Hi All,

My first post so please bear with me . I joined the forum as looks like a lot of knowledge here with regard to these types of systems. Therefore hoping someone might be able to assist with the problem described below that I’m experiencing with my biomass heating system. To say I’m beyond desperate to get this fixed is probably an understatement.

In summary, I had the system installed 2 years ago and it’s never worked properly. I have a 2200 litre Akvaterm thermal store which has the following circuits i.e. flow and return:

1. For the biomass cooker in my kitchen which has a 22kW back boiler (at about 80% efficiency so say 17kW) the towel radiators in our ensuite bathroom, main family bathroom and spare bedroom ensuite are also on this circuit.
2. Central heating i.e. all other radiators in the house
3. Domestic hot water
4. For the oil boiler which I kept for redundancy / convenience

The problem I'm experiencing in the winter months (i.e when I want to use it!) is that as the temperature of the store itself gets hotter, the water in the boiler of the biomass cooker itself (32 litre volume) seems more and more prone to overheating and as a result the safety valve built in to the cooker (PRV?) is coming on and dumping the water out. The end result is that we can’t use the cooker to heat the store or we can but the fire is so small it can’t produce any meaningful heat to raise the overall temp in the store. I’m probably not describing this very well but it is like the water in the boiler of the cooker isn’t getting moved quick enough and sort of sits in the cooker.

Thankfully I got the installer to do a drawing of the system for me. See image below.



I’ve tried to rack my brains as to what is causing this. The system is pressurised so I wondered if it was possibly the issue with the pump on the cooker return (above garage Black one as described on drawing above) either not being near enough to the cooker which is in the kitchen. Or an additional pump on the cookers flow near to the cooker itself is needed to more effectively pump the now heated water in the cooker’s boiler away quicker? My other thought was is there an issue with flow rate but the circuit from cooker to store/accumulator was put in at the same time as the store and pipe work all looks to be same size however the cooker circuit also has our towel radiators in the rooms as per point 1 above so I wondered if the smaller dimension pipework on these could be causing the problem?

Any help would be really, really appreciated.

P.S. The store was actually sized with the idea of a second 40kW batch boiler being added, hence why it is so big.

What controls the pump on the return to the cooker ?

Are there any automatic controls on the cooker fire or is it all manual operation ?

Edit to ask
What temp. is the store when the cooker over heats ?

Jacket84
What is the temp. at the top of the store when the cooker starts to over heat ?

Where is the pipe stat located ?

Is the cooker connected to the store via a coil or is it direct ?

By a coil I mean does the cooker go to a coil in the tank and so being an indirect connection.

Just to be a bit clearer, it's usually* illegal in the UK and dangerous to have a solid fuel boiler on a sealed system.

The building regs for sealed systems require two lines of defence against boiling: an electric safety overheat stat that instantly switches off the burner or immersion (separate from the normal temperature controller); and a mechanical safety valve as backup. The safety valve is like the airbags on a car, shouldn't ever go off!

However, solid fires cannot be switched off electrically. So it's important to clarify if the boiler circuit is separate and open vented and exchanging heat with the TS by a coil.

When the boiler was increased from 3kW to 22kW, the safety heat leak radiators should have been increased in proportion. Without this, the boiler will overheat frequently. Towel radiators are enough for a big boiler like this.

The boiler needs to be able to circulate freely (including by gravity during a power cut), so the pipe sizes would also need to be increased in proportion to the 22kW to allow heat to move to the store fast enough.

Here's a useful guide
https://solidfuel.co.uk/pdfs/central-heating-link-up.pdf

Would really be good to get a specialist solid fuel boiler installer to check the system and modify as above.


*there are a very few solid fuel boilers that come with an automatic safety quench system that avoids the safety valve needing to open, but in this case that's not happening

@Peter_in_Hungary no its direct to the store the only coil is for the domestic hot water

@philedge 1 the original installer came a significant way from the north east to where I'm based in Cheshire to install this and has ghosted me since I attempted to get him to address the issues described, which is frustrating as the system cost me a significant sum of money to install. It is interesting however that you mention the issue of the expansion vessel as several months ago I paid for another more local installer (who I didn't realise existed prior to the install else I would have commissioned him) visited and his opinion was that it was the expansion vessel not being large enough. I have just checked the gauges and the tow for the vessel are both reading as 0 bar however the two on the tank are also reading the same. I'm assuming this might be because the store is only programmed to come on in the summer when the tank drop to 35 degrees and heats back up to 40 degrees for our hot water needs

AFAIK if its a pressurised system, there should be pressure there when the system is cold so all your gauges reading 0 is likely incorrect....but that could be a legacy from the last time the cooker boiled and blew it's contents out through the relief valve??

