Well, for detailed answers to detailed questions a professional should be hired, for example a heating engineer.
 
Various things seem to be unlogic in the proposed design:
If there is a risk of over-heating of the thermal solar collectors then these are badly planned.
Installed at the correct angle there is no risk.
A log boiler of 15 kW is far to big to used for the same thermal storage tank as the 2m2 ST collector, these things do not fit together. Like wheel barrow tires on a motor scooter for high-way usage.
 
There are ST installers available who are no cowboys, who do the lay-out and sign for it responsible.A drain back ST system for example does not overheat nor does it freeze.
If using the ST system only for DHW in Summer then go for flat collectors, fix these horizontally to the wall.These are cheaper then evacuated tubes,they won't overheat in this position and they'll protect your wall, for example against thermal loss in Winter. Or against overheating in Summer....
 
There are heating engineers who will sign responsible for their job as well,the correct lay-out of a heating system...
 
And there are log boilers available which are capeable firing less then at 5 kW output...
 
Why did the PHPP came up with a recommendation for a 15 kW boiler???
 
 
 
 
Always ask for a full, detailed and signed calculation sheet, incl. ammortisation.
If a professional can't come up with this you know you're talking to a cowboy. Give him the cane. But not more.

I am not convinced that, if the boiler is simply loading the thermal store, that size is critical. Chopper, I am not an expert but it looks to me that the solar collector is too small for the store while the boiler may be too large. I think you ought to be looking at a larger solar collector area. Will you be using the boiler to charge the store? Perhaps daily in cold weather, every 2-3 days in spring and summer? A larger boiler will mean shorter burns less often while a smaller boiler will have to be burnt more often and for longer.

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This is not a 'design' as yet.
I won't have an installer, I'll be doing it myself with help.
I think you may have missed the point, I'm not concerned about the solar overheating.
I already have 2sqm of tubes feeding a 200ltr tank for the last six years with no trouble.
I don't want to buy more collectors, the 2sqm provides us with dhw perfectly.
 
For the new house, I have an abundance of wood and want this to be my primary fuel.
A 15kw log boiler is the smallest I can find and this would require a tank of 750ltrs plus to prevent overheating.
This tank would be very expensive and I can't heat it with my existing tube setup.
Heinbloed, I'd be very interested in info on a 5kw log boiler. That would be the answer to my problem but I can't seem to find one of this small size.
 
I'd like to avoid firing the boiler too much in the summer when there is no space heating requirement and the solar provides nearly all dhw.
 
I'm thinking that for now (as funds are low)I may install a 250ltr solar thermal store in the house and run it off my 2sqm of tubes and immersion.
I could then get a bog standard, low cost 15-25kw log boiler to heat a large buffer, 800ltrs plus and in turn heat the 250ltr tank from that.

@Julian:
Well, the term "thermal store" is confusing. A thermal store is the entire building, incl. radiators,pipes ,walls ect..
But I think the OP means the "thermal storage tank", so we'll look at this:
 
First of all:boilers are no stoves. They're rather unsightingly and dusty, maybe noisy as well. They need an extra boiler room.
There are stoves with a water pocket, these do heat the room in which they're placed AND via the water pocket the radiators/storage tank as well.
 
 
The OP should tell us if he/she is looking at such a stove instead of a "boiler".
 
The boiler will have 15 kW output as long as the OP sticks with the original plans. Heating up a simple tank containing 400-500 liters of water will take a very long time. Much longer then a smaller tank.
So it will take ages to get some hot domestic water unless another heat source (electric heating element, modulating electric boiler...) will be employed.
 
In the mean time the radiators will consume energy.
 
If now the total output demand at the space heating side is only low, the OP plans "3 very small radiators" - the smallest ones are around 1 kW- the demand for the radiators will be around 3 kW- if the rooms are cold, ready to take the load.
So we have a 3kW load at the radiators and an unknown load for the direct losses from the boiler to the air.
If the boiler (or water heating stove) is placed in a supposed-to-be-heated place then these losses are not realy losses. As long as they're welcome.
 
A 15 kW boiler/stove delivering at least 3 kW to the room where it is placed plus 3 kW to the radiators plus 1 kW loss by the plumbing (a rough estimate) will then deliver only 9 kW to storage tank.
Which would be in this case a good buffer storage if well insulated, but not suitable for a quick warming up of the DHW.
There are special thermal storage tanks available in the planned size which can layer the hot water at the top of the tank. But a standard storage/buffer tank wouldn't have a great layering capacity. Using pressure resistant tank (they come up to 1,000liters in household sizes) usually feeds the hot circulating water directly into the top of tank, releasing colder water back into the return of the boiler. These type of tanks create a good mixing of the different temperatures, depending on the speed of the flow.
The higher the output of the boiler the higher the flow rate must be kept.To avoid a steam explosion.
The created heat must be transported away.
 
