Top Half Radiators Hot, Bottom Half warm/cool?

Discussion in 'Plumbers' Talk' started by Chris101, Dec 1, 2013.

  1. Chris101

    Chris101 Member

    I've had a new central heating system installed in my house since July this year.
    Obviously we weren't able to fully test the heating for a long cycle/term until recently, as the temps have dipped.

    I have noticed for the last few days the rads are hotter at the top of the system and ever less hot on the bottom or 1/2 of them much cooler.

    If I increase the thermostat on the boiler to full, the rads get much hotter but there is still a noticeable difference between the top and bottoms half's of the radiators.

    I tried to bleed each radiator and then repressurise the boiler but this did not help.

    Any ideas what could be wrong?

    *The boiler, Pipes and rads were all newly installed in July.
  2. Hi Chris.

    Process of elimination...

    It's a new system, so all your rads and pipes are in top-notch condition - so no sludge or blockage (usually, a cold bottom means it's sludgy down there. Matron!)

    Also, the rad tops are hot, so that means no trapped air - so bleeding won't help.

    What does that leave? I dunno.

    Ok, the top will always be hotter than the bottom (hot water is lighter than cold), but it shouldn't be that significant since it's a pumped system.

    So, my guess is that the rads haven't been 'balanced' properly, and aren't actually getting as much water flow as they should. In other words, the flow coming in to these rads will be more of a trickle, so the water has time to float the hot up to the top. A faster flow would push all the water through more comprehensively. I think.

    Balancing rads is a slow tedious process, so it may not be a surprise if they haven't been done fully.

    You will notice that your rads have 2 valves, one at each end. One is adjustable - it has a TRV head or just an adjustable wheel. The other end is called a 'lockshield'. This is set and left. The idea is that the rads nearest the boiler, and which will get the greatest flow, should have their lockshields shut partially off so's the water flow is shared more evenly.

    Does your hot/cold situation apply to some rads more than others? If so, you could experiment with the cooler ones.

    If you do this, note down the current position so's you can always return it if it doesn't work. And turn only a quarter turn at a time.

    Bear in mind two things, tho' - rads will always be hotter at the top, and as TRVs shut off the flow, this difference could be more noticeable.
  3. tom.plum

    tom.plum Screwfix Select

    its not good, we can illuminate sludge so its either bad pipework or under estimated boiler output,
    is it copper pipework or plastic? if its plastic it needs to be 22mm from the boiler to the manifolds, if they've split to 15mm that might be the problem, :confused:
  4. Chris101

    Chris101 Member

    The boiler is the Potterton Titanium 33kw HE Condensing Combination Boiler and it's copper pipework.

    Some of the radiators bottom half are much cooler than the others especially when on a lower thermostat temperature.
  5. diymostthings

    diymostthings Well-Known Member

    Not sure if you can guarantee a sludge-free system judging by the appalling standard of installations I have noticed in recent new-builds. Worst thing I saw (amogst several others) was a 22 milli copper pipe which had obviously been bent by hand without a spring or bender - large kink and flattening on the reverse of the bend - just to move the end about 25mm to fit the cylinder boss. This was a pressurised system. I mentioned it to the sales lady who got her sales manager to speak to me. He was very indignant "its installed to current regulations sir and has all the appropriate safety certificates" I didn't say any more - what's the point (ROTD over) - so can we be certain the OP has a sludge free system - his symptoms are classic!

  6. "Some of the radiators bottom half are much cooler than the others especially when on a lower thermostat temperature."

    Well, yes, they will be.

    ALL rads will be cooler at their bottoms. Just how bad are yours? Are they ALL like this, or just some?

    Which 'thermostat' are you referring to above? If it's the room 'stat, then yes that'll make an effect. If the heating is going full-on, then the rads will be hot, hot, hot. What's the difference between hot and hot?

    If, however, the system as a whole is running cooler, then the difference may well 'feel' greater.

    If you mean the rad's own thermostat, then absolutely you'll notice a difference - as the water flow is reduced down to a trickle, the hot water will have more time to rise to the top.

    Are they actually failing to heat your rooms?

    Are the rad valves fully open? If so, try tweaking the lockshield on one rad open a quarter turn - then see how that one compares with the others (give it a half hour to settle).
  7. tom.plum

    tom.plum Screwfix Select

    right, its defo not undersized, and piped in copper it most likely piped up right, so....... your rooms are too big,
  8. Chris101

    Chris101 Member

    Just been to all the rads in the house -
    6 downstairs - 2 in the kitchen/diner, 1 in the bathroom, 1 double in the first reception, 1 in the 2nd reception, and 1 in the passage.
    All are maxed out on the TRV and open fully

    5 Upstairs - 1 in each bedroom inc 1 double in baby's room and 1 long bay window.
    All are maxed out on the TRV and open fully

    The thermostat I was referring to is for downstairs passage which is linked to the boiler.

    I have also double checked the radiators -

    Downstairs - There is definitely a big difference between the top of the radiators being hot and the bottom half much cooler/only warm.
    Upstairs - The Rads are less hot at the top and cool at the bottom half...

    The rads are heating the downstairs regardless of this problem but upstairs its failing to heat 2 rooms!
  9. Is there a rad in a bathroom which is steamin' hot?

