Woodburning Stove, Fire back boiler, Thermal store and DHW integration.

[wh00sher]

Hi, been reading the site and found it very informative with good info.

I am in the middle of designing a system to add a fireback boiler to my woodburning Stove.

I`m fortunate that I have an unlimited supply of FREE wood (Yes, I do know how lucky I am !   )  The Stove is lit most days from Autumn until Spring and during the summer on chilly evenings.

I`m planning on welding up a Mild Steel back-boiler/water jacket to cover the bottom half at the back of the stove. Outlets in 28mm and sealed where it will pass through the rear of the stove to keep the combustion chamber sealed.

Everything in the `purple` outlined section isn`t there yet.



My house was a selfbuild about 10 years ago. I have a  condensing system boiler which currently heats my DHW in an unvented cylinder and also supplies my warm water UFH system. There is a bypass loop (shown on the diagram as a radiator), this ensures the boiler is never `dead-ended`.

The flue / back of my fire is on a gable wall on the house, inside a garage with a pitched roof.

I`d be able to install the ~250l cylinder directly above the fire, the bottom of the cylinder would be around 2m above the heat exchanger, a straight forward vertical run with sweeping bends should give a nice thermosyphon in 28mm into the cylinder (heat store).

The vent from this cylinder would vent directly outside into a safe discharge point. I`d rather do this than back into the header tank, so if I had a power cut and the fire was burning, I wouldn`t keep recycling hotter and hotter water, by allowing the water to `boil off`, this would then allow cold water from the head tank to refill the loop. I`m unsure if some sort of `temperature` reflief valve would be necessary connected to the cylinder?

Once warm, a cylinder stat would activate both pump local to the plate heat exchanger, the aim of this being to transfer heat from the vented heat store into the unvented system.

The hot water from the PHE would then substitute the boiler output by means of the 3-way valve on the diagram, this would either heat the DHW or UFH. An unvalved radiator would provide an `always open` outlet for the heat, although the Stove output would never be more than the UFH demand. I actually doubt the Stove would ever get the UFH hot enough, but if it took off the chill, that would be sufficient. My primary goal is to heat the DHW and the UFH is a by-product.

I haven`t shown things like Relief valves on the diagram or the thermostatic control for the 3-way valves, pumps etc.

Just wondering what your views are on the proposed setup, suggetions for improvement ? Anything blindingly obvious that I`ve overlooked or not done ?

Nige.

[dhaslam]

Just a couple of points.   The  boiler in the stove probably should be high up in the stove  in order to  reach high combustion temperatures.   You might have to insulate  the combustion chamber with narrow fire bricks to  compensate for  the heat  loss to the boiler.   There may also be issues with the return water being too cold but that may not be such an issue if there is a lot of mixing in that cylinder.      An alternative  way to link the two cylinders might be to  heat the new cylinder from both old and new  boilers and to  transfer  heat  from the hotter upper section to the DHW cylinder and have a lower outlet to underfloor heating.  The  system boiler could be set  to come on by time and temperature controls  so that it runs  constantly when on.     

[wh00sher]

Thanks for the reply !

The rear of the stove is Steel, this is already covered with 1/2" thick insulating blocks (you can just make out the white material behind the Cast sides). This sheet is then covered with cast iron ribbed plates. These photos from when I was fitting the stove are all I have to hand.


My plan is to remove the plates from the rear of the stove and the insulation behind them. Make a back boiler that will cover the bottom half of the stove on the rear wall, the area marked in RED. I`ll then fit the insulation and cast iron plates to the top half of the stove, giving a vertical surface, but the bottom half (or 1/3) would be the water jacket. My plan was to put it here as this is the hottest section of the fire. The bottom glows red (as in the pic), but the top never gets as hot.


The jacket will have 2 outlets which will pass through the rear of the boiler and then be sealed to ensure the combustion chamber is airtight. The inlet will be at the bottom left, the outlet diagonally opposite at the top right. This will be connected by gravity thermosyphon to the thermal store 2M above the stove. From here I`ll be linking it to my DHW and CH system.

I`m not sure how I`d transfer the water from the vented thermal store to the unvented DHW cylinder and UFH without the PHE ?  

Why would the return water being to cold be an issue ? I could use a lower capacity PHE so the transfer would be less efficient, meaning the return to the cylinder wouldn`t have `lost` as much heat from the PHE, but obviously, I`d prefer to have a large PHE so I can get the heat from the Thermal store into the pressurised part of the my system as quickly as possible.

Losing some of the heat output of the Stove into the room isn`t a problem. It gives out more heat than we need when burning at full capacity. Once lit, we often close the draught to keep the output down. Recently, as the temperatures have been dropping, we`ve lit the fire, left the lounge door open and it has helped warm the house. When the back-boiler reduces the output to the room, that won`t be a problem. The main thing I need to keep in mind is that the fire chamber is still hot enough to get a clean burn. I don`t have any issues with tarring in the flue and obviously thats something I don`t want to introduce into my setup.

