ASHP linked to log burner boiler

[Lewis88]

Hi, ive been doing as much research as I can to see what heating system would suit my family. I’m currently looking down the route of ASHP however I have decided that I am using a log burner/boiler. 
 

the stove I have decided on can be linked to a  sealed system and I have schematics on how to link it to a combi boiler system (as per building regs) the stove has a 97 degree thermostat that will mix cold water and dump hot should that temperature be exceeded (doesn’t require electric for this). 
 

my question is can this type of system be linked to a ASHP thermal cylinder? My reasoning for this is myself and my partner will have the log burner lit in the afternoons and evenings when it is cold outside. As this is the time of year that ASHP is likely to struggle it seems perfect to marry the systems up. 
 

the stove itself is this:

https://www.modernstoves.co.uk/product/stoves-with-back-boiler/istove-lux-15kw-multi-fuel-wood-burning-stove-with-back-boiler-4-water-ports-for-vented-or-un-vented-central-heating-hot-water/
 

and the company has provided me with these schematics to link it to a combi system. Can anyone see any issues to linking it to an ASHP instead?

[Bruno]

I have a similar setup (which I regret already, I never wanted to burn another log after I have the AHSP running?)

 

This can be achieved either with a buffer tank which has 2 separate inputs (one from the burner and the other from the AHSP) or with "single-direction-flow" valves (sorry, no idea what's the English name for it) on the single input of the buffer tank (this was the option my installer chose in order to have a smaller buffer tank - separate input tanks are always bigger).

 

This, however, has a big problem which must be taken care of: usually the circulating pump after the buffer tank (which goes to the radiators) is controller by the heat pump. When you are using the log burner, the heat pump will stop and so will the circulating pump. This is dangerous as too much heat can build in the buffer thank.

It is therefore necessary to create a secondary command to the circulating pump: it is either activated by the AHSP or by the circulating pump of the log burner's water circuit.

 

edit: IMO don't do it

What you can do is take that money and replace your radiators with bigger ones, if possible ventilated. If you increase its emitting capacity you can work with lower flow temperatures and therefore increase efficiency.

And don't forget insulation. Any heating source that you choose will need to restore whatever energy the house looses, so less losses less heating requirements.

Edited January 28, 2022 by Bruno

[SteamyTea]

Do you know the high costs, finacial and enviromental, of sold fuel combustion?

A correctly sized, and opperated, ASHP should not be a problem in winter.

[Lewis88]

Thanks for the replies, yes  I’m aware that an ASHP would be sufficient for me on its own but I really want the log burner, I’ve grown up with one and find it quite therapeutic splitting and stacking logs. In terms of logs I realise that they are expensive however I do not normally pay for them. As far as environmental impact, I always season logs so as far as I’m aware it is carbon neutral?
 

The log burner I’ve selected only puts out 7kw to the room and 8 kw to water. The footprint of my house will be around 200m2 (all heated with UFH) with the room that the fire in being 55m2. I live about 370meters above sea level and the area is quite exposed. 
 

I'm hoping to use either or, for example if the log burner were lit, the ASHP would not be utilised thus saving in running costs. 

[Lewis88]

This will be my first self build, however there is experience in my family of self building. I don’t even know if 8kw heating from the log burner would be sufficient to be used instead of the ASHP. If it turns out to be more fuss than it’s worth I will probably instal a regular room sealed stove and an ASHP, just here for some experience of those that have done/considered similar 

[SteamyTea]

12 minutes ago, Lewis88 said:

far as I’m aware it is carbon neutral?

Think again.

Only dodgy accounting methods that make it look carbon neutral. 

[JohnMo]

If the tree has fallen over of a branch has fallen off, that would rot naturally and release CO2 back to atmosphere.  If using this work you be closer too carbon neutral, but anything taken down to burn, just released loads of CO2, so not carbon neutral.

 

I did consider going the same route as you and just settled on a normal log stove. To save complication.

 

You really need a thermal store to make solid fuel work, but they may not be best for ashp. I did see a German stove that could directly connect to an UFH loop to spread the heat further, but was huge money.

 

Trouble with the connection scheme they show for the combi is there is no storage for the hot water, once you light the fire most your house thermostat will switch and you will just be dumping heat.

 

If your house is well insulated, you won't have the fire on for long without melting yourself.  We have a single stick on at a time for about 30 mins.

[Lewis88]

I think I’ve gone at the principal backwards then, I do want the house to be well insulated but not too well insulated that lighting the stove would cause it to overheat. I think that the stove has been higher on my list of priorities than making it as efficient as I can. 
 

I am still at the planning phase so detailed drawings outlining cavities have not yet been drafted. Looking to use double skinned block with tender and stone, with an insulated slab. 
 

 I think I may just put a standard stove in and invest in MVHR to spread the heat around the house.

[Carrerahill]

1 hour ago, Lewis88 said:

...I have decided that I am using a log burner/boiler. 

 

About the only thing you will be able to afford - assuming DIY woodfuel, which is about the only way this should ever be done - to run in the near future and they cannot turn it off when the SHTF.

