Wood pellet/buffer tank/UFH

[derekhandforth]

I have a Gerkross 30kw pellet boiler heating a UFH system via a tank in tank buffer tank. The internal DHW tank is 250L plus another 300L as buffer. The boiler has a Laddomat 10 fitted with a 52deg C thermostat and the tank has a mixing thermostat so that the DHW is 50 deg C. The tank has two thermostats one at the UFH outlet the other at the UFH inlet. These turn the boiler and its pump on an off via a relay which allows the boiler to run until both thermostats are satisified an not to run until both thermostats are unsatisfied. This is to try and prevent boiler cycling. The UFH flow is 48degC and return 40degC controlled by a mixing valve on each of the two manifolds. The building is a new build bungalow of 2500 sq ft.

The tank has the DWH in and out at the top, with 5 outlets/inlets down the side and 2 for the solar coil at the bottom

1) I think that the connections to the tank are in the wrong place, the flow and return of the UFH are either side of the boiler return half way down the tank so despite the relay the the boiler is running nearly all the time. My pellet usage is 2 tonnes per month equiv to 1000L of oil.

2) What temperature should the Laddomat 10 thermostat be. I also have a 72degC one.
3) What temperature should the boiler be set at and also the tank thermostats.
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[Balenza]

What make is your buffer tank ?

 

[Kes]

Derek, I can't answer your questions but I am interested in wood pellet boilers, so I hope you don't mind if I bend this thread a little. I am an oil user with a similar kw boiler as you in a large old house. The news that oil will stabilise around 60p a litre, and the fact that I have to lie down in a dark corner when 2000 litres arrive along with a £1200 bill, sharpens my interest in wood pellets.

The problem is that, as with many new technologies, there's a lot of guff being spouted. Take this from The Guardian and other publications (possibly cribbed from some offshoot of the pellet industry). 'Each tree will produce about three tonnes of pellets - enough to keep 200 homes warm for a week.' Now a week in 200 homes is 200 weeks in one home. So three tons of wood pellets will warm a home for four years at a cost of little over £100 a year? I want some of that. But your use is two tonnes a month - are you burning the stuff? I do wonder why this cobblers ever gets into print.

Also oft quoted is that two tonnes of pellets are the equivalent to 1000 litres of oil. Great, as two tonnes of pellets is, what, less than £300? But what does equivalent to 1000 litres of oil mean? Energy output? If I used 1000 litres of oil a month (the 'equivalent' of Derek's use) I would have a fit and be on the phone for a boiler service sharpish. We use around 400 litres of oil a month. The only way I can equate the two is if the wood pellet boiler efficiency is around 40%, and that 60% of that energy goes up the chimney. Even at 60p a litre my 400 litres come to £240 - cheaper than two tonnes of pellets?

I wonder whether those who install wood pellet burners for the best of reasons are being misled? I think that if they believed all they read they would be rather miffed. This isn't in any way a criticism of your install Derek, just trying to sort the wheat from the chaff.

Rgds.

 

[Nixt]

The pellet usage can be worked out. Good pellets should produce at least 5kW per Kg. The ones I recommend quote 5.3kW/Kg. On an average winter's day the OP should burn 30-35Kg, which is certainly the case for one of my customers. The cost of pellets are about £150-£200/tonne in 15Kg bags, which calculated is £300-400 per 1000 ltr/kerosene. Bulk pellets are much cheaper but require a purpose made store and coupling for delivery vehicle hose.

OP- was the boiler commissioned because feed rates (adjustable) should be set according to flow/flue temperatures. Boilers are usually over 80% efficient.

 

[derekhandforth]

You have all highlighted my problem. The pellet boiler is set to use 6kg/hour. At 4.8kw/kg times 85% efficiency gives and output of 24.5kw. The heat load of the 230sqm bungalow is 9kw at an outside air temperature of -3degC(which is rare). I am burning 67kg/day (outside air temp +3degC) which is nearly 275kw. The bungalow is a new build with 270mm loft insulation/cavity wall insulation and double glazing. So where is all my energy going? The boiler has been checked 5 times it the last 5 months and is working properly. My only conclusion is that the connections on the buffer tank are incorrect as it is not filling properly.
Would it be better to have the boiler flow and UFH flow together at the very top, and the boiler return and UFH return at the very bottom. The UFH manifolds both have thermostatic mixing valves, so it does not matter how hot the flow is.[/img]

 

[Kes]

There's a similar discussion here ... //www.diynot.com/forums/viewtopic.php?p=529103#529103

I missed a trick in my previous post. I should have said 'Sort the wheat from the pellets'. Jokes are never as good served cold.

 

[Balenza]

I expect your heat load is much higher than 9 kw at -3 outside especially if you only have standard insulation in the walls.

I would still be interested in knowing the model of buffer tank.
Was it supplied by Gerkros ?.

The manufacturers of these products tend to give detailed information on the various tappings.