Sizing Radiator Supply Lines

JTMartin

Hey all - glad I found this site, already found some great info on here.

My question just looking for some starting direction on figuring this out. Looks like my boiler is dying so when I replace it I'm going back with the same Weil Mclain Ultra (have indirect tank that I think is fine) but looking to eliminate some of the old steam piping in the basement and switch the house to two zones (upper floor and lower living space).

Currently some monster pipes feed the thing. I've seen people commenting on running 3/4" pex al pex to 1/2" for each cast iron radiator. I could figure that out but I think I have a preference for running copper since I have a good bit on hand and will just wait to find it on marketplace.

So, the one riser to the second floor is 1" black pipe and feeds two radiators. The other three radiators have their own 3/4" risers. I will be tying into all the risers in the basement exposed ceiling.

Two radiators on the first floor (big guys for living room and dining room) have 1 1/4" black pipe supply and return. They all split out of 2 1/2" I'm guessing.

Anyone have input or resources for sizing the pipe on the header or supply and return lines? I was thinking to use 1 1/4 for the header above the boiler and size down as necessary.

Any input appreciated! I'll come with pics next time!

Thanks

mattmia2

is this currently steam or hot water? i assume hot water since a wm ultra is a mod con.

you really should do a room by room heat loss calculation as the first step. that would tell you the output you need from each radiator. from that you can figure the flow the radiator needs and what size pipe you need to carry that flow. there is no reason to replace the risers, just connect new pipe to it in the basement. the load and the flow dictate the pipe size.

EBEBRATT-Ed

You need to know the EDR of the radiators to size the pipe.

For copper I use the following"

Take the boiler BTU output example: 100000btu ouput:

20 degree TD (standard) take the 100,000output and divide it by 10,000= 10 gpm total flow

30 degree TD divide by 15,000=6.6 gpm

40 degree TD divide by 20,000= 5 gpm

For the pipe size"

1/2" copper 1.5 gpm

3/4" 4 gpm

1" 8 gpm

1 1/4" 15 gpm

1 1/2" 25 gpm

2" 40gpm

For the radiators a radiator with an EDR of 100= 100 x 150btu/edr=15,000btu

15,000 btu/20,000= .75 gpm= 1/2" copper for a 20 deg TD

15,000 btu/15,000=1 gpm= 1/2" copper for a 30 degree TD

Best way is to do a heat loos of the house room x room. Then get rad EDR of rads and see how low you can run the water temp and still heat the space. Then size pipe on what you need.

or take the easy way and use the 20 degree TD

Using lower water temp saves fuel but you have to circulate more water to get the same bTUs for the same size load. The rads are probably oversized so you can use lower water temp if you want.

mattmia2

i agree mostly but if you have a radiator that is 3x oversized you don't need the full output of that radiator and you want to keep things balanced so you can also consider heat loss.

JTMartin

You guys are awesome. Working on room by room calcs now I'll check back tomorrow.

Thanks again!

JTMartin

Okay guys reporting back with some details so far. Again I appreciate that help.

So I have EDR estimates for all the radiators. I used this file which was fairly helpful: https://images.greenbuildingadvisor.com/app/uploads/2019/01/05190952/Radiator-classic_sizing_how_to.pdf

One question I had about that before sharing more was it recommends using a 140 degree design temp for hot water system with 90BTUH output. That's what I did but didn't know if that was generally accepted.

For a simple example before doing more I'll use the two upstairs rooms that run off the same 1" riser (and split down to 3/4" under the floor I imagine before feeding the radiators).

Bedroom 1 (Master) - EDR 70 or 6300 BTUH

Heat Loss for Same room 2,478 BTUH

Bedroom 2 (Guest) - EDR 50 or 4500 BTUH

Heat Loss 1773 BTUH

For context house is a 100 yrs old. Has original sash windows with storms, no wall insulation but construction is cinder block with brick veneer on the outside of that. Attic is fairly well insulated with 4" foam and fiberglass laid over that. I came up with R of 23 for attic. Walls R value of 1.91. Windows R value of 2.

Intuitively do those numbers seem in the ball park to you guys? The EDR was easier to calculate with that table but the heat loss seemed a little more rough estimating.

How can I now use this information? @EBEBRATT-Ed I'm having a little trouble connecting your flow estimates with this information I've gathered.

Any more input helps! Thanks guys.

pecmsg

those radiators were rated at 180* supply water.
now if the structure was tightened then 140* may be good.
At the be least there will be times you won’t be condensing

mattmia2

the heat loss is an educated guess at the insulation of old wall assemblies and the infiltration. a mod con boiler would let you set and adjust an outdoor reset curve to come close to matching the heat loss and if your math is close it looks like it would be condensing most if not all of the time.

EBEBRATT-Ed

If your heat loss calculations are correct you could use a water temp below 140.

If it was me, I would calculate the heat loss with a couple of different methods to make sure it is accurate.

If it is then select the lowest water temperature that will heat the building with the radiation you have.

Don't forget the water temp in the chart is the " average water temp" so if it says 170 deg that is 180 supply and 160 return.

If you use a 30-degree td and average water temp was 140 you would supply at 155 and return at125