Aluminium radiators cold bottom-left. Can they suffer from sludge buildup?

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Italianconnectio | 22:37 Thu 24th Nov 2011 | Home & Garden
26 Answers
We have just moved into a house with existing combi boiler, and aluminium radiators.
The radiators are attached to 12mm copper piping top-right (flow-side) and bottom-right (lockshield). They all have the same symptom: they are mostly warm (not boiling), but cold in the bottom-left quarter of the radiator.

A plumber cam round and quoted over £1,200 to install a more powerful pump/expansion tank system as he said the piping was too narrow and probably restricted flow to the radiators too much. He did not check the actual radiators for cheaper alternatives such as balancing or sludge. Oh, and he said no guarantees it would work!
-When I spoke to the plumber about the possibility of sludge in the radiators, he said that as the rads were made of alumiunium, there would be no sludge.

-There are 10 radiators on the system, so I have tried turning off 9 of the radiators: this makes the one open radiator warmer, but its bottom-left quarter is still only lukewarm. Both the flow and return pipes are hot.
-After searching on this website, I carried out the instructions for balancing the one open radiator, but this does not fix the problem either.

is it true that sludge could not be the problem? is the plumber right with his £1000 pump/expansion tank idea?

Thank you so much for valuable help!


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So many possibilities, Signor. I've not heard of 12mm tube (except for tails on taps), but that size should be no problem.......... unless the whole system runs off one 12mm circuit. Impossible to say without inspection.
Maybe pump speed.
I'm fairly sure anything could get sludged up, whatever its made of ???
Try bleeding first.

Mainly, try and find a trustworthy heating engineer who'll inspect properly, and advise............ without jumping to unfounded, expensive conclusions ............ i.e. guessing!
To give further comment on sludge, since ‘normal’ sludge is an oxide of iron (magnetite) caused by dissolved oxygen in the water reacting with the inside of an iron radiator surface, there is little danger in my view that this is sludge. Aluminium radiators must have inhibitor added to the CH system as well and Fernox or Sentinel X100 are fine – but this is mainly to guard against the aluminium creating a catalytic reaction with the copper and brass components. The fact that all rads are the same (bottom corner close to water exit point) leads me to believe the basic problem is that rate of water flow through all the pipes isn’t large enough. This means either more oomph in the pump, bigger diameter pipes or checking optimum position of the lockshields re balance.
So to add the TB’s comments about pump speed / power, I’d be inclined to also check that the balancing has been done properly before forking out lot’s of money – the issue I’d be looking for is that the radiator presenting the greatest resistance to water-flow (that is the one with the longest pipework/narrowest pipe combination) has its lockshield valve RIGHT OPEN. This is always the start point for correct balancing, in that all other radiators are then closed down enough to ‘balance’ the water-flow through each rad. If the longest water path is already not fully opened to water flow, ALL either lockshields have to be progressively turned down more than they would otherwise have to be - this has the same impact as having a smaller pipebore, if you get my drift.
Balancing rads take time, because one has to wait a long time for the effect of a quarter turn in lockshield valve to take effect throughout the whole system – pipe that is money to heating technicians / plumbers – so it isn’t always done well. Worth spending time on before spending lots of dosh. Note the number of turns to max open of ALL the lockshields as you start the balancing – that way if you find you can’t get it right, you can at least return it to the original situation.
Question Author
Thanks to The Builder and mate for quick replies!

I've done some more measuring: it's 22mm piping from and to the boiler. This has to go 2 rooms across and 1 floor up. From here, the 22mm piping has two columns (one flow, one return), with 12mm piping which feeds the radiators to 3 rooms + 2 bathrooms.
(there is another one of these setups for the bottom floor to feed 4 radiators).

Correction to buildersmate: the cold part is not near the water exit point. entry and exit are both on the right, top and bottom. The cold spot of the rad is bottom LEFT, so furthest from the entry/exit points,

Regarding balancing the rads, I had a look at this possibility following a post I saw on this site by Gasman, and noticed that all lockshields where open to max. However, by completely closing all rads but one, (and trying different lockshield settings on this one rad), I would have expecting to fix the problem if it was indeed due to balancing?

I can confidently say that there is no inhibitor in the system, as both the plumber and the boiler technicians have drained the system and refilled it (for different reasons), and did not add or mention inhibitor. Now, the problem with the rads was before these interventions, however if the previous owner's plumber and boiler techs operated in the same way, we could probably assume that there was no inhibitor previously either!? What problems would this cause on an aluminium/copper system?

