Solar + Woodburner + Thermalstore design for comments

[beelbeebub]

I'm not entirely sure where this topic should go as it involves a number of bits, so apologies.

Right, on with the show....

I am proposing to provide hot water and heat to my house using a thermal store as the combination point.

The tank I am proposing is a akvaterm 500l solar plus

Specs are a little hard to find but the area of the solar coil is about 0.7m2 which I guesstimate using specs of other tanks to be about 20kw of capacity

The akvaterm solar coil is in fact 2 coils, one at the top and the other at the bottom of the tank.  A valve diverts the flow as follows

The flow temp is above 55C - Flow goes through top coil, then on to bottom coil.
The flow temp is below 55C - Flow is diverted past top coil and goes directly to bottom coil.

On the Output side of the system will be:

Domestic hot water (DHW) - from a coil in tank mixed to a safe temp via a thermostatic valve
Under floor heating - Using the thermal store water directly, temp controlled using a thermostatic valve
Heat dump - more on that later.

The input side of the system will be

Gas boiler - 15kw or so, using the thermal store water directly
Solar panels - about 5m2 of Velux collectors heating the tank via a solar coil
Wood burner - a Woodfire 12F, with a 10kw boiler (and quench coil)

I have attached a diagram.


The "diode" symbols are non-return valve to prevent back flow and "short circuits"
The red and purple valves (3 triangle pointing at each other) are thermostatic mixer valves
The black valve is a simple 3 port valve (manual or electric, but we'll get to that later)
The green valves are thermostatic diverter valves, that is they divert the flow between two outputs based on the flow temp
The pumps are black triangles on blue circles, the dotted lines represent the control thermostats (flue or collector)
The gas boiler is shown with dotted pipes for clarity
The heat dump rad is on the far left.

I've not shown expansion vessels, PRV's or the quench coil system for the wood burner.

The house is well insulated and projected to use somewhere between 4 and 8kw of heating on a cold winter night.

The idea

The gas boiler is connected to the top and middle of the tank.  This will fire to maintain a minimum level of hot water in the tank for days when it's cloudy/we can't be bothered to run the stove etc.  Essentially it is the backup device.

The solar panels will heat the tank from the top down.  The control system is supplied by Velux with pump station, sensor and logic all combined.

The wood burner will be connected to the solar circuit pretty much in parallel with the solar panels.  The idea is that as far as the solar circuit is concerned the wood burner doesn't exist.  The wood burner has it's own pump, activated by a flue stat.  To improve boiler performance a load control system is used.  This is a diverter valve that recirculates the fluid until it reaches a set temperature (say 60c)  at that point it starts to divert some of the output to the tank (via the coils).  The volume of +60 fluid exiting the recirculating loop is replaced by cool fluid from the lower coil.  It (should) work just like a laddomat, except instead of controlling the temp going into the boiler, I am controlling the temp exiting the boiler.

There is a reason for this, the heat dump radiator.   This radiator is placed after the boiler and is used when excess heat need to be removed.  It is not a safety heat dump, in that it cannot operate without the pump. The emergency condition is catered for by a quench coil.

The stove backs onto an external wall.  On the other side of the external wall is a conservatory (or to give it it's architect title, a "sun space", ahem). As the SS is not in the heated envelope of the building it does not have to comply with the building regs re: thermal performance.  Which is good as it would cost a bomb to comply.  However, it struck me that a little extra heating my be useful in there.  Now, plumbing the SS into the central heating would be a bit naughty an probably result in more gas being burnt and general un-greenness al around.  If I placed the heat dump radiator in the sun space, then I could divert some of the heat from the wood fire boiler into the SS. The diverter valve acting as the laddomat would regulate the flow from the boiler/rad combo so that I don't pump cool water into my tank (it would see the radiator as a poorly insulated pipe and adjust accordingly).

The diverter valve to the radiator would be an electric valve operated by a boiler thermostat when the boiler temp go too high (say 80C).  OF course there would also be a test switch that  I could use to test the system (ahem).

So that's the outline, no doubt I've left some areas unclear so please ask questions.

