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Heating and cooling: small Backboiler heat sink

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    Im plumbing in a simple primary radiator circuit off a wood burner back boiler that only puts out 3 Kw
    I will use a pump as I want a rad in the cellar as well as the ground floor.
    I only need a tiny heat sink radiator of a half a KW ? (in case of power failure)
    How much heat is lost on a 6 meter 28 ml copper pipe to the loft vent?
    does the heat sink radiator have to be the first radiator?
    can it be in the cellar on the return run?
    It seems like I am asking the impossible.
    Any thoughts would be appreciated.
    I am likely going to want to use the fire, without heating anywhere upstairs is my problem.
    Thanks
  2.  
    The heat sink rad has to run on gravity, which means the heat has to rise from the stove to the rad, so the rad must be above the stove. I would have thought 0.5kw might be a bit small. Think - if you have the stove running on full chat and the power drops, what will 500watts do with 3000watts to loose?
  3.  
    Thanks Peter
    That's how it seems to me, but they recommended 10-20% of the Boiler output on another thread and 20 foot of un insulated copper will drop about 250 Watts.
    I am going to stick a long low rad in the loft bedroom about 1.5 meters below the header tank .
    Ill try and get one rated as high as possible.
    A question For economy
    Can the fall and return be in Plastic pipe? until it gets to a few meters of the stove.
    click fit people seem to think its OK.
    Though I am using 28 ml copper to the loft, can the pipe diameter be reduced after the heat sink?
    It will be 15 at the pump .
    Thanks again.
  4.  
    Plastic pipe can be used after the first few meters from the stove (some installers say 1 meter) but one problem with plastic is that it sags so after the first firing of the stove the plastic pipe won't look very good.

    The trick with gravity circuits is the the pipe should all equal up, so 28mm would split into 18 and 15, or 22 and 15 then later 18 and 15 (or 15 and 15) then the return steps up in the same way as the flow devided. The essence is that you are dividing the flow with the pipe diameter in proportion to the flow you require, so if you reduce the pipe diameter after the heat sink then the flow will also be restricted. If you expect gravity to flow through the pump then I would use something a bit bigger than 15. (although I have had gravity (unintentionally) running on 6mm)

    Without a diagram it is difficult to comment properly from the comfort of my armchair.
    • CommentAuthorJonG
    • CommentTimeSep 19th 2016
     
    HETAS won't permit plastic on primaries in the UK, also the heat leak should have no valves on it, just elbows into the tappings and an auto air vent in place of the usual manual air vent.

    The pipework to the heat sink rad should only reduce to 15mm within the last 300mm.
  6.  
    A properly installed gravity circuit should be self venting otherwise you can never get rid of any air locks. And if it is self venting then an auto air vent on the heat leak rad. would not be needed. But I am not sure if the current UK regs. demand one.
    • CommentAuthorEd Davies
    • CommentTimeSep 19th 2016
     
    Does that mean that at least one of the connections needs to be to the top of the heat leak radiator?
  8.  
    Posted By: Ed DaviesDoes that mean that at least one of the connections needs to be to the top of the heat leak radiator?

    I would say - yes.
    IMO the best connection is in at the top and out at the bottom opposite side, but second best is in at the top and out at the bottom same side.
    On an ordinary gravity circuit it would work if both connections are at the bottom, providing the pipe runs have the correct gradient, but any air trapped in the rad would need a method of removing it and whilst an AAV would probably work I would not want to be reliant on an AAV for the correct working of my safety installation.
    • CommentAuthorowlman
    • CommentTimeSep 19th 2016
     
    Peter, I too don't trust normal/standard/cheap AAVs on gravity back boiler and some other circuits as the floats inside are often of plastic which with the frequent high temps. that can occur in these systems can cause deterioration of the float. If you go for an AAV look at the better quality ones, but they are expensive.
    • CommentAuthorJonG
    • CommentTimeSep 20th 2016
     
    Yes connections are top entry bottom exit and pipework should rise on the flow to the rad and fall away from it.

    Building regs do call for an aav on the bleed tapping, but as you point out there is some naievety about their longevity and effectiveness.

    Sadly this is just symptomatic of building regs generally.

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