One final thing: the towel racks in the bathrooms are warm throughout, although obviously it is easier for the water to pass through these.

'install a more powerful pump/expansion tank system as he said the piping was too narrow and probably restricted flow to the radiators too much,.

I would have assumed it must have worked when it was first the problem must be something that has happened since, by the way i have a large 4 bed detached running on 8mm !! works perfectly ok with this bore and a consensing boiler.
Question Author
Having spoken to the previous owners, they say that the system never worked well. They did move boilers around and things, and previously to them the house was divided into two: so the heating system was also divided into two. (I am in one of these two!)
So I do not know at what stage of these divisions/changes the heaters ceased working correctly!!
I don't have much experience at designing systems but the pure physics of this tells me you are bound to get a cold area over the far side bottom corner (sorry I misread your OP).
Hot water is less dense than cold, so that is why a typical installation infeeds from one bottom corner, allows the hot water to rise up as it flows across the width of the radiator, the cooler elements start to fall and the coldest water exits over the far side. I believe alternative designs are possible but they use baffles inside the rad to force the flow.
If you are saying that the entry and exit points are on the same side, it seems logical that colder water is going to collect over the far bottom corner unless there are baffles to force an alternative flow. Sounds like there aren't to me. Is it clear that the rads were designed to have the entry / exit points on the same side?
Most importantly, is the heat comfort level in the each of the rooms acceptable? If it is, is this actually causing you a problem.
Borrow a thermometer and measure the input temperature at the pipe on each of the rads - if it is about equal on all rads than no amount of rebalancing is going to change anything.
Question Author
BM, I agree with the physics, but our neighbour (the original house was made into a semi) has the exact same radiators and they are hot all over.

The lack of heat is causing huge discomfort, especially for a north-facing room. Just to get this room to an acceptable temperature I have turned the boiler to max, closed all of the other radiators, and the thermostat never reached its cut off so heating is on all day. (and it's still 10°c outside, so not that cold yet!).
This probably isn't very good for the boiler pump which is running continuously, and not very efficient either... :-(
But I am reluctant to spend £1,000 for something that might not work!!

Are there certain types of radiators, with larger internal conducts (I'm thinking about how the towel rails on the system do work) and high BTU, that might solve the problem for this particular room? Fixing this room only "might" be sufficient as the others are south-facing, guest room, or bathroom.
8mm microbore (god forbid) would give enough circulation for loads of rads.
Modern rads and TRV's just don't need or require massive flows.
Also .. the pump is not a pump .. it is a circulator. It does produce a head of water tho .. as one would expect. A standard size circulator .. like a Grundfoss .. would be fine on anything up to a mix of 15 rads or so. It's common practice to use a primary circuit on small systems in 22mm to HW tank/zone/mid-pos valve, then 15mm from there. Less volume in systems is good.
Unless this is a weird foreign system, the copper can be (microbore) 8mm, 10mm, or normal copper 15mm, 22mm, 28mm. Take your pick.
Cold spots in rads are feasible if one does NOT use common end to end or diagonal feeds. The rad needs removing to check the exchanger .. maybe it does have contaminants in it. You could wash out with some descaler .. but as aluminium, they must be washed out afterwards.
Plumber seems to define this as a gravity system .. He would be right to recommend upgrade to pressurised system with pressure vessel, vent, etc, which would do away with header heating water tank completely. My recommendation is to pipe all rads diagonally or end to end (bottom) as common-place.
Sorry I have no experience of aluminium rads, Melvin. I'm glad BM clarified the sludge issue. That makes sense now.
Anyway, since next door has the same system, I think we can rule the rads out of the problem.
The alarm bell goes off when you mention an original whole building system being "cut in half" when the building was divided.
In certain cases, it would be quite simple to isolate the two separate systems. It all depends on how the pipework was arranged in the first place. I would guess that it was not as easy as that with the original layout ....... hence the fact that next door has the short runs near to their boiler (possibly where the original boiler for the whole building was) ....... and you're left with what are known as "dead legs".

Perhaps, when they divided, they simply tapped into the circuit with 22mm Flow & Returns, and hoped for the best.
Perhaps too many rads are simply "tee-ed off" from one 12mm run. This would create one or more "dead legs"......... especially if the pipe run is long.