 

It can be about carbon neutral as a fuel where you are a self wood fuel producer - you must however allow for some chainsaw fuel. If you bring it in from external sources then the C02 starts to add up a bit, keep it local. All the wood I burn is from within about 2 miles of the house, I reckon a litre of diesel to haul it in, then maybe a litre of petrol for the chainsaw. I do sometimes bring in wood from about 25 miles away but I am going there anyway. I know people who got stoves and then buy heaps of kiln dried stuff from Latvia... seriously!

 

I have no issue with the many many many stoves which the media like to include in their false pollution figures that get burnt at Christmas and the odd cold night in January. They are £2500 sculptures that emit very very little by way of pollution. 

 

The only argument is the actual particulate, depends where you live, if you live isolated from other houses/people then the argument looses its gravitas. The figures that are banded about are totally inaccurate and tell a narrative that enviro types want to tell. The pollution in the room argument is one of my favourites. 

[JohnMo]

MVHR will not spread the heat around effectively, you can still have very hot and quite cold areas, you are doing less than 0.5 air changes an hour.

[Carrerahill]

11 minutes ago, JohnMo said:

If the tree has fallen over of a branch has fallen off, that would rot naturally and release CO2 back to atmosphere.  If using this work you be closer too carbon neutral, but anything taken down to burn, just released loads of CO2, so not carbon neutral.

 

If a tree is cut down, because someone is building a house for example, and it gets chipped or cut up and taken away, the chances are it is going to either get burnt, or allowed to rot somewhere. Either way CO2 will be returned to the atmosphere, as a cycle is is fairly short in the grand scheme of things. Tree's are being planted at a great rate, there are more trees in the northern hemisphere now than 100 years ago, all taking in CO2, CO2 which previously was not embodied in a tree, if that tree is planted for fuel, then the CO2 was just borrowed from the atmosphere to grow the tree. Had that tree not been grown for wood fuel the CO2 would never have been taken out the atmosphere anyway.

 

We must apply the law of conservation of mass. As a process it must be carbon neutral, you cannot create or destroy the carbon, in this instance it is just the speed in which the carbon is released into the atmosphere changes from slow to pretty damn fast. It is however by definition carbon neutral - processes excluded. 

 

For every tree used as fuel about 1.1 is used for construction. Therefore for every 2.1 tree's planted only 47% of the Carbon is quickly released into the atmosphere, i.e. within 1-2 years typically which means 53% is stored long term and the construction timber value is increasing year on year. 

 

If you include haulage and preparation of firewood, then you add some CO2 to the process, but most energy for heating currently releases CO2, wood fuel just has a benefit that it is renewable and the CO2 is cyclic. 

 

Interestingly, if you grow a carrot and eat it, you have taken CO2 out the atmosphere stored it, you eat the carrot, the human body uses the energy, or maybe stores some as fat, you burn the energy and breath most of the CO2 back out, some remains as solid waste. If you grew that carrot in your garden, then that carrot is pretty much CO2 neutral as a food stuff. 

[Carrerahill]

33 minutes ago, Lewis88 said:

I think I’ve gone at the principal backwards then, I do want the house to be well insulated but not too well insulated that lighting the stove would cause it to overheat. I think that the stove has been higher on my list of priorities than making it as efficient as I can. 
 

I am still at the planning phase so detailed drawings outlining cavities have not yet been drafted. Looking to use double skinned block with tender and stone, with an insulated slab. 
 

 I think I may just put a standard stove in and invest in MVHR to spread the heat around the house.

Can you make the house super well insulated, then put in a wood boiler in an outhouse or garage or utility room to charge a big buffer tank which you can then use to feed rads? You can then control the amount of heat that goes into your home so you don't overheat. You could also then have a little 5kW stove if you want "the look" and use it on coldest nights but still heat with wood fuel.

 

 

[ProDave]

I would forget the back boiler in the WBS that adds so many complications as it's an "uncontrolled" heat source so there must always be somewhere for the heat to go preferably by convection so not relying on a pump.

 

I would still fit a WBS (I have) with a few caveats:

 

Only fit a room sealed stove, that draws it's combustion air from the outside directly via a duct and make sure it is one that draws primary AND secondary air from the duct (not all do)

 

And site the stove in a place that heat can circulate to a lot of the house by convection.  We achieved that by double doors from the kitchen / diner with the stove to the central stair well where heat can circulate upstairs and to the living room across the hall also with double doors.

 

Our house has a worse case heat loss of 2.5kW so our 5kW stove will comfortable heat the whole house, in fact if you burn it too long it will overheat the whole house.  It is important that the heat can circulate to most of the house otherwise it would very quickly overheat just the room that it is in.

[SteamyTea]

3 minutes ago, Carrerahill said:

allowed to rot somewhere

Or most likely sequester into the soil.  Partly how soil is made after all.

[Bruno]

1 hour ago, Lewis88 said:

I really want the log burner, I’ve grown up with one and find it quite therapeutic splitting and stacking logs.

