Log burner with boiler connections and controls

[overlander matt]

Are there any members with a log burner with integrated back boiler that could share a few details of their set up, connections and controls.

I've had a lot of help here, particularly Fernicarry, to get our system of thermal stores, underfloor heating, radiator zones and solar thermal installed and operational. There's just one more element to go - an Arada Stratford EB12 boiler stove which was purchased some time ago before these devices were phased out. I'm finally working on the room where the stove will be positioned and looking forward to seeing some heat from it soon!



I've attached a photo showing the room the radiator store is in (excuse the mess). The stove will be fitted on the other side of the wall shown in the photo with the majority of the pipework. It's on the same level so a pump will be needed. I've briefly looked at ladomat type pump arrangement but was advised it is overkill for a small boiler...

Any advice or suggestions would be very much appreciated. Thanks.

 

[fernicarry]

Hi Matt, mess? That's a very neat install.

The big problem I have with the wood stove is that if the gas boiler is on at the same time then it (gas) tends to keep the store topped up to its set temperature with ease due to its much higher power output. Then the wood stove can't get rid of the heat in its water and this can mean that the air intake, which is thermostatically controlled by the water temperature on the EB12, closes down and the fire goes very low and smoky. Because of this I usually turn the gas boiler off once I've had the stove lit for a bit but of course the stove on its own can't heat the whole house so its a bit of a catch 22 -- smoky stove or rest of house cold...

Ideally what you want is for the stove to work exclusively on the cooler water at the bottom of the tank, and arrange for the gas to only come on to top up the top of the tank as needed. So long as the return temperature from your radiator loop is lower than the stove flow temperature there will always be some work for it to do. To separate the two you would take the gas return from the middle of the tank rather than the bottom (some stores switch between both points anyway) and have the stats controlling it only see the top of the tank. You could detect that the stove is running with a pipe stat and switch the gas boiler into its alternative mode with zone valves and relays. As a simpler solution I've considered just using a pipe stat to hold off the gas boiler when the stove return leg is above a threshold but haven't done that yet, the manual option being good enough (I can control the gas boiler from the kitchen) for the small number of days I light the stove.

Its a general problem really as we are being directed away from combustion heating but realistically period properties need the high flow temps to meet peak demand that only combustion boilers can provide. A hybrid system where a HP provides the first temperature gain (whilst running at a decent COP) and then gas tops up the rest when demand is high would be a possiblity (albeit expensive and complicated...). However, you have the same problem of the two heat sources competing, one low grade (HP or wood -- lower flow temperature, lower instantaneous output) which needs to be given time and space to work and the other high grade (the gas) which easily dominates if not controlled.

In terms of my actual install, I do have a loading valve which does seems to spend a lot of its time in bypass unless you run the stove quite hard to get its water warm enough for the valve to divert into the store. My stove is on quite a long run from the front of the house to the store at the back of the house so that might be increasing the warm up time which is quite long in any case as its a big stove. On the other hand if it wasn't there and the store was already up to temperature then the stove would immediately see a high return temperature which could exacerbate the above problem.

The pump in the loading valve is controlled by a flue stat on the stove and also by a pipe stat close to the stove. The latter is a sort of fall-back in case the fire is too low for the flue stat but over time its still heating the water. I actually find the flue stat to be quite unreliable but the installers bodged that by just strapping it onto the outside of the flue, ask your installer for a better solution. The two stat bodies sit off to the side in a recessed cupboard with their capillary wires fed behind the plaster in copper pipes.

I think you have two tanks, right? That might give you more options for separating the heat sources so that they play more nicely together.

 

[a twig]

This might be a red herring but there are systems which are designed for the two, my parents had a Honeywell Sundial system installed in the 80s, which auto selected between the back boiler on their fire and demanding gas heat as per demand on hot water and CH.

Honeywell no longer sell it or the parts or provide any support whatsoever but there is a company that sells a better system now as an alternative, which I really must get round to installing in my parents' house
H2 Panel

With the H2 panel the solid fuel boiler can be used on its own and the automatic boiler stays cold or you can use the automatic boiler and a solid fuel appliance stays cold.

If you use both the solid fuel boiler and the automatic boiler at the same time the heat from the solid fuel appliance is injected into the return coming from the heating system going back to the automatic boiler and therefore pre heats the return water thus the automatic boiler only has to top up the return water to the necessary temperature for the flow.

If the flow temperature reaches a certain temperature which is set on the panel it switches off the automatic boiler and the solid fuel appliance carries on heating the hot water and central heating system by itself. This means that if the automatic boiler is a condensing type the condensing principle does not get compromised because the automatic boiler is switched off long before the condensing mode is prevented by the high temperature return.

 

[fernicarry]

Interesting that a device exists to solve this problem. I wonder how much it is.

I think its principle of operation may boil (sic.) down to basically what I'm imagining with the thermal store. The automatic (gas) boiler return tapping from the store is above the solid fuel (wood stove) boiler flow tapping so effectively the solid fuel is pre-heating the automatic return.

I think if you already have a thermal store that combines heating and HW then it will be easier to roll your own functionality.

 

[a twig]

The whole H2 panel was about £500 last time I got a quote 18 months or so ago. You could definitely roll your own for less but for me having a properly thought through system, properly documented and with compliant safety devices, overheat/pressure protection and failsafes is worth the money

There’s a control panel diagram here that makes it clear

 

[overlander matt]

Thanks for the replies! In our case we have two thermal stores - the one in the picture is the 300L winter store that feeds the radiators and preheats the incoming mains cold water for DHW. It is heated by the gas boiler and, hopefully soon, with the wood boiler. When we decide not to use the radiator heating we just turn the isolation valves on the boiler feed and return to bypass the winter store. It's not at all sophisticated but it works! The other store is used for hot water all year round and supplies the UFH in winter. We added solar thermal to it this year which has worked out ok. In the summer, when the store is just needed for DFH the gas boiler remains off, the solar thermal heats the 400L store to approx 60deg on a good day which provides us with enough for showering. If we need more then the boiler goes on for an extra hour but generally a 10min blast gets the store up to temperature. I had anticipated there being a similar manual element to the use of the boiler stove so interesting to hear about the H2 panel.

