Cooler water to the radiators farther away?

[TSN79]

I'm wondering about radiators. A system runs with a supply and return temperature of 90/70 degrees C. But won't only the first few radiators have this actual temperature since the water reaching the radiators far away will have cooled considerably on the way...? How does one make up for this?

 

[russ_watters]

With insulation, usually the loss will be negligible.

 

[NoTime]

I'm wondering about radiators. A system runs with a supply and return temperature of 90/70 degrees C. But won't only the first few radiators have this actual temperature since the water reaching the radiators far away will have cooled considerably on the way...? How does one make up for this?

Computer software is used to calculate heat transfer for each radiator, room size, windows, doors, exterior wall and insulation type. The physical radiator size is also adusted for different water temps at each stage of the loop.
The end result is (hopefully) a consistant temp througout the house.
There is a tremendous amount of calculation that goes into this.

 

[russ_watters]

Actually, since the units of R value of insulation are just hr*ft^2*F/btu, it isn't that hard to calculate the loss. You need the surface area of the pipe and the temperature difference between the water and air. Then using the specific heat of water, you can find how much it drops in temperature.

 

[NoTime]

Sure, its possible to do it by hand.
Probably take you about a week to do it though.

It takes an hour or two just to pump all the required parameters into the computer, depending on how complex the house is.

 

[Artman]

In most commercial and some residential settings, the supply water and the return water are separated in different pipes. So the water that has run through a near radiator then goes to return, while still hot water continues to the remote rads. Insulation on the pipes reduces the loss to the remote rads and they see nearly the same entering temp as close rads.

Check out "reverse return" and "direct return" piping systems.

Reverse return takes the return water from the farthest rad supplied back to the boiler first and the water returning from the closest rad goes back last.

Direct return is piped so that the farthest supply is also the farthest return, the closest supply is also the closest return.

 

[russ_watters]

Sure, its possible to do it by hand.
Probably take you about a week to do it though.

It takes an hour or two just to pump all the required parameters into the computer, depending on how complex the house is.

Naa, it's about 5 minutes and you just need a calculator.

Example:

Lets say the house has 200' of 1" piping. That's 52 sq ft. of surface and a volume of 13 cu ft.

If your insulation is R5, your water is 180 degrees, and your room is 70, that's 1144 btu/hr.

Since a btu is 1 degree of temperature rise per pound and there are 62.4 lb/cu ft, that's 1.4 degrees F per hour.

Negligible. [someone can check my math...]