Solar/WBS heating system - design advice MkII

[Salamagundy]

Hi Folks!  Sorry this post is so long…. I’ve found other threads very useful, especially Cornishben’s, in getting as far as I’ve got with my ideas. I thought it’s probably better not to muddy his thread with my problems, so although many of my issues are the same as his, I have started a new one.  Hope this is ok.

I’m very much a rookie so I’d be very grateful for any advice or views you may have on my proposed scheme to heat a  small, one storey converted cartshed with small modern extension and also any answers to my queries. 

The heat demand of the building is in the region of 4.4kW 

Wood burning stove with back boiler (7 kW to water, 2.5kW to room) sited in living/ dining area.

Pumped connection from boiler to central heating:
1 x 2.1kW radiator in living area and same in bedroom
1 x 0.5kW towel rail in bathroom

Gravity feed from boiler to accumulator tank, sited in a well-insulated (100mm celotex?) lean-to outside the building (impossible inside due to lack of space/vaulted ceilings.) 
Connection from boiler to accumulator tank running upwards at 45 degrees through a modern 300mm cavity wall and feeding directly into the accumulator tank (i.e. not through a coil)
1 x 20 tube solar panel feeding into a coil in the accumulator tank to provide DHW in the summer.
High and low limit pipe thermostats in gravity system controlling the CH pump
1 x 1kW unvalved radiator in the gravity system.
Solar panel feeding coil at the bottom of the accumulator tank
Mains pressure DHW coil and separate mains pressure shower coil further up
Possibly immersion heater in case all else fails!

That’s as far as I’ve got really.  Problems I think I still need to solve:

Obviously not ideal to have the accumulator tank outside, but will the heat loss be acceptable with 75mm insulation on the tank and 100mm in the lean-to?
Assuming average (?) DHW use for two people how big should the accumulator tank be? Where do I put the gravity radiator? Does it need to be higher than the accumulator tank?
How many feed and expansion tanks do I need and where in the system do they go?
Can I use plastic pipework anywhere in the system?
How elaborate should the control system be?
Is it better to have 3 or 4 connections to the woodburning stove?  (i.e. separate returns from CH and gravity circuit or combined.)
Will a system as described above be SAFE??!!

I expect there are other problems I haven’t even thought about!  Anyway, thanks very much in advance for any responses.

[lightfoot]

Hi Salamagundy,

The main problem with locating the store in a outside or unheated space is the possibilty of freezing, you can use a antifreeze water treatment but this can get quite expensive depending on the size of the store and level of protection required.  You could rely on the immersion heater as long as you don't have power cuts when your not there to light the stove.  Make sure the store itself and pipework is well insulated.

As for store size going by the info you supplied (I assume it's a two person household) I would say somewhere between 120 to 150 litres  would suit your hot water demands using 20 tubes with a thermal store as it's best to run a store at around 80 degC.  However it's a little bit on the small size for the 7 kw back boiler if it's running for any length of time and you may start to boil the store if you can't dump that heat fast enough. So you could use a bigger store to act as a heat dump and take advantage of any extra solar gain.

It's best to run your stove as hot as possible to avoid creosote, condensation and corrosion problems, so don't over size your stove.

If you are connecting directly to the store you will only need one feed and expansion tank.  You need to allow at least 5% of the total water content of the system (thats the store + all the water in the boiler, rads and pipework) for expansion and it needs to be a least 1 meter above the highest rad to avoid pump over.  Keep it covered and insulated and don't forget to flush the system and treat the water.

You could use suitable plastic pipework for your heating circuits and hot and cold water supplies but I would recommend you use copper for you primary flow and return pipework and you solar pipework. Remember to use a blending valve on you hot water supply to avoid scalding.

Not sure why you want a separate shower coil, do you have water pressure/flow problems ??

Your heat sink towel rail only needs to be higher than the boiler so the pipework can rise unrestricted to it.  It's not a bad idea to use top and bottom connections on the rad/towel rail if possible as this will help the thermosyphoning.

Keep the controls as simple as possible.  A cylinder stat can be used to override the heating pump to give the DHW priority, you could put TRV's on your rads as long as you have at least one open circuit or bypass or even better would be a room stat to control the pump, this will make the pump last longer and save on power.  The solar will need it's own set of controls.

I think I mentioned in another post to Cornish Ben the best way to pipe up your boiler, save me typing again.

If your not sure what you are doing, please seek professional help as hot water can be very dangerous.

Hope this has been of some use.

Good luck

Lightfoot.

[Salamagundy]

Thanks very much for all that, Lightfoot – much appreciated – lots of good, practical information, - it just about doubles my knowledge on this subject!

I take your point about the danger of uncontrolled hot water, so once I have a firm idea of how the system should be set up I’ll be going to a plumber.  The only problem is that most of the ones I’ve spoken to round here are not keen on getting involved with anything ‘alternative.’ I live in hope of finding one….