    Do the rads make a noise - a hissing?

    Ok, go to one of the worst culprits wielding an adjustable spanner. Pull the cap off the lockshield, place the spanner on the exposed spindle's flat. Note carefully how your spanner is pointing, and then turn it clockwise. Count the exact amount it takes to shut off - this may be less than a whole turn. Tell us exactly - how many turns and fractions thereof.

    Then open it back up to where it was - and then open a further half-turn.

    Come back on here and tell us how much it took to shut it off.

    And after a half-hour tell us if it's any hotter than the others, now it's a half-turn further open than before.
  10. Chris101

    Chris101 Member

    Apologies for the late response.

    The downstairs bathroom which is a few feet away from the boiler is steaming hot and the upstairs rad in the bathroom has the same issues as most of the other rads.

    The cap off the lockshield are turnable so do I still need a spanner? Currnetly all the caps on the lockshield are fully open...
  11. liamo23

    liamo23 New Member


    I know this is an old topic but I came across this while checking out my own problem. I have the same issue but I have a stove with back boiler which heats up to 8 rads at a time. The top half of 8 rads can get very hot but the bottom stays cool to slightly Luke warm meaning the heat into room is limited. this is only an issue when heating them with stove. If I turn gas on the same rads will get piping hot all over and give out noticeable heat in rooms. Can anyone help with this?

    Kind regards
  12. itchyspanner

    itchyspanner Member

    if the system is balanced correctly drom cold with all thermostats fully open all rads should heat up at a simular rate and have a temp difference of about 20 degrees between the flow side and the return side. When radiator thermostats turn off this can of course alter the way it works.

    the more important questions is ...... is the room to temp? the thermostatic valve is there to keep the room to a set temp not the radiator. Its not an uncommon complaint from customers about the rads not all being the same temp, but that how the valves work, saving you money when the rad does not need to be hot.

    Liam, your stove is probably limited to how much heat it can give out.
  13. liamo23

    liamo23 New Member

    Hi itchyspanner

    Thank you for reply. Ok so basically the stove will not always give same heat output as the gas will. It's a Stanley Erin stove that says can heat up to 8 radiators. Also are you saying that I should toggle with settings/thermostat on rads and fully open them when using stove as opposed to having them let's say 3/4 open when using gas? Because they work differently with stove as to turning gas on I need to change settings to suit? Sorry if that's not what you mean!

    Kind regards
  14. Ryluer

    Ryluer Well-Known Member

    The first thing you need to look at is how the stove is controlled. Very important to have Hi-Lo stats fitted which work in partnership with a gravity circuit. The gravity circuit usually encompassing the hot water cylinder and a heat leak rad.

    If its an interlink system then probably difficult to advise without seeing a detailed schematic of your system. Which should be compulsory for all interlink installers to provide once the system is completed imo.
  15. Ryluer

    Ryluer Well-Known Member

    Stanly provide detailed schematics for their interlink systems.
  16. kiaora

    kiaora Screwfix Select

    Going back to basics, the boiler gas or solid fuel, sending the water to the radiators at say, 75c

    When return temperature is say 70c, the boiler will turn off, but.... It doesn't know if only the nearest rad is hot or they all are?

    It's a question of looking at the radiator return pipe work, if the balance/ lockshield valve are not adjusted correctly you will have a system where some rads are sending back very hot water, and some sending back not do hot water?
    Result is boiler thinks all rads are hot!

    Hope this makes sense

  17. liamo23

    liamo23 New Member

    Thanks guys for this information,

    I do think it makes sense especially about the pipe work. I had a guy out a while ago just to look who believes the problem could ly with the pipe work. He wasn't too sure but thinks the return hot water is running back down the return pipe and away from the direction it should be moving. He said putting a valve or something to have the water redirected could solve the issue. I'll look into this further and let you's know!

  18. Ryluer

    Ryluer Well-Known Member

    The stove should be controlled by 2 stats. (Hi - Lo) One fitted on the flow in case of over heating. And the other fitted on the return to ensure low temperature water is not pumped into the boiler.
    Fitting valves to a solid fuel system can be risky.
  19. liamo23

    liamo23 New Member

    Ok I have noticed 2 things which I haven't mentioned. The first is I have my stove stat which was fitted upstairs in hot press set at 55 Celsius. So when the water temperature hits 55 the rads com on and start to heat up. However sometimes the temperature goes well above 60 Celsius even up to 70 and the boiler makes a constant humming sound ( too hot I take it? ) The other issue is with the rads and how different rads on different days heat up and others not too warm. For example if I light the stove today both rads in bedroom get very hot and on landing not so warm whereas tomorrow and without adjusting anything one of the rads does not get hot in bedroom but on landing it does. I'm confused as to why it's like this.

  20. Ryluer

    Ryluer Well-Known Member

    With only a stat on the flow the boiler will be constantly be flooded with cold water lowering the efficiency, it will allow the circuit to rob heat from the hot water cylinder, cause acid condensation to form on the boilers inside and create a cold flue/chimney and the associated condensation/tar problems.

    The high stat should be set between 85 and 95c. The low stat between 45 and 55c.

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