How does the size of a radiator impact its heat output?

[TSN79]

I'm involved in a project with central heating. Due to a miscalculation the apartment radiators are about 50% too large. The current system water flow is based on the assumption that the radiators are the correct size (i.e. smaller), even though they are not. Due to this the water takes longer to pass through the radiator, lowering both the return temperature and the overall surface temperature. This in turn also makes the heat output lower. What I can't wrap my head around is that a lower return temperature indicates that more heat was transferred - but how does that comply with the heating output being lowered...? What am I missing?

 

[BvU]

This in turn also makes the heat output lower

How do you conclude that ?

 

[TSN79]

How do you conclude that ?

Because the difference between room temperature and radiator surface determines the heat transfer. If they were the same there would be none. And because the surface temperature will get lower with a larger radiator, the heat transfer will decrease.

 

[anorlunda]

more heat was transferred - but how does that comply with the heating output being lowered.

You are contradicting yourself in those two statements.

Assume we bring hot water into a room. If we leave it in the room long enough, the room warms and the water cools to the same temperature. Maximum warming has been achieved.

If we leave the water in the room only a short time, it cools a little, the room warms a little, then we remove it from the room. Only a little of the water's heat has been transferred to the room, and the water we removed was still hot.

 

[BvU]

And because the surface temperature will get lower with a larger radiator, the heat transfer will decrease

Not true. If the water comes in with the same flow and temperature in a larger radiator the exit emperature may well come out lower, but that means more heat has been transferred. Especially in the area near the water entrance the situation will be more or less the same.

For your understanding: compare one radiator with two in series, same flow and T_in. First radiator will do about the same as before. Second radiator will also transfer heat. 2nd radiator exit T will be lower, averagee T also, but more heat is transferred.

If all else fails, resort to the equations. You have them at hand ?

 

[NTL2009]

Because the difference between room temperature and radiator surface determines the heat transfer. If they were the same there would be none. And because the surface temperature will get lower with a larger radiator, the heat transfer will decrease.

You can find the disconnect in what you wrote above.

First you say (correctly), that: "the difference between room temperature and radiator surface determines the heat transfer"

OK. And then you go on to say: "the surface temperature will get lower with a larger radiator" (so far so good..)

But... ", the heat transfer will decrease" Umm, why do you say that? Do you see you just ignored your own words that heat transfer is a function of both surface area and surface temperature. Surface temperature went down, but surface area went up. So therefore...

 

[russ_watters]

What do you mean by "too large"? (And what type of radiator?) Do you mean they are longer? If so, imagine the first 2/3 is the radiator you needed and the last 1/3 is a second radiator with a lower inlet temperature. The added heat transfer into the room is the heat transfer from the second radiator.

 

[TSN79]

You can find the disconnect in what you wrote above.

First you say (correctly), that: "the difference between room temperature and radiator surface determines the heat transfer"

OK. And then you go on to say: "the surface temperature will get lower with a larger radiator" (so far so good..)

But... ", the heat transfer will decrease" Umm, why do you say that? Do you see you just ignored your own words that heat transfer is a function of both surface area and surface temperature. Surface temperature went down, but surface area went up. So therefore...

When I said "...radiator surface determines the heat transfer", I meant the surface temperature, although I get that also the surface area plays a part. A radiator with a surface temperature of 70°C will provide more watts than if the surface was 50°C. And that's where my confusion begins - because with the larger radiator the surface temp will go down (giving less watts), but the water will cool more - making me believe it provides more heat to the room. Am I confusing watts with heat/energy?

 

[russ_watters]

When I said "...radiator surface determines the heat transfer", I meant the surface temperature, although I get that also the surface area plays a part. A radiator with a surface temperature of 70°C will provide more watts than if the surface was 50°C.

Your gut is failing you here and not properly evaluating the contradicting effects of lower temp and larger area.

And that's where my confusion begins - because with the larger radiator the surface temp will go down (giving less watts), but the water will cool more - making me believe it provides more heat to the room. Am I confusing watts with heat/energy?

Watts IS heat energy (power actually). The lower return temp does indeed tell you the whole story, telling you exactly how much more heat is being transferred.

 

[TSN79]

Your gut is failing you here and not properly evaluating the contradicting effects of lower temp and larger area.

Watts IS heat energy (power actually). The lower return temp does indeed tell you the whole story, telling you exactly how much more heat is being transferred.

So the bottom line is that the larger radiator (some of them twice the intended length) will provide more heat to the room even though it will feel cooler when you put your hand on it?

 

[anorlunda]

So the bottom line is that the larger radiator (some of them twice the intended length) will provide more heat to the room even though it will feel cooler when you put your hand on it?

Feel cooler on the return side only, not the supply side.

You do not have radiators connected in series do you? I mean where the return side for room A is supply side for room B.

 

[NTL2009]

Feel cooler on the return side only, not the supply side.

You do not have radiators connected in series do you? I mean where the return side for room A is supply side for room B.

I assumed that was the case (radiators in series), but I jumped to a conclusion there.

I think it would be helpful to the OP to provide a simple diagram of what he thinks it should be, and how it looks with the "too large" radiators.

 

[CWatters]

I'm involved in a project with central heating. Due to a miscalculation the apartment radiators are about 50% too large. The current system water flow is based on the assumption that the radiators are the correct size (i.e. smaller), even though they are not. Due to this the water takes longer to pass through the radiator, lowering both the return temperature and the overall surface temperature. This in turn also makes the heat output lower. What I can't wrap my head around is that a lower return temperature indicates that more heat was transferred - but how does that comply with the heating output being lowered...? What am I missing?

Is it actually causing a problem? Are people complaining and about what?

It's not clear what is fixed and what is variable in the system. If there are room thermostats then I wouldn't expect larger rads to make much difference. They will just be on for a smaller percentage of the time.

If people are complaining about being cold despite the rads being on all the time then likely the boiler/furnace is under size.

 

[CWatters]

Due to this the water takes longer to pass through the radiator, lowering both the return temperature and the overall surface temperature. This in turn also makes the heat output lower. What I can't wrap my head around is that a lower return temperature indicates that more heat was transferred - but how does that comply with the heating output being lowered...? What am I missing?

Heat output depends on both the temperature and the area of the rads. The average surface temperature might fall but the increase in area more than compensates.

The "radiators" in my house are at least TEN times "too big" and as a result the surface and return temperatures are pretty low. I have Under Floor Heating.