[wh00sher]

Me again 

Whilst I`m collecting info, what are thoughts on PHE sizes ? I`m looking at something like this :


There are many differing sizes available from 10 plates with 0,12m ² of Effective exchange surface for £40 all the way up to 3,6m ² of Effective exchange surface for £460 !

I was looking at something in the 0,60m² range ?

[Solal]

Seems a bit Heath Robinson.
Boiler/Stove manufacturers  put their   products  through testing procedures  so you'll  have   at least some idea  how the system will perform.
Do you know  what the output of this contraption  will be when its  finished?

[wh00sher]

Seems a bit Heath Robinson.

Do you know  what the output of this contraption  will be when its  finished?

Well, apart from throwing it all away and starting from scratch, what other option is there?   I`m trying to make the best of what I have with a limited budget and good DIY skills.

I have no idea what the output will be when finished.

My understand was the principles of thermosyphons, heat stores, fire back boilers and plate heat exchangers are pretty well known. I was asking for advice on modifying my existing setup to make use of the unlimited FREE wood supply I have.

Obviously, with unlimited funds I`d do it all from scratch, but here in my world, thats not going to happen.  

[Solal]

Whats your budget  on this if I may ask?

[wh00sher]

As little as necessary  

I already have 2 CH Pumps, pipe stats and the direct cylinder & header tank (from a work colleague who recently went to an unvented system, its a 12 month old Direct one), I have the necessary steel, fitting and welding equipment to make a back boiler from previous projects.

Realistically, all I need, apart from time, is some 28mm pipe, insulation and a heat exchanger.  

What I`ve done is collect as many parts as possible over time. I`ve done a fair bit of research into this first. I`ve been on forums where people have done no searching themselves and expect it all to be handed to them  , I`m a moderator on a large forum so didn`t want to do that, hence me posting my proposed setup for suggestions on things I`ve done wrong and ways to improve them 

[Solal]

I`m trying to make the best of what I have

Then leave it as its design was intended. (ie - no boiler)
And invest your money  in some solar panels.

[wh00sher]

I`m trying to make the best of what I have

Then leave it as its design was intended. (ie - no boiler)
And invest your money  in some solar panels.

Sorry, you`ve lost me.   No boiler? I already have a Condensing boiler as mentioned in my first post.

Solar panels are not an option on my property. I never mentioned them, why would you think I`d want to fit those when all I`m trying to do is maximise my current installation? I`m confused.

[Solal]

Is your stove designed for  boiler fitment?

[wh00sher]

Is your stove designed for  boiler fitment?

My stove will have the rear cast iron `plates` removes and a `clip in` fireback boiler fitted.

Am I missing something here ? I know I`m new to the forum, but you seem to be pretty negative on my plans. I`ve seen other, similar projects that don`t seem attract the negative vibe I`m feeling from you.  If I`ve misunderstood you, then I apologise.

If the fundamental idea won`t work then fine, thats why I posted here asking for people who have done it before and have more knowledge than me, but if it will work, even not necessarily as efficient as a brand new system with integral factor fitted backboiler then why not ? Isn`t making the most of your available resources what this forum is all about ?

[Solal]

I'm not a fan of this kind of bodging  but if you can make it work  then good luck.

[dhaslam]

The boiler  doesnt need to have a very high temperature   around it  but the fire itself does.  You need to have  something like 800C to  get the fire to burn properly but only  300C  or less around the boiler.   I think you would be better to leave the stove intact and use  a flue boiler, particularly if the flue gas can  reach 300C.    The stove seems to be well designed  but doesn't have a very big firebox so the output would be a bit limited and even more so if it can't reach high enough temperature to  burn the  woodgas completely.     

[wh00sher]

Not sure if anyone is interested, but I thought i`d post my findings anyway  

Initial backboiler was made to replace the top half of the fire-back. This was too small and took hours to heat the cylinder above. Remade it to be twice as high and it now covers the rear of the firebox, this is now much more efficient and heats up the cylinder above by gravity to 75degrees in around 3 hours.

I have various thermostats and timers which enable me to run it on `full auto`, `Fire only`, `Boiler override` etc.

For the last few weeks, the house has been entirely heated by the Stove  , Hot water about 50/50. It retains some heat overnight but needs `topping up` by the Gas boiler on a morning for us all to have a shower, however, during the day it heats the DHW Unvented cylinder to full temperature.

The beneficial side effect has been to reduce the boiler output to the lounge. I knew this would happen and its meant the lounge is now pretty hot rather than roasting. We used to have to leave the lounge door open once the stove was hot to let some heat out, now its much more bearable as some of the heat is being taken by the back-boiler.

Bodgetastic.