My idea was to put the towel rail in the pumped CH circuit because it’s about as far away from the stove as it can be.  So I’d then planned to have a separate 1kW unvalved rad closer to the stove, both flow and return connections would probably have to be higher than the accumulator tank.  I wasn’t very happy with this as it puts heat into a part of the building where it’s not really needed, but couldn’t work out another way of doing it.  (See attached diagram, from Charnwood.)  Any ideas?

Also, the whole idea of ‘dumping’ heat seems unsatisfactory because it seems so wasteful,  but I guess it’s an unavoidable part of the system.  I was hoping that having a  radiator capacity in the pumped circuit of 4.7kW would mean that I wouldn’t be dumping too often. 

I start to go dizzy when I try to understand control systems, so your advice to keep it simple is exactly right for me!  If I’ve understood correctly, the system as suggested by Charnwood would give priority to the accumulator tank until the temp. is high enough and would then automatically start the CH system.  In the meantime the stove would be giving 2.5kW to the room.  Would a cylinder stat, as you suggest, be better?

Re. the solar circuit – I read on one of the other threads a  suggestion to have the panel mounted on the ground below the accumulator tank,  which would save on controls and electricity .  This appealed to me as it’s simpler, though, as Cornishben pointed out to me, this might be more liable to frost damage – more antifreeze?

Re. output of boiler: our experience with a 4 – 6kW Hunter stove (no boiler) is that it doesn’t seem to produce as much heat as claimed so I ‘geuestimated’ that the optimal 7.0 kW claimed for dhw and ch would be about right for a 4.4kW heat demand plus hot water.  May be wildly wrong….  As you say, the mismatch between heat from the solar panels and the heat from the boiler could be solved by a bigger accumulator tank and more tubes (and more money….)

The idea for the separate shower came from ignorance, probably…    I think our pressure problem will be that we have too much – 8.5bar.  I just thought that the shower pressure would be less affected if the washbasin, sink and dishwasher were all running at once

I’ll check out Cornish Ben’s thread again to look at piping.

Once again, many thanks!

Salamagundy

[lightfoot]

It's not always easy to combine wood stoves with solar hot water and more often than not a compromise has to be made.

As I mentioned before to get the most out of any kind of wood burning boiler it's best to run them hot and not just ticking over a lot of the time.  This dose mean you need to be able to use this heat in some useful way and the best way is to provide a buffer in the form of a well insulated thermal store or accumulator to hold all this energy until you can make best use of it either in the form of DHW or space heating. The same goes for solar.

I agree that heat dumps are wasteful in that they don't always put the heat where you want or when you want it and I think a system should be designed so that they only come into play in extreme circumstances as a fail safe device like a quench coil.  If renewables are going to stand any chance of making a real difference I think we should try and save and reuse as much energy as possible rather than dumping it when it's not convenient.  Water has a very high specific heat capacity (4.2 kJ/kgK)  or it takes 4.2 kJ of energy to raise 1 litre of water by 1 deg C, which basically means it makes a excellent storage medium,  so this is where a larger accumulator tank comes in handy and once it's up to temperature, if well insulated it will keep that energy for quite along time and you therefore only have to put in what you take out.

If the wood burning stove is going to be your main source of space heating and DHW then once you have selected a suitable size of stove/boiler to meet the needs of the household, I would then size the accumulator to match the output of the stove/boiler acting as a buffer so as to avoid boiling and therefore the constant need of a heat dump. I would then use as many solar panels as you could.  In the summer you may need to fire your stove or use the immersion heater to boost the temperature if you have limitations on the amount of solar panels you can have or if the sun don't shine.

The key to any thermal storage system (that includes hot water cylinders) is insulation and lots of it, see  http://www.accumulatortanks.co.uk/FAQ.htm  Good thermal layering or stratification can make a big differance too   http://www.termoventiler.se/default.asp?webb_ID=110&webbsida_ID=71

Yes you could use the charnwood suggested set up for giving the DHW priority. The low stat is keeps the heating pump off until the gravity return temp rises and if the gravity flow gets too hot the high stat turns the pump on to dump heat to via the rads etc.

They are not the cheapest but but in my humble opinion the clearview stoves take a lot of beating.

I think in most cases in this country it's best to run your solar as a pumped sealed (with antifreeze) pressurised system.  As thermo-syphon solar systems are best suited to places like the Med etc.

High static water pressure and high flow rates don't always go hand in hand.  For a good mains pressure system you should be able to open at least two or three taps in the house without a big drop in flow rate.  For safety you should always use a thermostatic shower mixer to accommodate any small change in flow rate.

All the best

Lightfoot.

[Salamagundy]

Thanks once again for all the advice and the links, Lightfoot.  I'll check out the Clearview stoves, have a think about everything you've suggested (steep learning curve for me...) and be back after the weekend.

Cheers!

[Ivan]

Check out the woodwarm stoves that Navitron sell - these are as clean-burning as the clearview, but without the pricetag

Ivan