Heat dump possible option - using woodburner boiler circuit?

[Eco Andrew]

My planned set-up is to use solar panels in summer and woodburner boiler in winter to heat a thermal store.  The system is a little over-sized (three 20-tube panels into a 300 litre store with fairly modest hot water usage) so I need a heat dump.  I sounded out my idea with Ivan (at one of his recent - excellent - workshops), which is to put a pump on the woodburner circuit to come into operation when the store starts to overheat.  This will take heat out of the cylinder and dissipate it through the woodburner's boiler.

Now, my builder/plumber (who is currently doing our loft conversion) reckons that when the pump comes into operation it will end up pumping water up the vent pipe into the header/expansion tank.

Who is right?

Or is it a case of 'it depends on the system' - maybe contingent on factors such as height of expansion tank above the boiler coil, speed of pump, direction of pump? (just guessing - I don't have a clue really - no plumbing experience ).

Urgent response needed please - he's almost ready to install the pipework and thermal store.

It he's wrong, what can I tell him to convince him?

[Greenbeast]

i think you'd need something like the laddomat 21 which allows the water to thermosyphon from the boiler in the event of power failure. and yes pumpover will depend on the configuration of the pipework, just like any boiler installation.

also i don't know how much heat you'd realistically lose from the internal boiler, unless you're intending to pump round the radiators, in which case, that works great.

[Mike McMillan]

I have six 15 tube panels heating 250 liters of water. I do exactly that, a pipe thermostat on the wood burner coil kicks in at 80 and circulates through the wood burner back boiler (which is pretty big) and takes the tank temperature down to 70. Works a treat! Even today with a cloudless sky and haze. There is a little splash into the header tank, but that presumably syphons back into the circuit. It has never over flowed.

Mike McMillan

Sunny Isle of Wight

[desperate]

Hi Mike

That little splash needs to be stopped ASAP as it is taking oxygenated water down into the system and will be corroding all the steel double quick, To go back to Eco Andrews point about pump over, you need to arrange the feed and expansion pipes to be at the neutral point of the system (in relation to the pump)to prevent this. I have seen no end of problems caused by pump over rotting out the heating system, it really is essential to sort it.


Desperate

PS Andrew

To answer your original question, the most reliable way to prevent pump over is to have a seperate feed and expnsion pipe connected directly to the boiler and take a feed and return from the other pair of tappings, assuming you have 4 tappings on the boiler, if you only have 2 tappings the feed and ex pipes should be teed off as close as poss to the boiler from the flow pipe, and then carry on to the system controls/pump etc. there should be no valves etc between the f/ex pipes and the boiler.

I hope this gibber makes sense.


Desperate

[Eco Andrew]

These posts are very helpful and informative. Thanks.  I think I'm making sense of the terminology.

Desperate, I do have 4 tappings from the boiler so what you suggest should be possible.  When you say 'take a feed and return from the other pair of tappings' I assume you mean 'take a flow and return from the other tappings'.  Also when you say 'then carry on to the system controls/pump' I take it you are referring to the flow pipe carrying on, not the f/ex pipes.  Sorry, but I need things to be idiot-proof!

Would the feed pipe be connected to a bottom tapping and the expansion/vent pipe be connected to a top tapping on the woodburner boiler or the other way round?

Any advice about the position of the pump on the circuit?  I was planning to locate it near the thermal store circulating in the same direction as the gravity (thermosiphon) flow occurs when the fire is lit, and using a low speed setting.

Greenbeast, when the woodburner is lit it will work by thermosiphon to heat the thermal store.  It is only when the solar panels are providing excessive heat to the thermal store that the pump will come into operation.  Judging by Mike's experience this should be effective in cooling the store.

Thanks again to all.

[Mike McMillan]

ooh, desperate, there's always more to plumbing than meets the eye.. I thought I had it sorted. It's such a tiny amount of water. As the header tank does not get replenished from the ball cock, is it still oxygenated? How about putting some inhibitor in the system, that help? I might have to blank off a panel otherwise.

Mike McMillan

[Justme]

How about setting up a clothes drying room as a heat dump?