That was exactly my reasoning.  I grew with fireplaces all my life, and I got a log burner with boiler for the heating quickly after buying my apartment, which was used for years.

And then I installed the ASHP last year. And then I had the house comfortable, no mess with the wood, cleaning the floor and the ashes, the smell of smoke inside the living room... and I never used wood again

 

That "magic" of the fireplace might disappear quickly once you replace it with comfort and cleanliness. Just saying

Anyway if you really want the wood burner I would second @ProDave's comment.

Edited January 28, 2022 by Bruno

[Carrerahill]

2 minutes ago, Bruno said:

the smell of smoke inside the living room... 

 

What was wrong with it? The firebox should be in negative pressure.

 

 

[Bruno]

Well, most of the times that we open the door to stick more wood in some smoke does come out. Or when for some reason the fire is not "ramping up" (the wood is not always in the best condition) and you need to do something about it, there's usually a large amount of smoke inside which tends to come out as soon as the door is opened.

In days of extreme wind it's not uncommon to have the smoke pushed inside (I assume that this might be caused by a bad construction of the chimney, although I know this from many many fireplaces in different houses).

[Lewis88]

In response to ProDave, I can situate the LBS directly underneath a thermal store etc. the store would be on the second floor. I did see that the stove I had chosen could be used with vented or unvented cylinders so could an ASHP be linked to a vented cylinder without the same issues?

[Lewis88]

27 minutes ago, Bruno said:

That was exactly my reasoning.  I grew with fireplaces all my life, and I got a log burner with boiler for the heating quickly after buying my apartment, which was used for years.

And then I installed the ASHP last year. And then I had the house comfortable, no mess with the wood, cleaning the floor and the ashes, the smell of smoke inside the living room... and I never used wood again

 

That "magic" of the fireplace might disappear quickly once you replace it with comfort and cleanliness. Just saying

Anyway if you really want the wood burner I would second @ProDave's comment.

The magic of the fireplace for me is watching is watching the flames and listening to the crackle,  and to some degree the smell too. I don’t want to be completely reliant on it for heat, but if it’s lit anyway I see if no reason that it can’t heat a thermal store of some description with some forethought. Obviously this doesn’t come without it’s difficulties, I’m just trying to weigh up if all the fuss will be worth it long term. 
 

 

[Kevm]

I have a wood burning stove, a coal burning stove and an ASHP.  The ASHP will comfortably heat the house on its own and runs constantly on weather compensation rather than using room thermostats.  When I light either or both stoves, it therefore makes the house warmer than usual so if I want to, I can just dial the weather compensation curve downwards a degree or two.  This is easy to do on my ASHP. 

 

The wood burner had a back boiler before I got the ASHP but I had it disconnected. IMO hybrid systems can be over complicated and more trouble than they're worth.  

 

I will say that we use the burners a lot less since we got the ASHP to replace the existing storage heaters.  

[TW9]

Not insulating a house as well as you can afford to avoid it over heating is simply insane. You're effectively saying I want to waste money every day because I think wood burners look pretty (ignoring the environmental impact of wasting fuel).

 

Hundreds of people are heading into fuel poverty and you're trying to waste fuel ?

[TonyT]

Back in the day, the dunsley neutraliser was talked about in situations with WBS.

 

https://www.solidfuelappliancespares.co.uk/product/c-type-neutralizer/

 

 

[Ralph]

Our last couple of houses have been old and badly insulated, the winters would have been very cold without the woodburners. We had been planning on one in the new house but have decided to wait and see how warm the house is with just the ASHP. 

 

The power outages after storm Arwen have got me thinking about it again. And I have to say there is nothing like sitting in front of a roaring log stove with a whisky and a book while the weather rages outside. 

[Lewis88]

4 hours ago, TW9 said:

Not insulating a house as well as you can afford to avoid it over heating is simply insane. You're effectively saying I want to waste money every day because I think wood burners look pretty (ignoring the environmental impact of wasting fuel).

 

Hundreds of people are heading into fuel poverty and you're trying to waste fuel ?

More naivety than anything, I was pretty much aiming for a home that was comfortable without the stove lit but toasty with it lit. I’m not looking for passive house standards but I do want a home that is relatively energy efficient but again the wood is free and as far as I was aware, carbon neutral. I had no plans to waste any fuel, just to choose which fuel I utilised. 

[ProDave]

11 minutes ago, Lewis88 said:

More naivety than anything, I was pretty much aiming for a home that was comfortable without the stove lit but toasty with it lit. I’m not looking for passive house standards but I do want a home that is relatively energy efficient but again the wood is free

Pretty much like us.  The ASHP does most of the heating and keeps the house at 20 degrees, but it is nice when it is grey and wet outside to indulge and get the living rooms up to 24 degrees at no cost with the WBS, something we would not want to pay to do by just turning up the thermostats.

 

And of course it then takes a while to cool down before the thermostats click on again and the ASHP comes on so it saves electricity.

 

But it is worth striving to get close to passive house standards, particularly the air tightness and an MVHR ventilation system.  That is a revalation to live in.