How to properly balance a hot water radiator system

JPL

Can someone recommend good reference material on how to properly balance a hot water radiator system?

I have a system with three zones – one Grundfoss 15-55FC circulator and 3 zone valves. The system has a total of ten old cast iron radiators with heat output ratings ranging from about 6,000 BTUH to 15,000 BTUH and one 4' cast iron radiator. Based on the heat load calculation, there is too much radiator capacity in some rooms while others should be just about right. My challenge is to balance the flows to ensure that I’m delivering the right amount to each area.

In related question, can I trust the GPM readings on the Grundfoss circulator? I have it set at 6 ft constant head. I would think then that the GPM in each zone would be the same regardless of how many zones are open. The readings I am seeing are as follows (note this is with all radiator valves wide open, so no balancing):

Zone 1 alone = 6 GPM

Zone 2 alone = 4 GPM

Zone 3 alone 4 GPM

Zone 1 + Zone 2 = 7 GPM (seems low)

Zone 2 + Zone 3 = 8 GPM (makes sense)

Zone 1 + Zone 3 = 7 GPM (seems low)

All Zones open = 8 GPM (seems low)

I’m wondering if I need a different circulator? If I use the back-of-envelope calculation for head = distance to furthest radiator x 1.5 x .04, it should be sufficient. The distance to the furthest radiators in each zone are as follows. Everything has been re-piped in copper from the boiler to each radiator.

Zone 1 = 45 ft (1” copper) ... 2.7 ft head; total heat load is 37K BTUH (3.7 GPM)

Zone 2 = 35 ft (¾” copper) … 2.1 ft head; total heat load is 11K BTUH (1,1 GPM)

Zone 3 = 70’ (45’of 1”and 25’ of ¾”) … 4.2 ft head; total heat load is 17K BTUH (1.7 GPM)

Any advice/guidance is sincerely appreciated!!

mattmia2

the gpm readings on the circulator are just approximate, i wouldn't try to calculate anything with them.

the easiest way to balance it is to close the radiator valve a little on rooms that are too warm. still too warm, close it some more, too cool, open it a little and move around the system over a couple weeks tweaking it. i suppose if you have people that play with the valves you could figure out orifice plates to put in some of the radiators, but tweaking the valves is the normal way.

EdTheHeaterMan

@mattmia2 has noted the correct method, however people can't understand that, and will "do you a favor" and open any partially closed valve in order to get as much heat as they need in a particular room regardless of what it does th the rest of the system.  I find the only way to get people to understand balancing is to place the thermostat in the coldest room and let the rooms that overheat happen.  When the people realize that the room is overheating, then the partially closed valve makes sense and you will get the system balanced much sooner.  

There is a psychology element to our industry.  Tell people why you do things, may not work for them.  Make them uncomfortable and show how to get comfortable and they understand better.   If it is going to cost them money, they understand quickly.  If it is free then the understanding is useless and forgotten.   You have to give folks an incentive to help you do your job.   

I installed TRVs in a building that saved a building owner  who rented rooms to the Coast Guard personnel in Cape May NJ.  Many rooms overheated and the windows were left wide open to compensate.  The TRVs made that stop and the fuel usage was cut by more than half.   I put the thermostat in the coldest section of the building and everyone else was able to turn their temperature down.  It would not work any other way.

mattmia2

I think they may have re-piped it home run to a manifold, in that case they can use the valves on the manifold to balance and leave the radiator valves wide open.

EBEBRATT-Ed

You may or may not have enough pump. Get a bunch of cheap thermometers and hang them in the rooms. Open all valve wide open. Then leave it alone for a few hours.

Then check the temps and record them. Start throttling the warmest rooms but only a little. Every time you move a valve it will take a few hours to settle out. Record all the temps and see if the colder rooms are coming up and the warmer rooms dropping. Also you can't have the thermostat in the warmer rooms.

I would do this a zone at a time for starters.

If things seem ok doing it zone x zone and go nutty when several zone are on it could well be pump capacity.

JPL

Thanks! I'll get a bunch of thermometers and start the process. I assume it is better to do one room in a zone at a time, rather than trying to do a couple in tandem. Is that correct?

Also, the TRV is a good thought. I have one room that gets tons of sunlight during the day which confounds the problem. A TRV there would probably help immensely. Don't know why I didn't think of that! :(