Wood Burner and Hot Water

[Ballynoes]

Sorry if this is in the wrong topic, but I can't quite see where it fits.

 

We are currently drawing up the design for the new build, and we plan to install a wood burner stove, as we only work 3 days a week, so have the luxury of being at home for 4 days.

 

Underfloor heating will take care of the general heat, bit I wanted a wood burner for those toasty lazy evenings.

 

The question is, how easy is it to connect a wood burner to a hot water storage tank, as hot water will probably be our biggest use of energy.

 

Not really going the Solar thermal or PV route as I don’t think I will get my investment back for quite some time.

 

The wood burner is designed in so I’m sure it won’t take muck to hook it up the hot water store.

 

Or are there issues I haven’t thought about with this setup.

 

Will be using an ASHP also,

 

Thanks in advance.

[jamieled]

As long as your stove is suited to the type of DHW system you have then there are plenty around. Consider boiler stoves, rather than wood burners with jackets as the boiler stoves are far more efficient (take the heat from the flue, rather than the burn chamber). When you say the stove is designed in, is it one of the inset type stoves or is it more a mass heater type thing?

 

One of the more important aspects to consider is the ratio of heat between air and water. Get this wrong and you potentially end up with an overheated house and not enough DHW or a cold house with a lot of hot water. I've seen setups where around 80% of the produced heat goes into hot water, rather than air. This avoids  directly overheating the house, and you have a store of hot water to use when you need it (for either heating or hot water).

 

I think if you're using an ASHP, there needs to be a bit more thought as to how the overall system will work together.

[PeterW]

38 minutes ago, Ballynoes said:

Will be using an ASHP also,

 

There is your issue. 

 

Ordinarily you would use a thermal store for this sort of setup - and a big one too - to take the output from the stove on a long burn. These tend to work at the 65-75c tank temperatures and have either coils or plate heat exchangers for the hot water provision. 

 

However, if you’re thinking of ASHP for hot water, then you won’t get anything like 65c out of an ASHP so you need to consider either an unvented cylinder or how to supplement the DHW tank heating. Summer DHW use will be the challenge as you will either end up with a thermal store that provides a lot of background heat to the building in summer due to losses, or having to use an alternate heat source such as direct electric to heat water at a higher cost. 

 

How much space have you got for tanks, and what’s the plan for the build fabric insulation levels..?

 

 

[bassanclan]

You can look at the H2 panel if you don't want a thermal store, or there are a number of similar system link etc from Ireland. The good thing about the H2 panel is when you ring up for technical advice you actually speak to the man that invented it.

 

If you are in a smoke control zone you only have the choice of a few boiler stoves, all expensive. There are not many Hetas fitters who can do wet boilers so that is expensive too!

[Ballynoes]

33 minutes ago, PeterW said:

 

How much space have you got for tanks, and what’s the plan for the build fabric insulation levels..?

 

 

 

As the house is being built from scratch, I can have as much space as I need, insulation levels will be "over the top", as I have spent 30 years in a cold drafty 200 year old house, hence the new build.

 

I checked and there are no smoke control zones anywhere near us, thankfully.

Edited March 8, 2019 by Ballynoes

[recoveringbuilder]

We had originally planned for a wood burning stove with back boiler to run all our heating and dhw through a thermal store, however during the planning of this it came to light that the government were starting to make rumblings about wood burning stoves and not wanting them used.

we eventually went for a pellet stove which gives us everything the wood burner would have and it lights itself , it is also legible for the rhi, caused us a few problems to begin with as we hadn’t been given enough information about how to use it but once we got round this it’s working out great , 

[ProDave]

We faced the same decisions.

 

We decided for the sheer simplicity the WBS is space heating only.

 

We have a well insulated house (and as a new build I would hope you do as well) so the space heating requirements are low.  We only light the stove in very cold weather and then it only works with all the doors open so it is able to heat the whole house.  With the doors shut it would very quickly overheat the room it is in.

 

So with the stove only burning for a few hours once in a while it would not make a significant contribution to water heating, and the extra not inconsiderable challenges heating water with an "uncontrolled" heat source adds, we decided it was simply not worth it.

 

If you are going to use the ASHP for hot water, you need to be using an invented cylinder not a thermal store and I don't believe you are allowed to connect a WBS to an UVC.

 

[jamieled]

 

3 hours ago, ProDave said:

 

If you are going to use the ASHP for hot water, you need to be using an invented cylinder not a thermal store and I don't believe you are allowed to connect a WBS to an UVC.

 

While that's broadly true, there are a few stoves that can be connected to unvented systems. The essential bit is making sure the stove is designed for this.

 

[bassanclan]

It's not the stove that is incompatible with the uvc it's the method of linkup. You need a neutral point e.g Dunsley neutraliser, thermal store, h2 panel etc.

 

Worth looking into those pellet boilers though as the payback was something like 7 yrs with the RHI last time I looked, but you need plenty space to house the boiler and the pellets 

[jamieled]

True if you want to be able to connect any stove. But if you want to be able to connect a wb direct to an uvc you need to use one specified for this purpose as they have additional safety measures such as quench coils built in.

