice this too on our simple direct hot cylinder heated by immersion 1/3 of the way down. A different system, but I think the same effect. The trouble is that you have stratification but at this time of year the water coming in at the bottom of the tank is very cold, and so heat conducts down the tank quicker.

I might be getting my terms mixed up...

Robwiz · 15 December 2014 at 6:47PM

Smiley_Dan wrote: »

Are these types of tank direct (you use the actual water in the tank for DHW) or indirect (DHW comes through a coil)?

I notice this too on our simple direct hot cylinder heated by immersion 1/3 of the way down. A different system, but I think the same effect. The trouble is that you have stratification but at this time of year the water coming in at the bottom of the tank is very cold, and so heat conducts down the tank quicker.

I might be getting my terms mixed up...

Usually the arrangement is that the incoming mains water for DHW passes through a coil in the top third of the thermal store, heating up as it passes through. The water in the thermal store is circulated around the radiator circuit – often with a blend valve on the return.

So many ways of doing it though.

Biomass log boiler, accumulator tank problems

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