I have recently replaced my open fire (with back boiler) with a multi fuel stove with integrated boiler. It all seems to be working well apart from a small aspect which I'm not sure is how all gravity feed systems work. I'm not a plumber so I have probably got the terminology wrong.
The Integrated boiler has four 1' connections which I have plumbed in diagonally, 28mm for main feed and return and 22mm for C/H with pump. If I wait for the Main feed and return to heat up and then turn on C/H pump. All pipes eventually get hot but as the fire dies down the C/H pipes remain hot but the Main Feed and return are cold is this normal.
'Gravity' circulation relies on natural convection, whereby hot water is less dense than cooler water and thus rises to the top, while cooler water falls to the bottom. The gravity circuit needs to connect the top of your boiler to the top of the coil in your (hopefully indirect!) hot water cylinder, with the return connecting the bottom of the coil to the bottom of the boiler. Hot water then rises from the boiler to the cylinder, gets cooled a bit by the water it is heating in the cylinder, and goes back down the return pipe to the boiler.
It is usual to have the flow and return pipes on the *same* side of the boiler. What was your rationale for connecting them diagonally? It's pretty likely that the central heating pump is interfering with the gravity flow - which is why your gravity pipes are cold when the CH is working. The gravity connections should be on one side of the boiler, and the CH connections on the other side. Furthermore, the CH pump should be on the return side, pumping water back into the bottom of the boiler.
An alternative is to combine the HW and CH return pipes just before the boiler - for which I believe you can get a special fitting - and just use 3 of the 4 boiler connections. If done right, the pumped CH flow helps to draw water from the gravity circuit, assisting circulation.
You sure about that? The instructions for at least two manufacturers say otherwise - eg "flow and return connection of any circuit on oppposite sides of the appliance" or "diagonal pairs should be connected for domestic hot water and central heating".
|> It is usual to have the flow and return pipes on the *same* side of the |> boiler. What was your rationale for connecting them diagonally? It's |> pretty likely that the central heating pump is interfering with the |> gravity flow - which is why your gravity pipes are cold when the CH is |> working. The gravity connections should be on one side of the boiler, and |> the CH connections on the other side. | |You sure about that? The instructions for at least two manufacturers say |otherwise - eg "flow and return connection of any circuit on oppposite sides |of the appliance" or "diagonal pairs should be connected for domestic hot |water and central heating".
Which would force the water *through* the boiler rather than just on one side. sounds sensible to me.
I had a look at the installation instructions for a multifuel fire with boiler at
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P7 the schematic matches your description Also see
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on P11
The examples shown have indirect cylinders - if you have a direct cylinder would that also make a difference?
If the water in the cylinder is hotter than the water circulating through the CH then if the cylinder is well insulated what would induce convection circulation in the pipes or putting it another way if you pumped cold water through a cold boiler could you cool down the hot water cylinder?
If he's got a direct cylinder, I strongly suspect he's installed it wrong.
One would hope not. So yes, it would be a good thing if the convection circuit stopped under those circumstances, and this would give the observed result (the pipes going to the HW cooling, since there's no flow).
I have to admit that my remarks are based on what was common - and on what I did successfully - about 30 years ago, so maybe the perceived wisdom has changed in that time. Certainly the document cited by PeterK in the next post shows diagonal connections - which I still find surprising.
It also shows an option of combining the two returns, using an injection tee - which is something to which I alluded in my previous post - and may well solve the OP's problem.
Using a direct cylinder would be a very *bad* idea! It would mean that the domestic hot water would pass directly through the boiler and radiators - rather than having separate primary and secondary circuits with indirect DHW heating via a heat exchanger - with the result that you couldn't use corrosion inhibitor. Sheet steel radiators wouldn't last very long, and nasty brown water would come out of the hot taps!
If the temperature of the water in the hot cylinder is the same as that in the boiler, you ain't going make it any hotter whatever happens - so it doesn't matter that gravity circulation stops under these circumstances. Indeed, if the boiler subsequently cools down, it's a good thing that there''s no gravity circulation - since this would otherwise dissipate the heat stored in the cylinder.
Just a last remark. Gravity hot water needs all the help it can get. To keep a correct temperature differential I have the feed pipe from the boiler heavily lagged and the return pipe unlagged.
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