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I recently had a woodburner with a small 10000btu boiler installed. Gravity fed to tank then pumped to radiators (upstairs only) the thing is no mater what the plumber tried it ony gets luke warm water. He repiped making sure no sharp bends or inclines so gravity goes up and return goes down all the way. I hear the tank gurgling like its trying to do some thing but no hot water. I know that 10000btu is small but I only wanted it for hot water when the woodburner is on, and radiators as a thermal dump etc, they also have a pump which i can switch on to pump to the radiators if i dont want hot water or it gets too hot( I wish !) . The boiler has only got 15mm outlets but it goes to 22mm and 28mm as required for gravity feed. He has checked everything and now says try it out for awhile to see if any airlocks clear, if not he feels it might need a new tank which I want to avoid doing before all else is checked. Should i try a pumped system all round or can anyone advise how I check for any further problems. Thanks
sounds like either a partial airlock somewhere, or because it's not flowing properly you're actually getting the water in the heat exchanger boiling, and that's what's causing the gurgling, which is definitely not what you want to be happening.
what's the set up as the hot / flow pipe goes out of the boiler? It needs to go either straight up, or fairly steeply upwards pretty much straight out of the boiler if you're going to get thermosyphon circulation going on IME.
Hi, IFrom the wood burner is it a swept T upto the cylinder? and then the down ward branch down to the pumped cicuit. This way all of the gravity flow is always into the cylinder. Cylinder might be furred up with scale - is the water when pumped the same temp. Older cylinder work better as they have a steeper slope on the coil inside so you getter a better garvity flow. Newer one in order to get more in (asists condensing by increasing heat tx surface) have a smaller pitch so slope of coil is flatter. this prevents good gravity flow. Try it with different pump speeds as well.
The 10k BTU is not a lot and that would be for the stove going at full wack and best wood etc etc so you may not actually be anywhere near that. you could be only getting 5k BTU.
some more thoughts... has your plumber definitely attached the flow and return to the correct ends of the coil and back boiler (hot at top on both)?
if all the above is correct, the next question is what size is the tank you're trying to heat up, and where in the tank is the coil? roughly speaking, even at full blast that back boiler's going to take 2 hours to heat 100 litres of water to 60 degrees, so more realistically 3-4 hours even with fairly regular stoking.
if it's the tank size that's the issue, you should be able to get hot water at the top of the tank by using a flat plate heat exchanger on the outside of the tank instead of the coil. Presuming the tank doesn't already have additional water draw off points (eg for directly heated water), you can achieve the same results by adding a surrey / york flange to the water inlet and outlet points on the tank and connect these to the flat plate heat exchanger so that it then uses thermosyphon action to draw cold water from the bottom of the tank and deliver hot water to the top of the tank. Make sure you put a flow restricting valve on the water side as you can use this to manually adjust the flow of the water through the heat exchanger to determine the temperature of the water delivered to the top of the tank. (note, you may need an additional temp/presure relief valve above the heat exchanger for safety)
The above set up would do nothing to actually increase the total amount of heat delivered by the way, it'd just mean that for the same amount of wood burnt you'd get 50 litres of usable 60 degree water instead of 100 litres of 30 degree water, or 200 litres of 15 degree water etc (approximately).
well thanks for all that food for thought. The gravity 28mm pipe is at an uphill angle but initially travels sideways , angled upwards out of the back of the woodburner where it then passes through the chimney breast and travels up the outside of the chimney breast in a straight line up to the tank. It has a swept t to keep the gravity and pumped systems separate. When the pump is on the water is the same temperature but the pump is only there to redirect to the radiators and they themselves dont get too hot so everything is a bit lukewarm at the moment. Also the actual water pressure is poor even though the bathroom is downstairs and the tank upstairs. The tank is not a big one, he described it as a standard size, foam lagged. I have burned it hard and have used smokeless fuel and on the following day tried wood some times for several hours, not a lot happening although if I run the tap for absolutley ages it finally gets a bit hotter but not bathing hot. There is a immersion heater I havnt used it yet, would this check out the coil and tank were ok if i used it to heat a tank of water ? Thanks for all you comments I will check all the points mentioned and report back.
Do the pipes get hot, if so where to? feel along to see how far it gets, if it is an air lock try attaching a hose pipe to the drain cock at the stove and give it a quick blast from the cold tap this will sometimes shift it!
Is there an open vent off the flow pipe where it enters the cylinder?
is there any control of airflow (eg sliding vents), or anything that alters the amount of heat being directed to the back boiler, and if so are they being used correctly?
Am I right in reading that as the swept T going to the radiators being at the top, or part way up the vertical pipe to the hot water tank? is it possible that the thermosyphon set up is actually partially thermosyphoning around the radiator system?
do as terrier suggests as best you can, and if it's hotter before the T leading to the radiator system than it is after it, and you can feel some heat on the radiator system, then that's your problem.
Thanks again for all the comments. fitter is back tomoro to discuss anyway forward before i give up the ghost and stop spending money on it. Gravity pipe is hot up the side of the chimney, swept t is up by the tank somewhere not sure where, I will get him to show me. I will light the woodburner again tomoro and follow the pipes up but I believe that they are hot right the way up. My feeling is that the cylinder coil may be at fault but i will try the suggestion terrier made(thanks) and attach hose to drain cock. I maybe need to establish that the coil is the standard type I have been told there are those that are more umbrella shaped. i will also get him to point out if there is an open vent off the flow pipe where it enters the cylinder. As far as the radiators go, even without the pump on the one in the bedroom closest to the tank becomes lukewarm, so I guess some thermosyphoning is going on there. Strangely with the pump switched on the same radiator does not get any hotter, whilst the one in the far bedroom is the only part of the whole syetem that gets truly hot. This radiator is 25 ft away from the tank across the landing.
bingo then, you're system must be thermosyphoning around the radiator system as well as the hot water tank, and some of the heat's dissipating out of the radiator.
you'll probably either need to fit an electronic valve that opens when the pump switches on, or possibly reverse the flow on the radiator circuit and fit a non return valve, but there'd probably be other problems with that.
Could also still be an air lock somewhere as well, and I presume you've bled the radiator that's not heating up properly?
Yes have followed through on all your advice, bled the radiators and tank etc, checked the vents and swept t's thanks to all for the help, after much deliberation and being perisihing cold and feeding the stove with masses of logs, I have reached a conclusion.Given that all the vents and pipework is in situ correctly, although I believe for gravity the 28mm pipe should have been more vertical. There is a good possibility that the coil in the tank may well be all silted up, and rather than go to the expense of new tanks or coils and further work, which still may not provide enough hot water or rads. I am going to cap off the boiler and have the woodburner as a stand alone. Then it will either be multi point and electric storage rads, or a small central heating combi. All the advice really helped trace through what the problems might be, this might seem a cop out, but I need to get the house up and running, thanks for all the advice.
Hi I can't quite understand why you did not put the pump on the return side to the boiler and have a 3 way valve fitted to the supply side to split the service to either supply hot water to the radiators or hot water to the calorifier (with automatic controls it is normal for hot water to take precedence, as it is for short durations and for heating to be provided at other times) - definitely not both systems running at the same time! - if there is no 3 way valve then it is likely that hot water to the calorifier will be pulled back by the pump when supplying hot water to the central heating system.
Suggest that before you do anything else you go back to the drawing board and get your plumber to technically qualify the installation.
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