Green Building Forum - Thermal Store with Back Boiler Stove Sun, 03 May 2026 10:09:59 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301498#Comment_301498 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301498#Comment_301498 Wed, 10 May 2023 09:32:18 +0100 Dan Barlow
Its a 210L Advanced Appliances multi fuel thermal store, its connected to Ideal 40kW combi boiler which also still supplies some of the hot taps and a fairly old 14kW (5 to room, 9 to water) Hunter Herald stove. The TS runs my central heating (radiators and underfloor) and the remainder of the DHW (mainly the bath). I will use the immersion heaters too once solar is fitted.

All seems to be working generally as it should it terms of controls, when the stove is lit the pump runs and depending where the thermostats are set the boiler switches off.
However the TS is suppose to operate at 75-80 degrees according to the instructions, but im struggling to get it up to that temperature using the boiler or the stove, 60-65 is more realistic. The boiler is set on max temperature (80 degrees) but unless i run it flat out for hours on end the TS seems happier at around 65 degrees.

Im not sure if i was expecting too much from the back boiler stove but even after several hours with a good fire the output temperature is still only 60 degrees. The TS has a blending valve which controls the flow back to the stove which according to the instructions should be set at 55-60 degrees. It is a multifuel stove so maybe im not getting anywhere near the maximum output burning wood and would need to burn coal which im reluctant to do. Ive experiments with various pump speeds for the stove pump and also tried closing the valves to the heat leak radiators which is plumbed between the flow and return but im still not able to achieve more than 60 degrees.

Any help or advice would be much appreciated

Details of the Thermal Store:

https://www.youtube.com/watch?v=UX1O1kcvo1Q&t=13s

https://www.advanceappliances.co.uk/product/70-multi-fuel-universal-thermal-store-sfuts/]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301499#Comment_301499 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301499#Comment_301499 Wed, 10 May 2023 13:23:16 +0100 Peter_in_Hungary
How much of the system was new,
Thermal store
Combi gas boiler
Wood burning stove

All of the above or just some as add ons?

You say the thermal store (TS) has a blending valve which controls the flow back to the stove - normally these are fitted to the stove usually at the back of the stove or at least very close to it. (What type is the blending valve)

A diagram would help.

Either of your combi or stove should heat your TS on their own. Whilst with poor firewood the stove may take some time e.g. lifting the TS from 15deg to 80 deg with the stove producing 6kW would take about 6 hours.

If the combi boiler is running at 40kW (or even close) then that heat must be going somewhere e.g. in the above example the combi would heat the TS in about 1/2 an hour at full chat.

Is there any load taking heat from the TS whilst you are trying to get it to 80 deg ?

What are the pipe temps
leaving and returning to the stove
leaving and returning to the combi
Ditto the TS
(An IR thermometer is useful for this).

Sorry it is all questions and no answers but a bit more info is needed]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301501#Comment_301501 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301501#Comment_301501 Wed, 10 May 2023 19:22:44 +0100 Dan Barlow The boiler is 2yrs old, the thermal store is new, the stove was bought second hand and must be around 8-10years old
The install is a combination of professional and DIY!! I’ve done a lot of the work but with input from professionals.
Did you see the video for the TS showing how the blending valve works?
I’m trying to heat the store with no load.
I will get an IR thermometer as currently just working from the temp gauge on the store and some clip on pipe gauges. All pipe work associated with the stove doesn’t really get above 60-65 degrees when it’s been lit for several hours. I need to do some work insulating pipe work as I suspect I’m losing heat in the combi pipe work.
The back boiler has 4 tappings and I’ve currently got the flow into the bottom and return opposite top.]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301506#Comment_301506 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301506#Comment_301506 Wed, 10 May 2023 23:25:16 +0100 WillInAberdeen
In summary (sorry for brevity):

35mm pipes are needed for a 40kW boiler, otherwise it needs to be restricted to lower kW;

Boilers expect the return pipe to be 60⁰ in order to make flow pipe 80⁰, so will struggle if TS is already hotter than 60⁰;

Boiler will run much more efficiently in condensing mode if you target lower temperatures, there's no need to store heat from a combi boiler;

5+9kW log burner is equivalent to feeding in another log every 3-5 minutes - is it actually getting through that many?

'Heat leak' safety radiator on log stove?

Measure all the pipe temperatures yourself.]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301507#Comment_301507 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301507#Comment_301507 Thu, 11 May 2023 00:12:46 +0100 Peter_in_Hungary Posted By: Dan BarlowDid you see the video for the TS showing how the blending valve works?
Seen that now, My guess is that they put it there as the only way to guarantee one is fitted. IMO better placed at the stove but there's no choice now.

Posted By: Dan BarlowI will get an IR thermometer as currently just working from the temp gauge on the store and some clip on pipe gauges.

Clip ons work, I suggested an IR thermometer as it saves buying lots of clip ons. If you have the clip ons all around then no need for an IR thermometer. (although I find an IR thermometer useful for all sorts of things).

Posted By: Dan BarlowI need to do some work insulating pipe work as I suspect I’m losing heat in the combi pipe work.

uninsulated pipes loose 60W/m at 55 deg and 40W/m at 40 deg for a 22mm pipe, 28mm pipe figures are 80W/m and 50W/m. you can calculate the losses from your pipes.

Posted By: Dan BarlowThe back boiler has 4 tappings and I’ve currently got the flow into the bottom and return opposite top.

The way I am reading this its a bit confusing to me.
The flow to the TS should be at the top and the return from the TS to the bottom of the stove.

