monster radiator (close to boiler), little heat

John Ketterman

> means the water doesn't spend enough time in the <BR>
> boiler or rads to pick up or drop off heat.<BR>

Yes, which is why the temperature drop across the radiator will be lower. Any given gallon of water loses less heat, but there are more gallons per minute, and these things tend to cancel each other out but not perfectly. If the supply temperature is the same but delta-T is lower, then the average radiator temperature is higher, which means there is slightly more heat output.

Speaking as an amateur, of course; I have some technical training, but I'm not in the heating business.

John Ketterman

One of our radiators is a behemoth, maybe 10' long (30" high, 15" wide). This was originally a gravity system, and this radiator is the first one off the mains (i.e. closest to the boiler). I am told that gravity circulation is weak close to the boiler, which is why such a big one was put in. The other reason is that it is under a row of windows in an enclosed porch.

In any case, now we have forced circulation, and it is not producing enough heat. Typically it can get the area up to about 64F when the rest of the house is 68F.

The mains are 3" pipe, and the radiator in question is the only one that is connected with 1-1/2" pipe (other rads on the first floor have 1" or 1-1/4" pipe). Our current system is a mod/con with outdoor reset, but the problem is not limited to mild weather, it is there at all water temperatures. The radiator does get warm all the way up, there's no air in it. I replaced the original valve because it was sticky, but this did not help.

I know that heat output is not very strongly related to flow rate, but what could it be other than inadequate flow rate? If it is flow rate, what can I do to increase it?

Oh, by the way, the radiator has a metal "biscuit-tin lid" type cover that is flat on top and comes down only a few inches at the sides (so the radiator is mostly open on the sides). Removing it would certainly increase heat output, but my wife won't let me do it; the large top surface is very useful given the 30" height. So...do they make these covers with mesh material to allow convection? Or would that be a custom job?

Mike T., Swampeast MO

If insulation/weatherization has been improved, my guess is that such resulted in a comparative improvement of loss to this "enclosed porch".

Such could however easily be a "sleeping porch" that was INTENDED to be cooler.

Sorry to sound like a broken record, but TRVs are the best comfort addition you can make to a gravity conversion system--and a condensing modulating boiler the best economy addition...

Steamhead (in transit)

Too MUCH flow?

This can be as bad as too little. Try closing the valve halfway and see if that helps. If it does, you're over-pumped.

If you can tell us the total EDR on that system and what circs you have, that would be another indication.

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R. Kalia_9

> Sorry to sound like a broken record,

> but TRVs are the best comfort addition you can

> make to a gravity conversion system--and a


It does have a TRV in place of the old valve (yes, a 1-1/2" TRV), but I don't see the relevance. As I understand it, it's not possible for a TRV to increase heat output, only decrease it.

R. Kalia_9

I don't actually know the EDR, but if it helps, I have a Grundfos 15-58 circ and I've used all three speeds, no difference.

How would too high a flow reduce the heat output? As far as I understand, if flow is too high, deltaT across the rad becomes smaller, which is wasteful but results in a small increase in heat output.

Steamhead (in transit)

Too much flow

means the water doesn't spend enough time in the boiler or rads to pick up or drop off heat. I know of one case where the circ was so badly oversized that the boiler metal overheated, expanded unevenly and caused a leak.

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Mike T., Swampeast MO

You're correct, TRVs can only LIMIT heat, not increase it.

Do all of the rest of the radiators have TRVs?

If so, you'll need to increase the supply temperature. Removing the cover may well solve the problem however... I've seen old perforated cast iron radiator covers on ebay, but never anything near that size. A decent metalworker could probably make something using perforated sheet metal--surely it's still available.

Steamhead (in transit)

But

the surface temp of the rad is a bit higher with a greater delta-t. Also, the incoming water at the boiler is cooler, which makes for a slightly greater delta-T between the flame and the water, so the heat transfer there is a bit more efficient.

And don't forget the extra energy that a bigger pump would use.

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