If your second installer thought the expansion vessel was undersized has that been remedied??

Posted By: philedgeIf your second installer thought the expansion vessel was undersized has that been remedied??

Yes, but an undersized expansion tank won't cause the cooker to overheat.

Posted By: Jacket84no its direct to the store the only coil is for the domestic hot water

Thats the cooker to store direct.
DHW on a coil and the rest direct (cooker and central heating) would be correct for the described system, so what is the expansion tank doing.

There is a pump on the flow from cooker to the store so the pipework size whilst it may not be enough for gravity circulation I would expect a pump to be able to overcome pipe size deficiencies.

Jacket84 said that the cooker starts to overheat with the store temp. at 55 deg. top and 45 deg. at the bottom should give enough capacity to takr additional heat

Jacket84 Is the pump running on max ? Is it a usual central heating pump ?

@ Peter_in_Hungary yes I think its just a central heating pump its a Grundfos one and I'm sure in the past I've changed the speed to max to see if that helped

Ah Jeez, I give up with uploading to this website.

photo??

Initial thoughts;

- I don't like using stats to control the switch on of pumps, as a fixed temp is not always appropriate. eg If the TS is 80oC, and the stat is set at 70oC, the pump will keep running, even with the oven off. I always use a differential controller. (that's not the issue here, but it's not good practice).
- the stat is on the return leg weirdly. The oven needs to heat the return pipe, initially by thermosyphon, or heat migrating up the return pipe. No likey.
- when the oven gets hot, even if the pump comes on, it will switch off quickly, as cooler water is drawn from the TS. That's going to lead to over-temp in the oven, and high pressure relief.
- the towel rad circuit could be taking all/most of the flow from the pump, if the press drop in that loop is less than the press drop thru the oven, starving the oven.

I think the stat is wrongly located, and I don't like the the towel rad influence, but to confirm that, I would do the following;

1. set the stat ON temp to 0oC, which will make the pump run all the time.
2. shut off/down the towel rad circuit. This needs to be done with an isolation valve, not at the rad valves, as it's likely a flow&bypass set-up. When you've done that successfully, the hot from the TS will flow through the oven (feel warm pipes in/out of oven), and NOT through the towel rad legs (stay cold) on flow and return at the branches with the oven.
3. Then run the oven, and keep eye on the flow and return pipes. If the flow (from the oven) gets way hotter than the return (to the oven via the pump), the flow rate is too low versus the heat input by the oven, so turn the pump speed UP. That should reduce the flow temp, to closer to the return temp.

Try the above, to at least start to ID the problem. Then we can look at resolutions.

cross posted with GreenPaddy

Jacket84 said the pump stat. is on the flow.

However doing a test run and noting pipe temps. and forcing the pump to run has to be a starting point.

The pump is "above the garage" - is that an up+over pipe loop, that can trap a pocket of air? (Ours does that)

Presume the heavy store and the kitchen cooker are down on ground floor in separate rooms, no syphon between them?

The accumulator needs to be sized for 2000+litres system volume, a standard CH system accumulator isn't enough.

Quench coil system should prevent overheat situation happening (like car brakes). It should prevent opening the safety valve - that reacts after situation is already happening (like car airbags).

Hopefully (?) it is difficult to shut off the safety heat leak radiators, eg they shouldn't have thermostatic valves.

When testing get an instant read out thermometer e.g. an IR thermometer and build a small fire only to check flow temps. on the pipes. When testing turn the pump controlling stat. to ON and then check that the pump is actually running. Physically check the pump by unscrewing the large screw on the back of the pump and check for rotation. If the pump is not rotating then turn the pump by means of the screw slot in the armature. They do get stuck sometimes. Oh and a few drops of water come out when the inspection screw is removed.

IMO careful testing with frequent monitoring during the test would not need the expansion vessel upgraded until the problem is found and a solution is planed. But be prepared to shut down the cooker or remove or douse the fire if things start to get out of hand. Increasing the expansion vessel size won't do anything for the problem, it will just stop the over pressure valve from blowing off, which should not happen if the testing is sufficiently controlled.

I still doubt that the cooker is capable of running a pressurised system as most solid fuel stoves are constructed from plate steel which is flat and won't take much pressure before bowing out of shape. Vessels designed to take pressure are almost always cylindrical. The cooker data needs checking. Akvaterm also make 2 types of stores, a 1,5 bar type for vented systems and a 3 bar for pressure systems - perhaps check this as well.