heating

[johnycarper]

can anyone advise me why one of out radiators has a cold spot on the bottom,i know when its cold at the top its air so i bleed them.I have  checked that the hot water is coming in from the valve end so the flow is correct but its still cold on the bottom.

Thanks

[Gluestick]

Sludge, probably.

How old is it?

[johnycarper]

very they are the old fluted cast iron ones,i did drain the rad down at the weekend and nothing came out except clean water.

[tonyv]

I would put it down to low flow rate. The water cools down sufficiently in the rad to create a temperature gradient.

Shut off all the other rads, and see whether this one reaches full temperature. If so, you need to start fiddling with the balancing valves to ensure an as much as possible equal flow through all rads. Can be tricky without specialist kit (basically, the temperature drop across each rad should be similar).

[Gluestick]

Probably then corrosion in the bottoms of the castings.

You could try closing them off from the circuit (i.e. capping the feed and return pipes), closing both valves and inserting de-corrosion fluid and leaving for a week or so: and then flushing the rad out.

It would need to be removed and probably inverted.

One danger is if the rad comprises separate sections, then they are joined top and bottom and have gaskets either side of the casting: the gaskets may well leak since they are probably currently sealed by corrosion!

Restorers strip these, dump them in a bath of strong acid (Usually Muriatic I suppose), reverse flush and rebuild, with new gaskets and then sandblast and stove.

See here:

[johnycarper]

Do you think it would help putting if possible a bigger and more powerful pump,i have a 30-18  tf110 if that makes any sense it was the biggest one i could see last year when i had to change the old one.

[Gluestick]

In a word,  probably no.

Sadly.

If the original pump was properly specified, and the whole system correctly designed, then the pump is really an assist to the natural process of the thermo-cycle.

Unless and accepting if the runs from and to the boiler are excessive, then the system would need some extra "Grunt".

Is the offending rad right at the end of the supply circuit?

Very long pipe runs are usually assisted by installing a secondary pump, at the last rad in circuit.

This is where "Zoning" comes in in design.

How many stories is the house?

[johnycarper]

   it is a big house and the runs are quite long but the rad in question is about two thirds of the way down the house,the pump is above the boiler in the cellar at the back of the house.It is only this rad and the next one in line that do not get hot so i guess it is as stated too far away for the pump to get a good flow.

[Gluestick]

Well, the first thing to try is "Balancing" the rads.

This is achieved by opening or closing the Lockshield Valves, until the temperature differential of the Flow and Return Pipes is within 20 Deg F.

For a rough rule of thumb, you can reasonably balance the system by closing all rad lockshield valves except the last in circuit; bringing the boiler up to temp: and then gradually opening up the lockshields, backwards, towards the first.

The core concept is the first rad in circuit will receive the hottest water: and the temperature of the circulating water will diminish as it progresses to the last.

In practice, you will find that the first rad will have the valve open a little: and each successive valve will be opened more.

Again, a rule of thumb is to test the temperature of each rad, in an identical place using a surface thermometer.

As a bonus, if the last rad becomes hot top and bottom, when it is the only one in circuit (At the beginning of the test), then clearly it cannot be suffering overmuch from corrosion.

See here:

[johnycarper]

i Spent the morning doing as you have suggested and turned them all off except the blocked one,it did get a bit warmer but not much.I then slowly turned them on one at a time and tried to get them equal.Would it be a good idea to put a new rad in place of the old one.

[Gluestick]

I suggest that if you turned all else off and left the suspect rad open, only, and it still refused to reach desired operating temp, then it really is Quad Erat Demonstrandum.

Corrosion.

It is a good idea to replace that rad with a cheap pressed item, perhaps temporarily, both for the Winter and until the defunct unit has been cleansed and restored.

These old cast iron rads are valuable, BTW, so don't be tempted to chuck it![:)]

[johnycarper]

Thanks for all your advice and i think this is the best optionm I am not sure how heat effective these old rads are against the new ones.I may leave it until the summer and try and remove the rad and clean it or just replace it.

Thanks again,

john

 

[Gluestick]

Cast Iron rads are just as effective as modern pressed steel items.

Provided the inner corrosion is removed.

They are much slower to reach operating temperature, obviously, since there is a much greater mass of material to heat: but the upside is they are slower to cool down, so are still releasing heat when the CH boiler has shut off.

Different material's efficiency at absorbing and radiating heat, is usually expressed as K (or Thermal Conductivity): additionally," thermal transmittance, incorporates the thermal conductance of a structure along with heat transfer due to convection and radiation.  It is measured in the same units as thermal conductance and is sometimes known as the composite thermal conductance. The term U-value is another synonym."

Cast iron has a K value of 55: whereas copper is 401: and Aluminium 250.

Thus the best rads of all for speedy heat up and heat transfer potential would be copper: not very robust though!

However if one starts at the basic precept that all CH systems are Heat Systems and that radiators are simply Heat Exchangers, then it's all really a matter of Heat In-Heat Out.

If a particular rad is not conducting heat effectively from the water to the air-surface (Where it can then radiate), then the heat of the water in the pipe feed stays very much the same: little difference between the temps of the feed and return.

Thus little energy is lost.

The effectiveness of any radiator is a simple function, really, of its surface area to air. Bigger rad (In area) then bigger potential BTU/Kw heat exchange capacity and thus output to room.

The great benefit of cast iron rads is longevity. There is a much greater thickness of metal to rust through!

And they look great, too.

[:)]

[johnycarper]

Thanks for that,i think the boss wants to keep the old one as they do look good so i will have to try and find somewhere that will clean it for me.

john