Woodburner with backboiler for DHW & solar

[alun]

I am planning to install a solar water heating system, and an 8KW woodburner with a small stainless steel backboiler (2.9KW).The aim is to use these heat sources to supply most of the domestic hot water. I have an oil central heating boiler, and a vented system. My plan is to fit a HW cylinder with 3 coils, one for solar, one for the woodburner, and one for the oil boiler.
The woodburning stove retailer has suggested connecting the backboiler directly to the HW cylinder, as this would be more efficient, and would save the cost of a 3rd coil. Any views on this please?
My questions are regarding overheating.
If I connected the woodburner to a 3rd coil, would I need to install some form of heat dump e.g. a radiator?
Would overheating be more likely to occur if the woodburner was connected directly to the HW cylinder?
Both would be gravity fed systems.

[lightfoot]

Hi Alun,

As the back boiler is stainless steel, you can indeed connect it directly to the DHW cylinder, which is also more efficient and will heat the cylinder from the top down and won't cause it to overheat any more than if you use a coil.

What size is your DHW cylinder, how much hot water do you get through per day and how many panels did you intend using.

Try and keep the gravity pipework as short as possible, avoid any sudden changes in direction and long near horizontal pipe runs.

Good luck,

Lightfoot.

[alun]

Hi Lightfoot, thnks for the reply.
My present HWC is 162litres, which is aedquate for our needs(mainly showering). I was intending to fit a 172l tank, and a 30tube panel.
The pipe run from the woodburner to the HWC has a 4m long section which runs along the joists, so will only have a minimal gradient.

Alun

[lightfoot]

It takes around 10 kwh (excluding losses) to raise a 172 L cylinder by 50 degC, so around 3.5 hours with a good fire going in your stove and depending on how cold it was to start with, you could probably squeeze a little bit more in before it starts to boil.  So depending on how long and hard you want to keep the stove going for space heating etc, you may want to consider a small gravity fed heat dump/towel rail....or just have another bath 


Lightfoot.

[dhaslam]

Direct connection to the cylinder returns cold water to the boiler.  Not such a problem where only a percentage of the output goes to water but it can reduce combustion temperature and reduce boiler efficiency.

[Richard Owen]

A low return temperature is good for condensing boilers such as gas or oil, but is bad for solid fuel appliances for the reasons outlined above.

Time to introduce our old friend the laddomat 21 http://www.dunsterwoodfuels.co.uk/accumulatortank.html

It keeps the temperature in the boiler high, returns hot water to the store/tank and has a bypass in case of power failure.

[alun]

Thanks for the helpful replies. As the percentage output of the woodburner going to heat water is about 36%, do you think having cold water returning would reduce the efficiency of the boiler significantly. I suppose the alternative would be to have the cheaper plain steel back boiler, and run it through a 3rd coil.

Alun

[Richard Owen]

I've only recently considered this issue (since being part of this forum), so I'm only a couple of steps further down this road than you.

This is the thinking:

The efficiency of the burning of solid fuel depends greatly on the temperature of the burning. The hotter it is, the better it is.

If we artificially cool the environment, by introducing cool water to a boiler, then we lower the temperature of the burn and lower its efficiency.

Putting the water through a coil rather than directly into a tank may raise the return temperature slightly. However, it is still going to be relatively cool until the bottom of the tank is hot.

Using a Laddomat means that the water in the boiler is as hot as practical, meaning that the combustion is always as efficient as possible.

There is an additional benefit: The water supplied to the tank from the laddomat is always hot, never warn. If the output of the laddomat is connected to the top of the store/tank, then, when the wood burner is on, you will always have some hot water.

If you connect via a coil, not only is the efficiency of the burner reduced for most of the cycle, you will have to heat the whole tank to get some hot water.

Hope that helps,
     Richard

[lightfoot]

In a gravity system, the flow rate and therefore the boiler temperature is regulated by the temperature/density difference in the system and generally the circulation is slow compared to a pumped system.  The system is self regulating and as the return water lowers the boiler temperature, the flow rate through the boiler will slow down, which will allow the boiler temperature to rise etc etc.  See this thread for more details...http://www.navitron.org.uk/forum/index.php/topic,4036.0.html

Personally I don't think you will have too much of a combustion temperature problem with this small DHW back boiler on that stove with a gravity circulated system.  A larger/fully pumped system is a different case, in which I would always recommend the use of a suitable loading valve/charging unit such as the Laddomat 21... http://www.termoventiler.se/default.asp?webb_ID=110&webbsida_ID=4   but I'm not sure you could justify it's use on a small system such as this.  You would be better off spending the money on, ensuring the flue is suitably sized/lined and insulated.