Now, there are also more sophisticated tanks. These have a better layering then the direct flow tanks.Either by a combination of circulation loops combined with thermo-valves or via a thermal lance. But they cost more as well.
 
For a domestic usage a stove with a water pocket toheat a.) the room in which it is placed and b.) the radiators which are connected and c.) using the ST storage tank as a buffer
would be more economical to run.
Most rooms will do with a direct input of 2-4 kw.
Plus the 3 radiators of 1 kW each and roughly 1 kW distribution loss will lead to a stove size of max. 8 kW.
Assume 3 kW for the room in which the stove is placed and 4 kW cover the radiators and distribution loss.
 
Such a water heating stove would be called then a 50/50 stove, maybe a 40/60 stove. 40% to 50% of the created heat straight into the room, 50%-60% to the radiators.
 
The warmer the circulation system's sourroundings get (the pipe's insulant and the rooms) the less thermal energy will be lost.
And here the ST thermal storage tank kicks in, it'll be the buffer tank to avoid the central heating from automatically being switched-off as an emergency solution. To avoid a steam explosion a valve would somewhere open, releasing the pressurised steam.
 
Designing a standard domestic central heating system to include a 400-500 liter buffer tank and keeping it at a minimum temperature (maessured at the bottom of the tank at the return pipe !) similar to the return temperature of the radiators means a huge overdimensioning.
 
It would be more economical to run ( and cheaper to purchase) to install a smaller boiler/stove combined with a smaller tank.
 
The boiler/stove should not be designed to heat up large quantities of DHW if these quantities are not used. And since a domestic heating system will run 24h per day during the heating season it'll be very costly to spend 3 quarters of the boiler's output on domestic hot water if a smaller fraction would be allright as well.
 
Better top-up the maybe luke warm water from the buffer/storage tank on demand.By an electric shower or a heating element within the tank.
Instead of keeping it warm for 150 days per year(the whole heating season !) whilest there might be only a demand for a total of 1-2 days per heating season when using 4 showers per day of 15 minutes each.
 
A heating engineer will deliver more precise, more individualised numbers, my calculations are based on rough estimates.
 
That is for the heating season.
 
But for the other times, for example a dark week in a bad summer a 15 kw output boiler would be far oversized.
All boilers work best at their maximum output. And a shower, fed with cold water, won't demand more then around 9 kW. If the water was pre-warmed to room temperature( for example coming from a ST storage tank situated within the house)then this would be only around 8 kW. Most electric showers run on 9 kW therefore.
 
So looking at all of these basic assumptions a 5-6 kW stove/boiler combined with a 200 liter ST storage tank and an electric shower would be more suitable.
Provided the ST collector's size/output isn't increased and there are no 'abnormal' demands for DHW ect..
 
Remember that fuel which costs money (here:timber) should not be wasted to heat up storage tanks.
Fuel which doesn't cost a penny (sunshine)can be wasted, here the thermal storage losses which don't cost are no real financial losses.
This is how mother nature handles fuel, we can learn a lot from this life-old principle: keep it small if you have to work for it.

Thanks Heinbloed.
There are a couple of things specific to my installation;
The entire building has a heatload of about 2.7-3kw (from memory without digging out phpp calcs).
The 'water heating' boiler will be in an out house to prevent over heating of the building.
This could occur on overcast Spring/Summer days when there is a dhw demand and the solar is not fully functioning.
 
I have a constant supply of offcut timber from my building company, I don't wish to use too much electricity for water heating due to the free fuel source I have.
 
I was thinking of having a large thermal store/tank in the outhouse which is kept continueously hot from the boiler.
Water can be drawn off this to heat the smaller tank in the house when necessary.
 
I have found it frequently hard to specify 'small' heating plants for these types of houses without using a heatpump.

Well, if you really want to have the boiler as well as the thermal storage tank outside then you need a larger output then the building itself demands, the energy wastage would be enormous.
I'm not a great fan of combustion, so I have little experience with modern fire technology.
Since you want to use your own dry timber in log shape why not going for a stove with a water pocket, heating radiators and/or thermal storage tanks? For example the type "Momo" from Wodtke, producing a max.output of 8kW, ca. 70 % for the central heating water system and 30% for the room.
Around 2,000 Euros. There are cheaper ones from other companies, starting at around 1,200 euros when bought in continental DIY markets.
There are many of these types of boiler stoves available from many manufacturers in a similar output class.Some are standard burners, others are gasifying.
 
