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Discuss Sanity check..., gravity hot water and pumped radiators, should they work together in the Plumbing Forum area at

  1. Matstones

    Matstones New Member


    I hope someone can comment on my setup and let me know if I have a problem or not !

    I have old gas central heating system, all was working:

    gravity fed hotwater heating
    radiators via pump.

    We then had back boiler fitted to wood burner and replacement cylinder with 2 coil element type fitted by professional plumber, so water can be heated by gas or wood.

    Can't seem to heat water from boiler

    Can gravity work fast enough to transfer the heat, from cold.

    When I try there is lots of scary banging and sound of water rushing through pipes, the input of cylinder gets too hot to touch, the bottom exit gets slightly warm, Water clearly not flowing through it properly.

    Even with the radiators on, hot water doesn't seem to get heated, but as the pump is pulling water through radiator circuit it's hard to see why it would flow down into the hot water circuit, but no banging or venting but water does not get heated

    As a layman I can think of three possibilities

    1/ The water heats up too quickly to create circulation current, boils and vents off
    2/ There is a Honeywell valve on exit - I replaced the head, and checked the spigot turns, this seems to be OK, and does activate when powered - and tried locked open - seems to be ok but could be blocked
    3/ An air block in hot water heating element

    What I want is to be able to heat water from boiler without radiators being on..

    Should gravity be enough for water heating only

    As there are two parallel circuits (rads/pumped+ water/gravity), it's hard to see how they can both exist together...

    If you made it this far - thanks for reading - any comments very welcome

  2. stani

    stani Plumber GSR

    You've had a motorised valve fitted to your cylinder, it is now NOT gravity.
    DO NOT USE until you have got the fitter back, you are heating water against a closed valve.
  3. Chalked

    Chalked Plumber GSR

    Nothing wrong with a motorised valve on a gravity hot water return.
    It should be wired up as a "c plan" incorperating a white wire in the head.
    • Like Like x 1
  4. stani

    stani Plumber GSR

    Your right, just never came across C plan, but do remember them.
    Still, banging noises there's a problem
    • Like Like x 1
  5. Matstones

    Matstones New Member

    When I set the controller to call for hot water it activates the valve so valve is open.

    The logic being in winter we can have radiators on without the boiler heating water. The wood burner produces more than enough heat to heat the water, but only heats lounge so we need radiators on...

    In summer, we want the opposite - no wood burner, no radiators, but gas heated water..

    So in summer, controller calls for hot water, valve does activate, boiler heats up, after maybe 2 minutes, clanging, whoosh, and boiler turns off due to it's thermostat.
    upper pipe too hot to touch, lower pipe slightly warm. I have tested with Honeywell head off and manually turned spigot anti-clockwise

    Should gravity work - or do I have blockage or air lock...
  6. Matstones

    Matstones New Member

    looking at drawings I do have a "C plan" system
  7. Best

    Best Trusted Plumber Top Contributor!!

    Why is the flow pipe rising from the tee to the coil?
    Sorry to tell you this, but your plumber doesn't know much about pipework
    Another thing is some cylinders have coils that only suit pumped water.
    You have to order a cylinder that is suitable for gravity circulation that solid fuel fires require
    • Winner Winner x 1
  8. Best

    Best Trusted Plumber Top Contributor!!

    Gravity should work no bother at all and fairly fast.
  9. Chuck

    Chuck Active Member

    Not all cylinders are suitable for gravity-circulation systems. Most of them these days assume a pumped system. Check you've had the right type installed.

    Air locks will, of course, prevent circulation but the installed pipework should have the correct gradients and vents to ensure they don't occur.

    Looking at your photo. I would expect to see the hot water rise from the boiler, turn right flow down a slight gradient (your's is upward) through the Honeywell (?) valve, into the top of the heat exchanger where it transfers heat to the cylinder, cools, and emerges from the return near the bottom.

    The upward gradient on the upper pipe, which you call 'return' and I think should be 'flow' would be a great place for an air lock to form.

    If anything I've guessed/suggested conflicts with the manufacturer's installation instructions then I'm wrong, follow the MI!

    Edit: While I was typing this Best posted essentially the same points. He was more economical with his words, and I paused to get a mug of tea midway, so he won.
    • Funny Funny x 1
    Last edited: May 11, 2017
  10. Matstones

    Matstones New Member

    I am not a plumber but understand the pipe has to rise for gravity to work. Around the back of the boiler (photo doesn't show) are similar 28mm pipes for wood burner circuit which works well.
  11. Best

    Best Trusted Plumber Top Contributor!!

    You have me confused. I thought you said the solid fuel stove won't heat the hot water?
  12. Chuck

    Chuck Active Member

    It's more complicated. One needs the correct combination of temperature gradients, pipe gradients and points of heat extraction and input for 'gravity to work'.