Posted By: Dan BarlowThe install is a combination of professional and DIY!! I’ve done a lot of the work but with input from professionals.

The problem with combined DIY and professional work is the apportionment of blame when things go wrong. Unless the professionals are good and have a care it is easy for them to walk away blaming the other guy (you) when things go wrong. - Have you asked them for their opinion as to why the TS fails to get up to temperature?

Posted By: WillInAberdeenthere's no need to store heat from a combi boiler;

Correct but I don't see a 250 ltr tank as a useful TS for heating (mine is 2000 lts) This one is more of a way of combining various heat sources to do one job prioritising cheap or renewable heat over fossil fuel heat.

Posted By: WillInAberdeen'Heat leak' safety radiator on log stove?

Yes - not really needed as as if things start to get over hot the assumption I would make is that the intelligent owner would stop piling on the logs.

When the combi is trying to heat the TS is is running all the time or is it cycling on and off?

The overheat thermostats could be stopping the supply of heat to the TS if they are faulty or wired incorrectly. To what temp. are they set.]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301508#Comment_301508 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301508#Comment_301508 Thu, 11 May 2023 07:05:34 +0100 Dan Barlow Pipe work to the boiler and stove is all 28mm so that could be one cause.
I’d normally fill the stove with wood and then leave it for an hour or two. I suspect it’s not producing anywhere near the maximum output as I’m burning wood not coal.
Yes the heat leak radiator is on the stove circuit as required by the TS manufacturer. Currently the valves are turned off though.
I plan to contact the manufacturer for advice. The problem this time of year is we aren’t using the TS as the CH is not on and our boiler covers our DHW requirements, I’m not lighting the stove as it’s not necessary at the min so I’d be heating the TS just to experiment with temperatures etc which isn’t actually cost effective!!]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301510#Comment_301510 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301510#Comment_301510 Thu, 11 May 2023 08:46:10 +0100 Peter_in_Hungary Posted By: Dan BarlowI’d normally fill the stove with wood and then leave it for an hour or two. I suspect it’s not producing anywhere near the maximum output as I’m burning wood not coal.
Stove outputs quoted by manufactures are typically the best achievable to give their stoves consumer appeal. Which means that the rated output will be with good quality firewood at mid-burn. when just lit, after reloading or near the end of a load the output will be very much reduced.

Firewood is very variable in its heating capabilities but a reasonable calculation is about 3.5 kWh / kg. Even if you burn coal you will still be subject to low output on start up, at reloading and towards the end of the load.

Typical household coal is about 7 kWh / kg. which will give longer times between refuelling and more time at max output.

Don't expect to be able to run your stove at 9 kW to water for very long because at that rate you will be putting 5 kW to air and the room in which the stove sits will become unbearably hot after an hour or two. This is one of the problems with wood stoves that have back boilers.

Your combi should get the TS up to temp. fairly quickly. In your place I would be addressing the issue of the combi not heating the TS properly as a first step as it is easier to run without putting unwanted heat into the house. Once you have cracked this one then address the stove problem - you may find the same reasons apply.

With the combi running measure the pipe temps and note the burn cycling and pump run times. Most combi boilers have an integrated CH pump and I note that the TS also has a pump for the gas boiler circulation. Are you inadvertently using 2 pumps or is one out of the circuit.

IMO you should try to the resolve the issue now whilst the installation is new. It is much better to talk to the manufactures and professionals now rather than in the autumn when you have to start off by saying "Do you remember that system you installed months ago............."]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301511#Comment_301511 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301511#Comment_301511 Thu, 11 May 2023 09:01:05 +0100 WillInAberdeen
The overheat thermostat has no effect for log stoves. If it did trigger it should lock out the immersion or gas boiler until reset with a screwdriver. I had a scare and now test mine each year with a kettle.

Suggest to set the timers so that the stove or solar get first chance to heat up the store, if they haven't done so by the time when heat is needed, then combi tops it up to 50⁰-60⁰ condensing mode.

Suggest to measure temperature at both ends of every pipe using the same thermometer, to find where the heat goes, and to check all the stats and clip ons are all calibrated the same. IR thermometer well worth having as Peter said, or one of those digital cooking thermometers with a spike probe, hose-clip that onto pipe and wrap insulation over.]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301513#Comment_301513 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301513#Comment_301513 Thu, 11 May 2023 12:12:32 +0100 Dan Barlow The boiler circuit is just using the boiler pump, the pump on the TS store has been removed.
It is a sealed system with expansion vessel (no F&E tank).
I will ensure that the heat leak radiator is not left isolated this was done to eliminate this from soaking up all of the stoves output.

I clearly need to do a little more investigating with pipe temperatures etc.
I take on board your comment about doing it now rather than in the autumn when nobody can remember the install and we are desperate for the heating!]]> Thermal Store with Back Boiler Stove https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301530#Comment_301530 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17832&Focus=301530#Comment_301530 Fri, 12 May 2023 08:33:58 +0100 GreenPaddy
I design these sort of systems. If you can draw a schematic diagram of what is installed, happy to try to help you resolve this. Don't need a beautiful CAD drawing, hand drawn does the same job. Show the TS, log boiler, gas boiler, rads, DHW outlet, pumps, control items, what you believe to be the water flow direction, temperature indicators.

Wordy descriptions can lead to misinterpretations, plus take too long to read through. Once we're certain the schematic is an accurate reflection, then a solution will appear. I've done dozens of TS's design with all manner of heat source inputs, so I'm sure this can be sorted, if there even is a problem.]]>