If you're running a low energy building/PH with limited space then the pellet stove-boiler "Therm" from GUNTAMATIC might make sense.A wall hung pellet boiler modulating between 2-7 kW with a min.output of 38 degrees.Similar to wall hung gas boilers.
Pellets are more expensive then "free" logs but your energy demand seems to be on the very low side anyhow.
 
If you have only limited space for the thermal storage tank then consider a flexible thermal storage tank.These tanks fit into any eve, any corner, round or square. In principle these are much cheaper then the pressurised standard cone-shaped storage tanks, all you buy is the ready-made tank (empty in a flat pack, made to order) and the heat exchanger. The box around it to keep it in shape incl. insulation is a DIY job.
Keep the energy in the house, this is more economical then transporting off-cuts and heating the environment.
Fuel timber cost money, people pay for dry off-cuts. What you save you could sell.

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@Chopper:
(Yesterday's posts came in short conesequences, so I didn't answer)
All modern log boilers are controlled, they can operate on the max. output but don't have to.
A thermo sensor and/or a lambda probe will control the airflow, the combustion: regulating the thermal output(kW).
 
This regulation is demanded by various laws, environment regulations.
 
For a heat load as you have figured out only the very smallest boilers are suitable.
Here a list of a few, by far not complete:
 
ORANIER "Polar",(max.9kW)
BUDERUS "blueline"(max.8kW)
WAMSLER "Aquarello"(max.8kW)
WODTKE "Momo"(max.8kW)
 
HAAS+SOHN "Vestre" (cheap! 3.5-10kW)
HAAS+SOHN "Warburg" (same as Vestre)
 
Check out how far these can be down regulated to get your water-side output of 3-4 kW.
 
A PH with a 3kW heatload must be very big....are you sure the heatload was correctly calculated?

Chopper, I hope you don't plan on burning treated timber.

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A hanky for Julian...
A building company uses dry timber, the EU legislations for loadbearing timber doesn't allow for green or wet timber to be used.
Max. permitted moisture content for this timber is 18%, this is the max. permitted moisture. Timber can get wet of course, therefore the standard building material is kiln dried to a max. of 15%, usually less. To allow for a rain shower on-site and the up-take of atmospherical moisture when stored.
 
Off-cuts are usually occuring when dry timber is cut to dimension. It makes no sense to cut wet timber and see it shrinking in consequence.The structure would fall apart.
 
And that an intelligent person doesn't burn hazardous waste I take for granted.
 
Off-cuts from saw mills/wood work shops are classified as fuel, provided they're dry and 'natural' when going into the fire. Pellets are made from this material.
Of course a different story it'll be if the timber is green(fresh)and not seasoned or kept outside for a prolonged time....
Or if it contains raisins, like birch or pine or other evergreens.Then it can only be used in gasification boilers or turned into pellets, according to EU anti-airpollution legislations for habitated areas.
With the process of pellet making the timber will be exposed to high pressure.What causes it to heat up and to dry, to lose it's evaporable contents like water and turpentine.
Pellet makers/factories have strict airpollution controls, turpentine and dust are closely monitored. But maybe not in GB.
 
The greenbuilding magazine should highlight these basic facts for it's eager readers, it seems to be necessary.

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On another thread here recently it was stated of a 3m2 solar thermal collector;
"It was a waste of money, of resources. It caused more demage (sic) then good. Toy installations of a few m2 are of course a waste of energy, everyone will be able to get the SHPP and calculate the demage caused by undersizing the heating system."

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@ Julian:
Yes, you're missing a lot...
Chopper HAS already a ST installation, as he stated.It would be a waste not to use it anymore.
Better start reading the thread again before you further ridicule yourself.
There came no answer to the OPs questions from yourself, propably for a good reason?
I hope you understand the issue on how to make a fire by now?

@ Julian
I reccomend not getting too deeply involved with Heinbloed.
Although he is obviously extremely knowledgeable and has a lot to offer, he needs to measure his responses and learn to be less arrogant with his opinions.
I started posting on this forum for some impartial advice after getting unreasonably hammered by him on the Construct Ireland forum.
I wasn't delighted to see that the first responce I got was from Heinbloed.
Online forums can be a great tool for sharing knowledge but they also allow people to anonymously throw there interlectual wieght around often intimidating and scaring others off from the discussion.

Hi Chopper
I have a 25kw log boiler and 2000L accumulator - the point was made to me that I could have put in a 35kw boiler which would have heated the store more quickly since (as I guess you know) it is the store that heats the radiators and hot water cylinder rather than the boiler directly.
Good luck with your enquiries.
julian

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Heinbloed and Julian need to stop arguing please. It is of no help to other forum users.