Radiators- Upstairs hot/Downstairs mostly cold

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20 Dec 2007
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United Kingdom
Hi, I contacted this forum a few months ago about problems with some radiators being cold (mostly downstairs) and it was suggested that sludge was blocking these rads. From the good advice i got, I used a litre of sentinel X400 in the system for a month, flushed it out and refilled. I also took off 3 easily accessible downstairs rads and hosed them through with mains water outside. After bleeding the rads I still have most of the downstairs rads cold or slightly warm ( including the ones flushed out). Upstairs ok. The bathroom rad upstairs at the end of the run is very hot, as is the hot water tank, also at the end of the run. The first rads nearest the boiler are hot (Both upstairs and down stairs)
The systems uses 28mm pipe from an Oil boiler with a gauge showing the water temperature at 70C. This pipework runs from the pump near the boiler to the first floor, with 15mm pipework feeding these upstairs rads and 15mm pipes looping down to each ground floor rads. I've even changed the pump (Grundos 15-60) to no avail. Most rads have TRV's and I've tried turning all upstairs off to try and force water tho downstairs rad with no success either. The return 28mm pipe is warm so presumably water is circulating and not blocked, so why doesn't the downstairs rads get equally hot ?/any ideas out there ?/
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Balancing. See eg FAQ no 2.
But try
1) turn OFF at the lockshield valves, all the rads which work well. Fully open the main/thermostatic valves. See if the others heat up. Should only take a couple of minutes.
Keep turning rads off until the last one gets hot. If it doesn't, (check its valves are open) you have a pipework problem.

Right. Remember that a valve a couple of turns open is just about FULLY open.
Small rads need less than big rads
Rads near the pump need less than rads far away.
A small rad right by the pump may need its valve only just detectably open.
2) open the smallest, closest-to pump rads half a turn. Open further bigger rads perhaps one full turn, and the furthest 2 or more turns

See how it goes (heating on full).
Thanks for reply ChrisR
I've done as u suggested, turned off lockshield valves on hot rads, but other rads do not seem to have improved. The boiler is near the house in an old shed, and I can hear noise through the pump pipework, sounds like air or something, though I've bled all rads. 3 rads seem to be affected ( all downstairs), though only one is stone cold.( furtherest from boiler.) There is no bleed or themerostat on the hot water tank, which is getting very hot and at the end of the run.
Nearly all of the return 15mm pipework is very cold on affected rads. Is it likely to be an obstruction of somekind ?
do you think there is a bypass? If the pipes are hot, feel along them to see where the heat is going
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Where is the bypass likely to be, near the boiler? What will it look like ?
I can't follow the heating pipework due to it being layed under 1st floor, covered by boards and carpet, but it seems to end furthest from the boiler at the hotwater tank, which is fed from 28 or 22mm pipe which is very hot in and out. The main problem rad splits from this hotwater feed pipework, and is still cold going in and out. ( this rad has been removed and flushed & bled) If you run a lot of water from the bleed valve it gets slightly hot, but then goes cold when the bleed valve is closed.
... the hotwater tank, which is fed from 28 or 22mm pipe which is very hot in and out. ... .

sounds like the cylinder is stealing the flow. What causes it to stop taking heat when it is hot enough? Does it have a thermostat strapped to it, and is there a three-way valve near the pump, both with cables coming out of them?

How long ago did the system last work properly?
There does not appear to be a thermostat on the hotwater cylinder, neither are there 3way valves ( no wiring apart from immersion heater). I presume is set to the boiler temp which switches on & off till it reaches the boiler setting. The system is over 20 years old, we've moved here about 3 years ago. It never seemed to be working 100% but seems to have got worse over the last 12months.
photos please of all the pipework round the cylinder, the pump, and the boiler

Especially any valves or odd-looking elbows near the cylinder

if the cylinder is taking all the flow there will not be much left for the rads

Thanks JohnD for web link to photos

photos please of all the pipework round the cylinder, the pump, and the boiler

Photos as follows:







looks like you have the pump permanently running :eek:

so it looks like you need a way of restricting flow to the coil in the cylinder when the cylinder is up to temp.

This is usually done by having a thermostat strapped to the cylinder, which controls a three-way valve that diverts the primary hot flow from the boiler, and sends it to the cylinder, or to the radiators, or to both.

I do not see a flow restriction on the flow to the cylinder coil, usually the flow pipe is throttled down to 15mm to restrict it, so that it does not steal all the hot flow, or you could use a gate valve and scrw it down to partially restrict the flow. You might aready hace such a valve, though I can't see one. You could tinker with it a bit.

Another method is to fit a thermostatic valve on the return from the cylinder, such as a Cyltrol or Stoptap (?) but these are pretty expensive and not much used these days.

the 28mm pipe into and through the cylinder offers very little resistance to flow, so, as you have found, most of the primary flow lazily runs through there instead of marching all the way round the radiators.

p.s. it is usual to have a CH Programmer with a timer and wire into it the boiler, the room stat, the cylinder stat, the 3-way valve and the pump so that all get switched on and off automatically.
looks like you have the pump permanently running

Thanks for reply and advice JohnD, The Boiler & Pump is controlled from a programmer, with a thermostat switching boiler off when it reached temp. Any idea why 3 radiators have poor performance (all downstairs, and have been disconnected and flushed out) ? Also why is there a pipe from the right hand side of the pump going down (via gate valve) to the return feed to the boiler ? I will try and restrict/control the cylinder temp as you suggested. Thanks again for you interest.
That pipe you ask about is the by pass pipe and if the valve is fully open water will take that path back to boiler rather than go through radiators.
I met similar problem last week. With valve about 1/4 open and restricting the flow your radiators will receive more flow.

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