    Whether a pipe rises or falls depends on which direction the water is travelling inside it.
    • Agree Agree x 1
    Last edited: May 11, 2017
  13. Matstones

    Matstones New Member

    gravity fed wood burner circuit works
    (can't see pipework on photo - on back of cylinder, same 28mm fittings)

    gravity DHW does not seem to work - seems circuit is blocked

    I understand the logic and location of the airlock on heat exchanger input at top

    To remove I need to force water through the coil ?. Does it matter which direction (ie from vent, or supply in attic). Isn't there a danger I'll just flush water through the boiler, bypassing the cylinder, as it offers lower resistance

    Would cracking open the bolt remove airlock - I suppose not as not much pressure to push air out, unless I heat the system, but knowing me if I did that I'd spray boiling steam into my face, so not too keen in that idea..
  14. Chuck

    Chuck Active Member

    If a pro fitted the cylinder you need to get them back to fix the problem. This will involve first checking that the cylinder is suitable for gravity circulation. If so then I would expect them to move the tee feeding the top part of the heat exchanger upwards to get the gradient correct. If it's not a gravity cylinder, they need to fit a circulating pump for the HW or replace the cylinder depending on which you are happy with (and paid for).

    Musings: This is a strange mistake for a pro to make. They've used end-feed fittings and and the soldering looks okay but they've left an inverted trap in a gravity-circulation system. Did they assume that your boiler had a pump on the HW circuit, perhaps?

    . . . and while you were screaming in pain water would continue to pour out of the open joint causing a remarkable amount of damage to your house in the process. Damage that probably wouldn't be covered by your home insurance policy.
    • Agree Agree x 1
    Last edited: May 11, 2017
  15. Best

    Best Trusted Plumber Top Contributor!!

    I still remain confused! You say the "gravity fed wood burner" circuit is connected to the rear coil and is working. But then you say the " gravity DHW " isn't working? There is no such term as that in a heating system. Your coil that isn't heating must be from another boiler, - gas I think you said? Gravity circuits not allowed on new gas or oil installs now.
    But as you are not a plumber, you could make a mistake.
    The job has clearly not been done by someone who understands the basics of connecting to a cylinder coil. Even if that front coil was on a pumped circuit, it is wrong and air will be trapped in top of coil.
    I would get the plumber back who did the job to see what he says is wrong with his work. The job probably needs another plumber who is experienced in gravity circuits and dual heating link ups to check all the pipework.
    Last edited: May 12, 2017
  16. Matstones

    Matstones New Member

    There are two heat sources and two coils both for domestic hot water (DHW), wood and gas.

    The system was installed many years ago, I did test it then, but not really used gas for hot water much due to wood burner and unlimited free wood. We also use the immersion heater as we have PV solar power that provides free electricity. (sometimes!)

    Anyway I slackened off the 28mm input to cylinder and air hissed out and carefully heated up the system to create more pressure.

    It seems to all work now - I can hear the boiler boil but it doesn't vent, and both ends of the coil now get hot soon after system is turned on.

    After about 5 minutes the boiler turns off due to thermostat control, and when cools due to heat transfer it comes on again. I guess this regular on/off/on/off is an unavoidable fact for gravity fed systems as there is not much water in the transfer system when the radiators aren't being used

    So a massive thanks to everyone who commented.

    I now have a working system and a much better understanding of how my C-Plan system works
  17. Chuck

    Chuck Active Member

    That was a dangerous thing to do. You are lucky you didn't injure yourself.

    You shouldn't normally be able to hear water boiling inside a 'boiler'.

    One end should get hot, typically 75°C. The other should start cold and gradually rise to 60°C over a period or an hour or so, at which point the boiler should stop supplying heat.

    If the system is installed and working correctly the cylinder should heat constantly without the short-cycling you observe.

    In my opinion, your system is not properly installed and is neither safe nor efficient. You really should get a competent heating engineer to review it and fix any issues.

    Also, it doesn't normally make financial sense to use PV-generated energy to heat water if you can use gas instead and sell the surplus electricity.
    • Agree Agree x 1
  18. Matstones

    Matstones New Member

    Points taken

    However with solar power you are paid for all the power you generate, whether you use it locally or feed it back to grid, so not a choice to sell or use, you either sell and use or sell and don't use...

    So on a sunny day it produces 3Kw. I am paid for 3Kw regardless. I have choice to dump it in immersion heater, or if I don't use it it flows into grid. Either way makes no difference in terms of £, but one provides tank full of hot water.
  19. Chuck

    Chuck Active Member

    Interesting. My comment was based on this:

    Feed-in tariffs: get money for generating your own electricity: Overview - GOV.UK

    , which seems to describe a generation-tariff (as you describe) plus a 4.85p/kWhr export-tariff, which is more than gas costs at the moment. Evidently you either don't have an export meter or have another type of feed-in agreement.
  20. Matstones

    Matstones New Member

    My solar system was installed 8 years ago - the contracts were much more "interesting" then, and the contract holds for 25 years.

    Would be a very different contract for a new system today
  21. Best

    Best Trusted Plumber Top Contributor!!

    Boiler is coming under pressure and forcing gravity flow to work, I reckon. You removing air was only a temporary sort of fix, but not correct or a solution. Get a heating engineer in to repipe the fault we mentioned and anything else they find wrong
  22. zzzjim

    zzzjim Well-Known Member

    Another GOV scheme to get the ball rolling .
    (Won't be wanting a Smart meter (IF it can cope !))
    . Combining heat sources was expensive to do right ..(Safely)
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