Topic: Wood boiler + Solar + In-floor heating

Wood boiler + Solar + In-floor heating 

We’re planning a new build, and would like to incorporate a 20kW wood boiler (Thermalux Linea 20) and solar panels, to heat a 300L+ H/W cylinder that will supply not only domestic hot water but also exchange heat for hydronic in-floor heating.

The idea is to have the vented wood boiler system connected to one heat exchanger coil in the cylinder. The hydronic heating system would be connected to a second coil, and the solar heating would be an open system, (or possibly connect via a third coil.

Due to the layout and positioning of the house, an internal cylinder would be impractical, and an external standing cylinder would be aesthetically unpleasing because it would have to be on the front wall of the house. The ceilings are raked so placing it in the roof space isn't an option either.

So we're thinking of using a horizontal roof mounted cylinder, but these don't lend themselves very well to heat exchange between one coil and the next, because the coils are generally placed side-by-side instead of one above the other.

So I've come up with a setup where a vertical cylinder is placed inside a the partially enclosed alcove of the wood boiler (see attachment). The downside of this setup is that the pump noise would be audible from the open aclove, and access to the cylinder would either be very difficult, unless we were to put an ugly door in the middel of our lounge feature wall. An access door in the external brick veneer wall would also be ugly and difficult.

Any ideas on how to resolve this issue?

Re: Wood boiler + Solar + In-floor heating 

Oops...
Forgot to attach the diagram :-)

Re: Wood boiler + Solar + In-floor heating 

You could to talk to these people http://www.warmtec.co.nz/

Re: Wood boiler + Solar + In-floor heating 

Trying to do hot water and underfloor with this set up isn't going to work very well as you simply don't have enough storage.

Either it will need constant monitoring, or you will run out of hot water, or the floor will be cold.

You would need at least a separate 500 litre storage cylinder apart from one for hot water.

Do you have free wood? If not I would suggest the following:

1. Heat pump or solar hot water.
2. Log burner totally separate to anything else.
3. Heat pump for underfloor.

Wet back systems are to a large extent an anachronism.

So, free wood or not?

JK

Re: Wood boiler + Solar + In-floor heating 

First of all you need a heat demand calculation.
A good thermal envelope is also essential to achive a comfortable house without too much heatload.
Full perimeter isulation in the concrete floor, 6x2 framing, airtight membrane and insulated windows (at least R 0.7 plus!!!!!!) definitly none of the traditional aluminium windows.
Other factors are house and window orientation and shape of the house.
The warmtech people have expertise in the heating field which you don't find very often.

Re: Wood boiler + Solar + In-floor heating 

Woodboilers work better and more efficient with large buffer capacities of 1000 -3000 litres depending on the sizeof the boiler.

Re: Wood boiler + Solar + In-floor heating 

Thanks.
I've sent them an email and am waiting on a reply.

Re: Wood boiler + Solar + In-floor heating 

Thank you to those who have responded so far.

Yes, we have access to unlimited free firewood, which is the main reason for opting for wood fuelled heating.

With regard to insulation, we are installing a MAXRaft concrete slab, which is fully insulated and thermally broken (www.maxr.co.nz). Wall and ceiling insulation will be upgraded to respectively R2.6 and R4.0, and thermally broken joinery with double glazing installed (if we can afford it).

The lounge is north facing with large sliders and windows to allow the polished concrete floor to capture passive solar heat.

www.centralheating.co.nz kindly did a heat load calculation for us, and indicated that we'd require about 9kW at start-up and then 4-6kW to maintain the desired temperature. They have done many heated floors with solid fuel boilers and indicated that at 20kW water heating capacity + 8kW radiant space heating capacity, the wood boiler should be more than capable of supplying the required heat for both hydronic heating and domestic use. The only real downside is that you have to remember to keep the fire fed. They saw no issue with a single shared storage tank, though they did recommend upsizing to at least 500L.

The main advantage of implementation just one shared storage tank is that it allows for shared heating from multiple heat sources (e.g. wood boiler, solar, and an electric element as backup), an keeps the plumbing setup as simple and cheap as possible. Though admittedly in winter the solar panels won’t provide much heat, every little bit of extra heat counts, especially when it’s free. In summer, when there’s no need for floor heating, the solar panels will most likely provide more than enough heat for our domestic use.

To avoid the risk of running out of domestic hot water in winter, a thermal switch will automatically cut off the floor heating when the tank temperature goes below 65 degrees. If in practice we find that this means our floor will get too cold, then we’ll simply retrofit an LPG instant hot water heater in the domestic hot water line (after the tank), so that the tank can be dedicated to the floor heating in winter and to domestic hot water in summer.

Unfortunately we're still left with the challenge of where/how to install the storage tank without it being an eye-sore and/or ineffective.

