piping size for old radiators

nogomomo

Replacing the heater in the old farmhouse. I tore out most of the old gravity feed piping. I have free standing radiators on the 2nd floor, 24" high, 28 to 34 columns. Two of the rooms are fed with 1/2 copper, coming out of the basement off of a 2 inch main. One room is fed off of 1" black pipe, supply and return to the basement and the other two rooms are fed off of a 1" black pipe , feeds two radiators, pipe tees, then feeds both, then back down to basement. The gravity feed piping is old, a oil burner had been installed 35 years ago and a circulator was currently being used. The first floor is all cast iron baseboard, being fed by 1/2 supply lines. Is it okay to run 3/4" lines to the takeoffs for these 2nd floor radiators? Boiler will have 1" supply, that I will reduce to 3/4" to feed out to opposite ends of the house. Were the 1" feeds to the 2nd floor, from when it was a gravity system?? looking for any thoughts or advice. thanks, dave

Ironman

Pipe sizing is based upon the btu requirement. What size boiler are you installing?

Generally speaking, a 3/4" line is sufficient for most residential size rads.

I believe that there are radiation tables from the resources on this site. If not, go to uticaboilers.com and find their radiation tables and use them to determine the square footage (EDR) of your rads. Then multiply that x 150 for the btu output at 170* average water temp.

With a standard 20* delta T design, 1 gpm = 10k btu's; 2 gpm = 20k btus, etc.

For residential hydronics, a 1/2" line carries 1.5 gpm; a 3/4" line carries 4 gpm; a 1" line carries 8 gpm; a 1.25" line carries 14 gpm. This will keep the velocity at 3-4 feet per second which is where we want it.

I would not have removed the large gravity piping unless there was an issue: it gives buffer to the system which prevents short cycling when a low mass mod/con boiler is used.

You really need to do a detailed load calculation to determine the proper size boiler. Trying to size it from the previous one or from the radiation is a sure way to come up way over-sized.

nogomomo

thanks Bob, \i am installing a Slantfin modulating propane 30 to 120, 92k net. When I run the heat load calculations, I come up with 60K, but there are a number of voids in the walls, where we will have plenty of infiltration. i am going to use outdoor air reset.

SWEI

nogomomo said:

i am installing a Slantfin modulating propane 30 to 120, 92k net. When I run the heat load calculations, I come up with 60K

Why are you oversizing so much?

nogomomo

i really like the combi boiler, the unknowns in a farmhouse are really tough to estimate, i used infared gun to look at walls, doors, etc. , before insulation is installed in walls, ceilings,

Ironman

You need to trust the load calculation - if it was done correctly. In the 35 years that I've been doing load calcs, I've never seen one come up short if the correct data was input. To the contrary, they always gave more than sufficient heat at design temp. The same for cooling.

Over-sizing a low mass mod/con will cause short cycling which will decrease its efficiency and life expectancy. Not to mention a larger boiler costs more.

Bigger is NOT better with boilers.