Now some queries

Can I connect the wood burner to the solar circuit?
In other words can I use solar fluid in a wood boiler.  I assume solar fluid is also a corrosion inhibitor as well as anti freeze.
Is viscosity a problem?
Will the boiler cause any problems with the collectors (will I need some sort of sludge trap/filter)

In the same vein, will connecting a radiator to the solar circuit be a problem?  The anti freeze properties of the fluid will be very useful as the radiator (in the unheated SS) may get below freezing sometimes.

Will the Solar coil have enough capacity for the boiler (10kw) - my calcs indicate the coil has a capacity of 20kw but have I made a mistake?  There is a chance that if I have the boiler on full blast and it's a really sunny day the combination of 10kw boiler and 5m2 of collector may just exceed the coil's capacity but I judge that a bit of an unlikely scenario. And if it occurs, that's what the heat dump rad is for.


Any comments etc. happily received

Thanks.

[Brandon]

WHY?

why not put the stove in direct?

you could also pipe the gas return to the bottom, and control how much of the store that it heats by setting the stat at the height you want (thus you can select different volumes as you require).  I would advise that you look at the resol MX for this, as it will do a lot better for overall control ability than the velux offering.

I have drawings for this sort of install if you want to take a look.

I would avoid linking the stove and solar on the same hydraulic circuit.

Brandon

[beelbeebub]

Hi Brandon, thanks for the feedback.

Why not put the stove in direct?  Well, that is an option, in fact that is the current plan that I'll implement if this idea proves to be a non-starter.

The reason for connecting the solar and WB circuits is simply that it would allow the stove to run using the solar (and therefore freeze proof) fluid.  This, in turn, allows me to use the "heat dump" rad to transfer some heat from the WB to the sun space.  Essentially it's a bit of a fudge to allow me to get some heating into the SS, without officially heating it.  The rad needs the v.hot WB output to work, if the WB ran on the store water it wouldn't be freeze proof so I couldn't run the rad direct off the WB.  The solar pipe work etc all runs near the stove so there wouldn't be any extra complexity.  However it is a bit unusual.  I did ask Resol if I could run the system as an East/West solar system with the East panels bening the WB and the West the Solar.  They seemed to think it would work, but would be a bit unusual!

What reasons would you give for not linking the WB and solar?

I would very much like to see any drawings etc you have.

[Countrypaul]

You don't mention whether the system would be open vented or pressurised, though you mention you have not shown any expansion vessels. Is the WBS you are planning suitable for use on a pressurised system as I  understood most are not. I am under the impression (not sure from where) that solar circuits are usually pressurised and if so there may be a significant incompatability between the solar and WBS.

Paul

[Brandon]

1. I would not want gummy glycol in my WBS.
2. The stove will load the store better if plumbed direct.
3. It is rather overcomplicating the situation in my eyes.

I can see that the control would work happily enough as a e/w, and if you can get the PWM on the wood burner, you could get it to serve you as back end protection too.

[beelbeebub]

@CountryPaul

The stove is fitted with a quench coil and is suitable for connection to pressurised systems.

@Brandon

1. What is the concern with glycol?  As I understand it, glycol is more viscous and the thermal characteristics mean it's not as efficient as water. The increased viscosity and lower specific heat of the glycol could be compensated for with a higher pump setting, although this would increase the power requirement.
2. I thought the top to bottom coil would give good stratification and loading?  There wouldn't be any flow related disturbance in the tank, only some convection currents.  As the hot water enters the coil at the top and flows to the bottom it would effectively heat from the top down.  In your opinion, is this more or less efficient than physically pumping the water into the top of the cylinder?
3. You could be right, although the amount of piping, number of pumps etc is the same for both systems.  The major complication in the linked WB/Solar system is that it's unusual.

Is there any documentation about bad effects of glycol in a WB?  I've found some info on US sites about WB boilers installed outside but little else.

That being said I'm still very much undecided about which way to go, if I can't be sure about the combined system then I'll go with the "standard" system (attached)

[Brandon]

If and when your solar stagnates (and it will) the glycol will start to denature, at which point it becomes gloopy, and with extra exposure to elevated temperatures it becomes more and more so to the point where a circulating pump will no longer pump it.  The chemicals that you need to dissolve it out of the system are expensive, and I have never really been convinced of their efficacy.

There are a couple of points that I would alter on your standard drawing too.

PM me, and I can email you some schematics over.

HTH

Brandon.