Re: Wood boiler + Solar + In-floor heating 

Well if you install a larger storage tank you have hot water without having the fire going all the time.
The wood boiler has a longer lifespan if you have a decent fire for periods of time rather than smouldering along all day with a low fire.
I doubt that it is a good idea to place a 500 L plus tank in the space above the fire. It is hard to access even with a door and very costly to construct , or carry out maintenance, repairs or one day replacement. Imagine the construction effort to stop this massive weight coming down in an earthquake.You might have to place it elsewhere in the laundry or so and use a pump to to move the water from the boiler.
If your budget is tight you should not economise on the thermal envelope and windows. Even with the free firewood it is a comfort factor and I would rather sell some of the firewood to recover costs than using poor insulation or windows.
The double glazing is wasted in anything less than thermally broken aluminium. The European joinery systems are more value for money as they come with
very well performing hardware and the whole concept is designed to bear the weight of insulated glazing (DG or TG) The NZ aluminium companies don't have the grasp yet on quality hardware which seals the window well, can be adjusted and easy to operate.
They still do very strange things like two handles on one window.
And one more thing with regards to your woodboiler. I would definitly use one with external air source.

Re: Wood boiler + Solar + In-floor heating 

Just some more thoughts/ideas. I have installed a wood boiler together with a 1200L thermal store feeding the in floor heating in the concrete slab and radiators in bedrooms upstairs. The house is large and well insulated so i can heat the place with a 14kw boiler (HDG SL14). I've installed the wood boiler in a small lean to shed to keep and smoke and dirt outside the house. The boiler will start loading the store once the return temperature is 60deg C and will run until all wood it burned. Re-fill as required. It's filled from the top and once empty the thermostat in the chimney will stop the pump. Installing a larger boiler (more mass) would also mean larger residual heat loss once empty. The thermal store is inside and well insulated. Any heat loss stays inside the house. I can fire the boiler in the morning and use the heat from the store in the evening or have it sitting there for a day or two.

Re: Wood boiler + Solar + In-floor heating 

Good point evert.
Smoke, dust and ashes from a wood boilder can make a mess inside the house.

Re: Wood boiler + Solar + In-floor heating 

Great project. I agree with 'Guess what'. Definitely invest well in the best thermal performance materials that you can.

I'd recommend getting another opinion/model done independently for your heat load. A Homestar Practitioner http://homestar.org.nz/assessment-types/homestar-practitioner could do this for you.

Alternatively, now that ArchiCAD 16 has been released, lots of CAD technicians should be able to help.

MAXRaft looks like a great solution and a vast improvement on some other raft systems.

Re: Wood boiler + Solar + In-floor heating 

I'm with Evert. It is becoming a no-no to install a fireplace or boiler within the house envelope of an energy efficient house.

At night and in the shoulder months (when the fire is not lit), the cold chimney and firebox act like a large cold heat exchanger.

This effect means that you must burn wood for longer in spring and autumn, and more wood through winter. And you can still wake up to a cold house in the morning after the fire goes out!

Beware that a heat load calculation may not have the resolution to see this effect. ie: it may look at the building envelope but will probably ignore the chimney as a cooling source.

Also in the diagram above, the location of hw tank does not look ideal. In cold months the tank will probably cool quickly at night - due to the proximity to the chimney.

Re: Wood boiler + Solar + In-floor heating 

Thanks again for all your thoughts.

If cost was irrelevant, then we would certainly be choosing the best insulation and most efficient options available. Unfortunately however, we are building on a "shoestring" budget, so we're trying to come to the best balance of compromises.

The most important contributing factor to the choice of heating system, is the fact that we have access to firewood, and that we would have to pay for all other forms of heating no matter how much more efficient or convenient they might be. But even if we didn’t have access to free firewood, we would still install a wood fire in the lounge, simply for the ambience.

So the use of wood as our primary heat source seems to be a no brainer in our situation, and since carpets are out, so is the use of floor heating as a heat distribution method.

Since wetbacks on regular wood fires don’t have sufficient capacity to power floor heating, the obvious choice would be a purpose-built wood boiler. We simply can’t afford one of those unattractive high tech automatic boilers that you usually hide away in a garage, basement, or shed, and having to keep trudging out to one of those locations to keep the fire fed doesn’t sound appealing to us either.

Since we want a wood fire in the lounge anyway, (and not a solid fuel kitchen stove with built-in boiler), a purpose-built wood boiler for in the lounge seems the most logical choice. To our knowledge, the Thermalux Linea 20 is the only such boiler on the NZ market, but we’d be most interested to learn of other similar models.

So our only real challenge at the moment is where to locate the hot water storage tank.

I was under the impression that in order to work effectively, the tank would need to be within 3m of the boiler. But last week I was told by someone from Solarmaster that that only applied to systems that rely on thermo siphoning for their water circulation, and that when using a circulation pump you could locate the tank much more remotely.
Can anyone here confirm this and if possible provide an authoritative source to back that up?