[ProDave]

11 minutes ago, jamieled said:

True if you want to be able to connect any stove. But if you want to be able to connect a wb direct to an uvc you need to use one specified for this purpose as they have additional safety measures such as quench coils built in.

Care to link to an example of such a stove?

[dpmiller]

Woodfire Passiv and CX12 are both setup for pressurised systems and quench coils etc. 

[ProDave]

You mean this one https://www.stovesonline.co.uk/wood_burning_stoves/Woodfire-CX12-boiler-stove.html

 

Oddly no mention of being able to drive an UVC , you would think they would want you to know?

 

I am pretty sure that was the one fitted to the straw bale house I wired a number of years ago. It was linked to a massive thermal store to provide DDHW and UFH, the theory being you only light the stove once every few days to heat up the thermal store.  But that was the only heating.  I never did figure what they would do for DHW in summer.

[dpmiller]

Yup. Look at the MIs, it's quoted up to 2.5bar and is set up with quench coil, thermopocket for Watts quench valve and safety valve.

[Ballynoes]

I'm slightly disappointed that you can’t simply connect a wood burning stove, to the hot water cylinder which the ASHP will also heat.

 

Back when I was a lad, we had an immersion heated water cylinder ALSO connected to the coal fire back boiler, so when the fire was lit, no need for the electric immersion heater.

 

This sounds like a step backwards ??

 

To answer the points above, this is a new build, fully insulated with an ASHP and underfloor heating.

 

I was only looking to fit a log burner for those long cold evenings. Also during these times in winter the ASHP would have to work overtime to heat the water as well, when it is least efficient, due to the ambient temperature.

 

The log burner I hoped would supplement the water heating during these times, and just leave the ASHP to deal with the UFH.

 

Am I missing something…. probably.

 

 

 

[Declan52]

When I was looking into mine I also wanted to be able to use a wbs to supplement the heating demand.  The problem that was explained to me is you can't just turn a wbs of.  So how do you deal with the fact you want the  stove to be still lit but not cause the tank to overheat. . My plumber had spec a 2nd tank in the loft as a heat dump plus a radiator as a backup dump.  There where lots of other plumbing bits but it soon all added up in cost and complexity that's why I ruled it out. 

I just have a normal wbs which when lit it will heat the complete house through convection.  This heat will stay in the house for 2-3 days depending on how cold it is outside. 

Don't underestimate how much wood you will need if you have to heat the water as well. 

An ashp will still deliver heat in the winter but if you want water heated to over 50 degrees it will start to lose efficiency. 

I use a pellet boiler for my main heating but I'm not sure you would want one as just a room heater.  They just blow out hot air through grills at the top.  Plus they don't have the same look as a log smouldering away. 

[Jeremy Harris]

45 minutes ago, Ballynoes said:

I was only looking to fit a log burner for those long cold evenings. Also during these times in winter the ASHP would have to work overtime to heat the water as well, when it is least efficient, due to the ambient temperature.

 

 

 

There's very little, if any, noticeable performance reduction from an ASHP when it's cold outside, at least over the range of normal UK winter temperature.  They actually tend to work slightly better when it's cold (sub-zero) and dry than they do when it's cool (just above zero) and damp.

 

There's not much difference in the heat available between air at -10°C and air at +10°C, anyway, and not enough to make any real difference to ASHP performance (assuming 100% for the heat capacity of air at 0°C then at -10°C it would still have 96.34% of the heat capacity, at +10°C it would have 103.66% of the heat capacity).

[ProDave]

You are probably not realising how you will actually use a WBS in a low energy house.

 

There should be no notion of the ASHP struggling to keep the house warm, if it can work for ours here in the Highlands, it will work anywhere in the UK climate.

 

We fitted a WBS, against the advice of most on here.  The main reason for having it is the abundance of free wood. I would not entertain having one if I had to buy fuel for it.

 

We don't use the stove often.  The 2 things that make us light it are if we want a little more heat downstairs to "indulge" or if we need a bit more heat in the (unheated) bedrooms.  Firing up the stove for a few hours heats the  whole downstairs to about 25 degrees.  I would never personally choose to pay to heat the house that hot, but for an occasional indulgence it is nice, particularly on one of those grey miserable wet days when the ambience  feels cold, even if the thermometer says it is not.  All that extra heat downstairs soon finds it's way upstairs so gives a boost to the bedroom temperatures some while later.

 

I repeat my caution that we can only use the stove with all the doors open so heat can circulate around the whole house. Any attempt to use it just for one room would very rapidly overheat that room (and I mean hotter than the indulgence 25 degrees). If your house layout does not lend itself to heat circulation you might need to think carefully.  We have double doors from both living rooms to the central hall and stairwell so there is an easy circulation and convection path for heat to spread througout the whole house from the stove.

 

The original plans for this house had 2 stoves. We very rapidly realised fitting a second one in the snug living room would be a silly idea.

[epsilonGreedy]

43 minutes ago, JSHarris said:

There's very little, if any, noticeable performance reduction from an ASHP when it's cold outside, at least over the range of normal UK winter temperature.  They actually tend to work slightly better when it's cold (sub-zero) and dry than they do when it's cool (just above zero) and damp.