In terms of basic controls, a pump that is activated by a thermostat on the output of the boiler seems to be as simple as it gets. Is this how your system works Alan? A load valve or laddomat seems to provide a bit more control over the water temperature being returned to the store but I'm not sure how necessary this would be in our case where the boiler capacity is small and pipe runs are fairly short.

The H2 arrangement looks interesting. It might have been useful to know about that a couple of years ago when we were setting out the layout. I would struggle to fit that in the boiler room but it looks interesting and maybe the concept could be adapted for use in our case.

To start with I think I would prefer to go with a simpler arrangement to see how well it works and modify it if necessary once we get to use it and work out the limitations.

Thanks again.

 

[fernicarry]

In terms of basic controls, a pump that is activated by a thermostat on the output of the boiler seems to be as simple as it gets.

Yes, basically. My pump is integral to the loading valve but it could be a standalone pump. The value of the loading value is for black starting the store using just the stove. It lets the stove get its own water jacket up to temperature as quickly as it can then slowly blends in the cold return from the store at a rate that the stove can maintain without going stone cold again (bad for the stove).

The only time I've done this is when the gas boiler was out of action. The wood burner was a life saver but it took an immense amount of stoking to get the store warm enough for a quick shower!

I think you will still need to consider some way of throttling back the gas boiler. If the gas boiler is heating the store to a higher temp than the wood burner can reach then the store will actually be heating the wood burner's boiler and that will definitely upset the air intake on the EB12. Will depend on your tapping points and the temperature gradient in the store.

Do you have two stats on the store to control the boiler, one top and one bottom (or middle-ish)? You could back off the bottom stat to just below the stove's flow temperature to stop the boiler reaching that temp and give the stove some "headroom". Assumption is that the bottom stat is reading roughly the flow temperature of the stove. If the store can't provide enough temperature gradient then the top stat may never be satisfied and the boiler will run in short bursts (driven only by the bottom stat, not an efficient mode for it), so you might need to reduce the top one at the same time. Bit of a faff to have to run out to the store every time you light the stove but lets you establish the parameters with a view to automating. I'll try to remember to have a tinker with this myself next time I light the stove. We're getting down to low single figures here some evenings....

 

[overlander matt]

Thanks Alan. I suspect that we will also normally have a warm store that is topped-up by the wood boiler rather than heat up the store from cold unless there was some form of emergency... In this case would you still go with a loading valve? Could I ask what you use? I believe you have a larger boiler than ours - we had to go a bit smaller as there's a wooden fire surround. Apologies that the questions are fairly basic - I've found that it's not until the system gets installed that you fully understand how it all works! There was a lot of procrastination over the solar thermal and now it's time for the wood boiler!

I found an arrangement below sold by Advance Appliances for their stores. Is this a similar arrangement to what you have?



In terms of our setup with two stores, each store has two stats for the gas boiler - one at the top and one where the boiler feeds the store (about half way up). The problem with the current arrangement of two stores is that the boiler flow passes firstly into the primary store and then into the winter store whether the winter store is calling for it or not. I never got round to installing any temperature controlled valves to bypass the winter store - there are manual isolation valves that do this job outside of winter. I wonder whether a solution to throttling back the gas boiler to the winter store is to use a three port valve to bypass the store under certain conditions ie when the stove pump is running or the temperature in the store is above a certain temperature.

At this stage I just don't know how hot the boiler stove will heat the store. The EB12 isn't exactly the biggest boiler - perhaps 8kW to the boiler and 4kW to the room? I suspect there will be a fair bit of experimentation and optimisation...

Thanks again Alan.

 

[fernicarry]

I have an ESBE LTC300 I think. Which I notice is being sold at sensational prices these days. Definitely not what I paid for it...

The picture looks like it would achieve the same thing. The cut over temperature is adjustable which would be useful for tinkering purposes, the ESBE one being fixed. I presume the other end of the capillary tube goes on the stove flow?

The temperature dials on the ESBE one are somewhat useful as you can see how hot the water is from the stove and to what extent its diverting into the store so worth adding.

As you say if you have a diverter valve that stops the boiler feeding the store then that gets you on your way towards the controls you need for the stove.

 

[overlander matt]

Thanks again Alan. I notice the ESBE valves come in fixed temperature options (50/55/60) for the LTC341 units. I agree the temperature dials are useful - that's one thing missing from the thermal stores but something I can see on the solar thermal controller. The Mk1 hand has become reasonably good at measuring temperature over the past couple of years.

I'll PM you if that's ok with a few detail questions?

 

[overlander matt]

A year has almost passed by on the thermal store based system here and there is progress to report.


At long last, the boiler stove is operational following installation of a Laddomat loading valve on the wall behind the fireplace. This has been a long time coming. The idea is that this boiler adds a bit of heat to a 300L store that feeds the radiators in winter. There is also a gas boiler to feed the store but this hopefully provides some additional heat. Time will tell how effective it is.


Many thanks to Fernicarry for all his help from the earliest days sharing his experience and expertise in designing the system through the various phases of the project to today. Thank you Alan.

Whether it works well we will see!

 

[fernicarry]

Looks great!

 

[a twig]

This also reminds me - finally got round to installing that H2 panel at my parent’s house over the summer. Will have to take photos and share the info next time I’m over there