A heating specialist from Waterware told us that thermo siphoning wouldn’t be fast enough for a 20kW system, so we’d need to install a circulation pump anyway. So, if the above is true, then we could potentially install the tank in the linen cupboard or in a corner of the garage, and thus solve the whole location issue.

The question regarding how large a tank to install still has me wondering though. Although a larger tank would mean the floor could be kept at the desired temperature for longer without constantly keeping the boiler burning, it would also mean it would take much longer to heat the tank.

Of course it would be great to have a tank large enough so that once heated it would never drop below the required floor heating temperature, even overnight without the boiler running. But how big would such a tank need to be, and where would you practically put it?

Considering that our solar heating system would be connected to the same tank, which would be used exclusively for domestic use in summer, I also wonder if a tank of that size would work. I imagine that you’d need quite a few extra (expensive) solar panels to bring and keep a 1000-3000L tank at 65 degrees; many more than for a 300-500L tank, which would be more than sufficient for domestic use.

In other words, to achieve the luxury of running the boiler frequently in winter, you’d need to spend a lot more on an extra large storage tank, sacrifice living space to house it, and spend a lot more on an unnecessarily large solar heating system. Maybe I’m missing something, but that sounds a whole lot more expensive than we can afford, and on the whole not a very efficient use of energy.

The alternative would be separate systems and tanks for floor heating in winter, and for domestic solar heating in summer. But there again I wonder if the benefit would outweigh the considerable extra cost

Re: Wood boiler + Solar + In-floor heating 

Sorry, I just noticed two typos in my post above.

The first sentence of the second paragraph should read “…the fact that we have access to FREE firewood…”
The first sentence of the second-to-last paragraph should read “…to achieve the luxury of running the boiler LESS frequently in winter…”

Re: Wood boiler + Solar + In-floor heating 

By the way, we're building in Waitetuna valley, in the Waikato, with fairly mild winter temperatures. So heat loss would be less extreme than more southern based locations.

Re: Wood boiler + Solar + In-floor heating 

im going through the same thing as you right now.with a 300 litre tank it will boil over then dump half the tank of water.ive got a friend who has recently installed a wetback 20 m away from his hwc, on a pump ,and its been signed off by the council.make sure your plumber is familiar with this type thing ,as, for example if theres a power cut and your pump stops,can the boiling hot water still circulate? if not youve just created a steam bomb!
heres some calculations i found to give you an idea of what your boiler will do.
Heating 1 liter of water by 1 Kelvin(1 degrees Celsius) demands 4.19kJ. Or 116 Watts.
1 kWh = 3,600 kJ or 3.6 MJ
So figure out the efficient output of your boiler and check if this sums up with energy being stored in the tank.

say You have a 1000 liter tank.
To bring the temperature from 20 degrees to 40 degrees it would take 1000l x 20K = 20000 lK x 4.19 kJ = 83800 lKkJ = ca. 23.28 kWh
So it would take your boiler going full blast (20kW) about 1 hour and 16 minutes to raise the temperature by 20 dgrees Celsius.
Minus the losses versus the insulation (tank and piping).

Check my numbers twice, there is no guarantee.
i recomend getting a specialist to design your system,as most plumbers ive talked to struggle with it.they will be able to join every thing together,its just the design they struggle with.i got a simple sketched design done by a specialist plumber for around 400 dollars.
hope it helps you,
cheers,casey

Re: Wood boiler + Solar + In-floor heating 

Waitetuna is where you are building - you have a neighbour on this forum - you should talk to him - he is building a house that needs next to no heating.

Re: Wood boiler + Solar + In-floor heating 

Hi Case - thanks for sharing those insights.

A technician from Waterworks.co.nz supplied me with the following basic draft schematic, and the generic one below that was supplied by a technician from centralteating.co.nz. As you can see they are very similar, although the bottom one includes a little more detail (and shows wall radiators instead of floor coils). Both companies indicated they have successfully used this setups many times.

The use of a header tank in the boiler cicuit prevents the system from turning into a steam bomb. If the water in the system reaches boiling point it will escape up the pipe into the header tank, where it will be cooled down again. The bottom of the header tank connects to the cold return of the boiler circuit to ensure the circuit doesn’t run dry, and a float valve (like in a toilet cistern), keeps the header tank topped up. The H/W storage tank is fitted with a standard pressure release, in case the tank should over heat.

Re: Wood boiler + Solar + In-floor heating 

So let me run this by you to see if I’m on the right track…
Based on the calculations you provided, it would take about 5 hours of running our 20kW boiler at full capacity before a 1000L tank would reach boiling, but only if no heat were to be drawn from it during that time.