Chas mentions 8mm. Nothing wrong with that, BUT his whole system would not work well unless larger bore is carried around the house first ..... possibly for distribution at a manifold. Maybe not, but it obviously has been designed properly from the start. It would seem that your system has not been designed ....... just modified.

If bleeding, balancing etc have no effect, then I'd go back to my first post. You do need someone to trace the layout of the system BEFORE they start to draw conclusions. Who knows? It's quite possible that simply running a couple of 22mm pipes from the boiler to a "starved" point somewhere in the system, might be all it needs.
Have you done heatloss calculations for the building and sized the boiler?
What boiler and model is it?
Is it heating HW cylinder through zone or mid-pos valve .. That working OK?

You can get a rough output in watts or btu from rads by measuring height, length, and if single panel plain, single with emitter, double plain, double with emitter)
Failing this, I can do calcs if you can post room/construction details that I would need.
Draw a diagram and layout .. even tho it may mean looking under floorboards.
Ha .............. cross-posted with Al
No matter, between us I think we've given you plenty to think about!
Just bleed them and save yourself £1,200, your plumber sounds like a cowboy!
Bleed them?! lol

If only it was that easy. Better get around there then, hadn't you?!)
To upgrade a gravity system to pressurised system with Robo kit and expansion vessel .. say 16-18L, take out all in-needed stuff .. and improve some piping and rad work .. £1200 is probably not so far off.
I may have missed it but I haven't seen any mention of water temperature. If your water isn't hot to start with you won't get hot rads. It could be a faulty boiler thermostat or wrong setting. It could be a misplaced restrictor in the pipework. It would be worth checking the wattage of the pump.
For The Builder.. on the continent copper pipe is available at nearly all even millimetre intervals ..except of course 15mm. anyone want my collection of UK fittings?
Jomifl........ another good point. Melvin did mention that the boiler is at max, so maybe a thermostat problem. The towel rads are hot though.

Anyway, as you can see from all the replies, it could be one of many problems. Inspection, tracing etc needed first, I think.
Question Author
Thank you to all the points made, I'll try to address all the questions:
(I am new to all the terminology so some of the comments are helping me better understand and describe the heating system!)

-You are correct, the boiler is feeding HW(*), then 2 zones with valves in 22mm. From each zone, is where the 12mm tubes go to each rad and back again.
-You say gravity system, but there is a pump in the boiler, and I have to fill up the boiler to 1bar if it drops (although it will actually still run on 0.15 bar..)
-Boiler is 28kw http://www.solvgroup....NT_Thermo_compact.pdf but heatloss/building size is not initial problem here. I estimate the rads are working at half efficiency compared to neighbour's. and this is with all rads turned off apart from the 2 in the room I am trying to heat!! Turning on all rads makes those 2 rads colder, and cold zones larger too. If properly working rads did not heat up the room well enough then we'd be looking at the heatloss problem.
-HW: we can get HW in 3 ways: (1) direct from the boilder. (2) from an electric water heater upstairs. (3) there is a serpentine in the water heater that comes from the boiler. I've currently closed off that circuit, however this would in effect act like a hot water tank.
-Not sure how to attach a diagram on here? Also no floorboards, but terracotta tiling: can't look under!

The Builder:
-Yes, "not designed, just modified" seems like the Italian way of doing things!
-Running 22mm to a "starved point", if there is one, is not an easy option: flooring is not floorboard but terracotta tiling..

Water temp is very hot when it leaves the boiler. It cools a bit by the time it reaches the zones but still very acceptable. Like TB says, the towel rads are hot. (they feed off the same zone as non-working rads)

Thanks again for all the answers!
If the water 'cools a bit by the time it reaches the zones' then the flow rate must be very low as normally there would not be a detectable difference in temperature between the boiler and the rads. If the flames in the boiler are at the normal height for maximum (perhaps 50mm) then the heat must be going somewhere. If the flame height drops soon after ignition then you probably have too low a water flow rate. It may be that your circulating pump is faulty, or it is pumping against a high hydrostatic head due to an airlock, or series of airlocks which would have a cumulative effect. Restrictions in the pipework can sometimes be detected by listening for the turbulence caused by the restriction( a piece of 5mm plastic pipe, one end in your ear the other held against the pipe is quite effective)
Question Author
I'll try listening to the pipes, check flame height and if possible measure temperatures tomorrow, and report back! Thanks

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