Easy & cheap to fit lots of pipes (or even an off the shelf towel rad) to the walls.

I guess down side is it will only work in summer when the clothes shoud be on the line any way.

Justme

[Greenbeast]

Greenbeast, when the woodburner is lit it will work by thermosiphon to heat the thermal store.  It is only when the solar panels are providing excessive heat to the thermal store that the pump will come into operation.  Judging by Mike's experience this should be effective in cooling the store.

fair enough, its always handy when you find someone that has done what you're asking

[sleepybubble]

erm, so what you have worked out is great for dumping heat from the solar, but what happens when there is no sun and you're running the WBS how do you dump any excess heat from that?
Presuming the WBS has a dedicated heat dump facility cannot this be used to dump the solar heat?

[Eco Andrew]

Hi Justme
Yes, I hate to think of the heat going to waste - if only I had a spare room to use as a drying room!  We're in a 2-bed terrace (soon to be 3-bed when the loft conversion is finished).  Another idea I had was to have pipes running down into my cellar/storage space, where some of my excess wood is stored (current loft conversion has provided a few winter's worth!!)  That would keep the wood from getting damp, but again wouldn't be needed in the hot weather, when the heat dump is likely to operate.
Andrew

[Eco Andrew]

Hi sleepy bubble
Yes, I will have a radiator to act as a heat dump in the loft connected to the woodburner boiler circuit.  The loft is soon to become a bedroom and so this will have a benefit in helping to keep that room warm during cold months when the woodburner is in use.  However in summer when the weather is exceedingly hot my darling daughter will not thank me for the radiator coming on in her bedroom because the solar panels have overheated the thermal store!  Since the woodburner will not be lit in such weather I thought it would be a handy (and cheap) alternative.  The living room is colder being on the north side of the house and most of the heat will go up the chimney anyway so it shouldn't get uncomfortable.
Cunning plan eh?

[desperate]

Hi Eco Andrew

yes you are right I do mean flow and return from the other tappings, and also yes take the flow pipe to the other controls, sorry for the confusion. You are correct again about connecting the feed from the header tank to the bottom tapping and the expansion pipe to the top tapping, then if you get into a boil over situation the water and steam go around in a nice circular manner, with no real problem.
Regarding the pump position, I nearly always site the pump on the flow pipe from the boiler, this places most of the system under positive pressure, and if you have a leak you notice it as water escaping, if the pump is on the return you dont see a leak, but air can be drawn into the system to wreak its havoc unseen untill a radiator rusts out, This does put the pump in a hotter situation but they should be able to cope ok, very few pumps burn out due to overheating, mostly it,s down to air entrapment in the bearing or sludge causing it to stick.

Hope this helps


Desperate

PS

Mike

As per above it is all down to the position of the feed and expansion pipe in relation to the pump, I am afraid that warm little waterfall in the loft is where the oxygen is coming from, not the freshwater from the main. Inhibitor will help for a bit but it will soon be oxidised to uselessness.

BTW I have a caravan parked up about 1 mile south of the tesco superstore near St Hellens, we will be there for 2 weeks in August, Maybe meet up for a beer?


ATB


Desperate

[sleepybubble]

Regarding the pump position, I nearly always site the pump on the flow pipe from the boiler,

Circulator impellor made of plastic, in contact with steam from uncontrolled burn on WBS turns to extruded mess of stringy stuff up the flow pipe.... seen that one.

[Mike McMillan]

Hullo desperate, thanks for you reply. It is not a continuous flow through the header tank, a spoon full when the pump starts and a spoon full when it stops. Would be delighted for you to come round for a cup of tea when you are on the Island. I am away 12 - 28th Aug, boss wants to go cruising on his yacht in the Med.  Mike

[Eco Andrew]

Thanks for the clarification, Desperate - this is very helpful.  I'll show this to my builder and we'll all be happy.

Sleepybubble, I've chosen a pump with a high-temperature specification (115 degrees I think) so I guess this will cope with being on the flow pipe.

Again, thanks all round.

Andrew