Radiators with an 'arch of cool'

[Spamlet]

Prompted by 'do these products actually work' below:

I noticed, while balancing after fitting a smaller rad in a bedroom that had one much too big for the room, that the shorter rads with an area approaching square, don't seem to heat up in the middle.

I guess that what happens is that the hot flow rises as it enters the rad and then goes across the top and down and out at the other side, leaving an arch of cool in the middle that never quite seems to catch up.

These rads aren't blocked: swivelling them to horizontal heats them up evenly, but stand them up again and the arch reestablishes. I've tried all the pump speeds and valve positions, but as far as I can tell, this behaviour is just associated with the squarer shaped rads.

Most of the time I suppose we don't notice this as even the reduced hot area is sufficient to keep the room warm, but with the cooler than usual weather these ones can't quite keep up.

Anyone else noticed/solved this one?

Cheers,

S

[Ian_m]

You need to flush out the radiator it is blocked. I think you will find it is a black gel like substance, based on a copper/iron hydroxide that occurs when no inhibitor is used in a system.

Every radiator in my last house suffered from this when I moved in. I drained the system down, thick black water came out, removed each radiator one by one and flushed using a hose pipe until water ran clear. I put back all the radiators, filled the system and drained a few more times. I then ran for a week with Fernox system clenser and drained again.

I have heard of people acheiving good results by just adding the system cleaner for a week to save having to remove the radiators.

Refilled but with normal Fernox added.

All radiators heated evenly all over (starting at the top obviosly).

When working on system 6 years later, water was still clear.

[Spamlet]

The radiators are relatively new and joined in the main by plastic that I put in myself and completely cleaned out the few stretches of copper that still remain, with flushing, and following up with inhibitor. The 'water' which one drains out is now more or less clear, though there was still a bit of sludge in the big old rad that I have just removed. As I mentioned, I did lay one of the square rads down, as an experiment, and the water then passed through the 'blocked' area with no problem. (I might add that there are two of these small rads, and one is upstairs and the other down, the pump being upstairs - I would prefer down but didn't set it all up myself - both show the same effect so it is not a relative pressure/head of water thing.)

Cheers for the reply.

S

[John]

I don't think it matters where the circulator is (upstairs or downstairs). Perhaps less likely to get sludged if upstairs. My combi is upstairs and I have no issues over that aspect.

[Spamlet]

In our case there is not a lot of head of water between the pump and the top up tank and overflow, and the pump pumps downwards, so it can draw down air if left on full speed for long (not that we need to do this - but others might). I would prefer to have it downstairs and pumping horizontally, with the bleed upwards to make it easier to keep air out of the system (and save the bearing).

S

[chunkyoldcortina]

How do you add these treatments to pressurised systems with a permanent filling loop?