 

There's not much difference in the heat available between air at -10°C and air at +10°C, anyway, and not enough to make any real difference to ASHP performance (assuming 100% for the heat capacity of air at 0°C then at -10°C it would still have 96.34% of the heat capacity, at +10°C it would have 103.66% of the heat capacity).

 

 

This defies common sense.

 

Relative to absolute zero (-273) your maths looks plausible but surely what is significant to ASHP is the heat capacity available down to the minimum chilling operating range of an ASHP. If an ASHP can chill to -10, then the available heat capacity at -5 is half of that available at Zero.

[Ballynoes]

59 minutes ago, ProDave said:

 

 

I was think along the lines of ProDave's answer, in that why pay for electricity to run the ASHP, to heat a tank of hot water in the winter, when I can get free wood to run a Log Burner, which will heat the hot water and throw out extra heat into the whole house.

 

I am surrounded by woods and live opposite a sawmill, and each year I burn excess wood in the open just to get rid of it.

 

Yes the ASHP will come on automatically, like "central heating", when we need it, but I can "turn it off" when the log burner is producing what will effectively be free heat.

 

Very interesting replies all the same.

[ProDave]

If you want to go all wood, think of the system I mentioned before in a straw bale house that I wired.

 

One of those boiler stoves that puts 10Kw to water and 2Kw to a big thermal store.

 

The difficulty comes in deciding how to do DHW in the summer. You really don't want to be burning the stove and to heat such a massive thermal store any other way would be a waste.  It almost cries out for a completely separate smaller HW tank for the summer, which could be heated by an ASHP or excess solar PV.

 

Even today, where it is flitting between a few minutes of brilliant sunshine, then back to a downpour, the solar PV is helping to heat my hot water tank.

[Jeremy Harris]

57 minutes ago, epsilonGreedy said:

 

This defies common sense.

 

Relative to absolute zero (-273) your maths looks plausible but surely what is significant to ASHP is the heat capacity available down to the minimum chilling operating range of an ASHP. If an ASHP can chill to -10, then the available heat capacity at -5 is half of that available at Zero.

 

Within the operating region of the refrigerant used, then air source heat pump performance is largely governed by the available sensible heat in the surrounding air and the ability of the unit to deal effectively with icing, that may reduce the efficiency of the evaporator (in heating mode, it becomes the condenser in cooling mode). 

 

There is a limit on heat pump performance that is set by the nature of the refrigerant gas, under the working pressure that exists within the primary circuit, specifically the temperature saturation pressure characteristic, but generally the critical lower working temperature for R410a, when used at typical domestic heat pump primary circuit pressures and with an adequate throttling valve design, is a bit below the normal range of ambient temperatures experienced in the UK.  A typical ASHP that uses R410a will be able to operate reasonably linearly* in relation to the available heat capacity in the air being drawn through it, down to around -20°C or so, with the heat capacity of the surrounding air, together with the humidity, being the dominant factors.

 

*by reasonably linearly I mean that it will broadly follow the temperature saturation pressure characteristic curve for the refrigerant gas used.

 

PS:

 

if your theory was correct, that:

Quote

the available heat capacity at -5 is half of that available at Zero

 

then that would imply that the COP at -5°C would also be half that at 0°C (assuming the same humidity).

 

In reality, the COP is pretty constant, within about 5%, as best I can estimate, based on measured energy in and estimated energy out (from the constant flow and temperature at the output, so energy out is ~ proportional to flow time) at around 3.5 from 0°C down to about -6°C (not been able to check below this temperature).

[Ballynoes]

Hi ProDave,

 

I was thinking that in the summer months, with a fully insulated house, the ASHP would provide sufficient hot water, and only be running at a small percentage of its capacity, as it will probably not be heating the UFH, I just thought that in the winter when UFH demands are higher, I can supplement the DHW, with the log Burner.

 

After all, if I am building from scratch now is the time to install it all, even if it only gets used occasionally, as the Log Burner will essential be fed by free timber.

 

Just my thoughts.

Edited March 12, 2019 by Ballynoes

[Vijay]

If you have gas, would it not be cheaper to have a small combi boiler for the summer?

 

That's what I'm contemplating and if possible, have the heated water from PV go through the boiler, so the boiler tops up the temp if needed?

 

I'm hoping to use a wood boiler so very interested in this thread

Edited March 12, 2019 by Vijay

[Ballynoes]

13 minutes ago, Vijay said:

If you have gas, would it not be cheaper to have a small combi boiler for the summer?

 

That's what I'm contemplating and if possible, have the heated water from PV go through the boiler, so the boiler tops up the temp if needed?

 

I'm hoping to use a wood boiler so very interested in this thread

 

I wish we had gas, but live in the country, so don't even have sewage, or decent broadband, but that's the price you pay for piece and quiet surrounded by your own fields... not complaining though. ?

 

I still find it difficult to comprehend that you cant "feed" a hot water cylinder with two different form of heating, after all I was under the impression that you had to occasionally heat the hot water tank to a certain temperature, to make sure certain bacteria don't thrive... or is that another myth. ?