In actuality, the floor heating system would be drawing about half the boiler’s output from the tank during the first 4 hours to bring the concrete slab up to temperature, and then about a quarter of its capacity every hour after that to maintain the floor at operating temperature. This would mean that if we assume a starting temperature of 20 degrees, it would take about 7.5 hours of running the boiler at a constant maximum capacity before the tank would reach 95 degrees.

Realistically, the boiler would probably only run at close to maximum capacity for the first few hours in the morning until the house was warmed up, and then probably closer to an average of about half capacity during the rest of the day as logs would be added to it every so often. This would mean that it would take about 15 hours before the tank would reach 95 degrees, by which time we’d be heading off to bed again.

So if we were to install a 500L tank instead of a 1000L tank, everything would be about halved. This would mean it would still take about 7.5 hours before the tank would reach 95 degrees. If we were to also use the tank for domestic hot water, (showering primarily in the evenings when the tank is at its hottest), the chances of it every overheating would seem fairly remote. Just to be on the safe side though, I suspect it would be fairly simple to install an audible warning device to inform us to turn down the boiler before the tank should overheat.

Re: Wood boiler + Solar + In-floor heating 

what youve written sounds right to me dingeling.
im no expert,im only going off what ive been studying on the web.few more things to consider are the water feeding the slab will only need to be around 40 degrees or less,and will return to the boiler at a reasonable heat too.
in a large cylinder the heat will rise,so even if you dont raise the whole cylinder to a high temp,the water in the top (where your domestic hot water coil should be) will be nice and hot.
also the solar will be raising the temp a bit to help things along.
my concern with the steam was more with having a pump in the line from tank to boiler.if the power goes off does the pump still allow water to flow through it?
i think you would be fine with a 500 l tank,you may just have to feed your boiler a little more often with smaller loads

Re: Wood boiler + Solar + In-floor heating 

Hi , i had similar issues with my place , and ended up running with solar tubes heating the domestic supply 300 ltrs as an open circuit , (works fine in summer) with a second coil from the boiler to boost it in winter . The boiler is 35 kw for the underfloor via a 800 ltr buffer tank and is closed circuit (contains rust inhibitor) with pressure release etc and expansion vessel . i think you will be running that boiler pretty much full time in the winter if your tank is any smaller than 500 lts.

Re: Wood boiler + Solar + In-floor heating 

Thanks for sharing Gaffertape.

I’m trying to picture the details of your setup…

You have two tanks: a 300L domestic H/W tank (DHW), and an 800L buffer tank for the floor heating.

The 300L tank has two heat sources: solar, and boiler. So in winter the boiler basically heats both tanks, with the solar assisting in the heating of the 300L tank, and in summer the 300L tank is heated exclusively by solar.

You mention that the solar connection is an “open circuit”, and that the boiler connects via a “second coil”.

I suspect I might be getting confused by differences in terminology. Just so we’re on the same page, here’s my understanding of open and closed circuits/systems (but I stand to be corrected):

An open circuit/system is one that has the actual water from within the tank flowing through it.
A closed circuit/system is one where the water (or other liquid) is kept separated from the tank water by using a heat exchanger coil.

A vented system is one that has a header tank or some other open pressure release mechanism installed, which also means it’s a low pressure system.
An unvented system is a high pressure system that uses an expansion vat to compensate for some pressure fluctuation, but because it doesn’t actually have an open venting mechanism it cannot handle a constant increase of pressure.

Unvented systems work fine with an automatically regulated electronic heat source that can shut down if the pressure increases too much, but they aren’t suited to use with manually regulated solid fuel boilers because of the risk of the boiler continuing to heat the system beyond its maximum pressure threshold.

So if your solar connection is open to the 300L tank, and the boiler is connected via a “second” coil, what is the first coil in the 300L tank connected to?

How is the boiler setup to heat both tanks? Does it go into the inlet of the (bottom?) coil in the 300L tank, then from the outlet of that coil into the inlet of the coil in the 800L tank, and then from the outlet of that coil back to the boiler (or visa versa)?

Re: Wood boiler + Solar + In-floor heating 

Hi , sorry for the confusion ,you are right its just the 2nd input (the solar being the first) not a second coil. i will try and be clearer.... . the boiler primarily heats the 800l tank , in near the top and return at the bottom . the underfloor draws off at the top and returns to the tank about half way down.

The 300 l tank is between the boiler and the large tank , so its feed is a tee off the buffer tank feed, through the coil and back out to join the return flow from the buffer tank to the boiler.

when i say primary, its that the pipe is larger 25mm diameter from the boiler to buffer , and its reduced to 15 mm to feed the DHW coil .

Re: Wood boiler + Solar + In-floor heating 

Stay away from the Linea 20 if specs cant be provided
We have one and we are of the opinion its output is nowhere near the stated output
We have been in constant contact with the manufacturer with no satisfaction
I will post more info later, but beware.