Good luck,

Lightfoot.

[dhaslam]

There is a single valve that will do the job.   It shortcuts the  return water, taking water from the hot side until the temperature  builds up.   It has to be on the return side to leave the output side clear for expansion.     I am considering this even with a coil  because the coil surface is very large on the buffer tank I am going to use.   I am not sure how much the gravity flow regulates itself through a coil but in any case the fixed temperature return  thermostat would probably help keep the top of the cylinder hot as well as the boiler.       

http://www.esbe.se/nav5174/?prodid=4032

One problem with the stove using a small back boiler is that the room can be very hot.  5kw is a lot of heat to a well insulated room unless it is very large.     

[lightfoot]

There is a single valve that will do the job.   It shortcuts the  return water, taking water from the hot side until the temperature  builds up.   It has to be on the return side to leave the output side clear for expansion.     I am considering this even with a coil  because the coil surface is very large on the buffer tank I am going to use.   I am not sure how much the gravity flow regulates itself through a coil but in any case the fixed temperature return  thermostat would probably help keep the top of the cylinder hot as well as the boiler.       

http://www.esbe.se/nav5174/?prodid=4032
  

Here's another version of the same thing... http://www.acaso.se/english/produktblad/TERMOVAR%20Ladd-ventil%20EN%200502.pdf   I'm not saying it won't work....but I have never fitted or seen one on a unassisted, gravity system, so it would be worth checking that it was suitable, as you need to consider the resistance it adds to the system, because the available circulation pressure in a gravity system in very low compared to a pumped system, thus the need for larger, less torturous pipework etc.  Let us know how you get on.

The gravity flow will regulate itself the same way through a coil, except the coil will add further resistance to the system.


lightfoot.

[lightfoot]

Emmm....thinking about it, these bypass/loading valves won't work on a unassisted gravity system, ie without the aid of a pump.  In a unassisted gravity system, the only way to ensure/maintain the boiler flow temperature throughout the charging period, would be to restrict/reduce the flow rate thermostatically, therefore changing/increasing the temp rise across the boiler, ie by using a suitable valve on the return, controlled thermostatically according to the flow temperature.  It wouldn't need much restriction to slow the flow rate on a gravity system, but again as with the pipework etc you would need to consider it's resistance when fully open and size it accordingly, but even then I think building control may possibly be a little concerned with this setup. 

So I guess that brings us back full circle, to using a suitably controlled/pumped loading valve set-up or a all in one loading/charging unit, such as my old friend the Laddomat 21, especially on larger systems.....and ideally in conjunction with a proper, high efficiency, clean burning, log batch boiler etc 

Lightfoot.


PS, Alun, as mentioned before, I'm not sure you could warrant or even really need such a device for your proposed small DHW back boiler setup, it would help send hotter water to the top of store sooner and it would be a different story if it was a larger back boiler in relation to the stove's gross output, or if you was trying to charge a larger store, but you would need to weigh up the running costs of the pump against any increase in stove efficiency etc.

[Mr Pipples]

Thinking of a similar system. What boiler are you going to use?

[alun]

A Charnwood Country 8 with a clip in back boiler. Having given it further thought, as the boiler is not an integral part of the body of the stove, I wonder whether this lessens the cooling effect, and hence have less effect on it's efficiency. I have emailed Charnwood for tech advice, but havent received a reply yet.

[Mr Pipples]

I'm getting a Stratford 30 w.b.b. (7.5kw integral boiler) to hook up along side a gas ch loop and solar system to provide hot water in a vented cylinder . Will connect up the solar and gas indirectly through coils  - have to do gas that way as it's a pressurised loop from newish condenser boiler and I don't think it makes much sense to ditch it. Do people think it's best to fit/use an extra coil for w.b.b. or for the w.b.b. to heat the cylinder directly (and if the later, would a Ladomat 21 make sense) considering heat efficiency stuff.

Cheers.