ADVICE FOR BOILER SIZE/PIPE SIZE

FeelingColdColdCold

Looking to know if my thinking is right-help! Have a 25 year old cast iron Burnham 160Kinput works fine using 3" steel pipes feeding 18 cast iron radiators in 2700sf 100 year old brick house in Boston with only insulation being 6" cellulose on attic floor. Storm windows, no insulation in walls. Based on heating bills and therms used, 52K BTU would suffice so by adding 1.4 oversize factor I'm at 72K. Being safe, can go to 1.7x which is 89K. Replacing with another old cast iron boiler as too many plumbers stated they're servicing too many modcons so energy savings $ becomes plumber maintenance $. Plumber reco is to repipe with 1 1/2" copper (as want to remove the low hanging 3" pipe in basement to get some ceiling height). It's all 1 zone and 3" pitched pipe was gravity fed it seems and then they added a pump. Should I be worried about anything? Pipe size? Decreasing boiler size so much? Or just get some electric blankets if it all doesn't work?

Big Ed_4

Do you want to keep the cast iron radiators , then forget about repiping the supplies in the basement .

If you need the space , scrap the iron and go panel radiators and Pex piping .

Size the boiler to the heat loss .

Hot_water_fan

Upsizing by 70% is extremely conservative - you’ll be paying more upfront and every month on your bills. 

EBEBRATT-Ed

You need someone to do an accurate heat loss and size the boiler properly. Re-piping near the boiler can be downsized based to the new boiler sizing. Size the pipe for 10000btu/gpm based on the boiler output.

If you alter the piping to the CI rads out in the system you may throw the system out of balance.

EdTheHeaterMan

You can remove the 3" main pipes to all the radiator takeoffs. You may also need to remove/redesign the riser from the radiators that go down the the basement. Those riser pipes may be too low. You don't want to have the pipes go from the new main, point down to the end of the old riser then back up to the radiators. You may need to remove some of the riser piping also. See the Red pipe in the "New Main Pipe" on the illustration below. The supply is incorrectly using the original riser that ends below the new, smaller main pipe. The return is piped correctly, with a shorter riser so the water can flow more easily with less restriction and no thermal trap.
@EBEBRATT-Ed makes a point about the balance of the old design that repiping will most certainly cause. In order to compensate for the imbalance, it will probably be wise to also install new thermostatic radiator valves on each radiator. These valves will compensate for the imbalance caused by the new design.

OldawgBryant

@FeelingColdColdCold How did that job work-out for you?

OldawgBryant

@EdTheHeaterMan I am doing kind of a similar thing, but not messing with the supply/return for the radiators, only replacing the boiler & connecting to the piping. I've done the heat loss etc, and I'm just trying to figure-out how to size my near boiler piping and connect it to the old system. Most of it is 1-1/4" (coming out of the new boiler) and serving the existing house. What happens if you drop-down to 1" in between? Should I be doing a head loss calc for all the existing piping so my boiler gpm does not suffer?

EBEBRATT-Ed

Size the pipe coming off the boiler to the boiler output in BTUs

3/4" pipe 4,5000,btus=4.5 GPM
1" pipe 80,000,btus=8 gpm
1 1/4" pipe 140,0000 btus=14gpm

If your leaving all the old pipe you probably don't need a head loss calculation if you pick the right size boiler

EdTheHeaterMan

I would direct you to this publication: https://s3.amazonaws.com/s3.supplyhouse.com/product_files/108119-Reference Guide.pdf
I used this to teach a one day seminar on Hydronics. The first 4 pages of the text (3,4,5 & 6) give you the information you need for selecting the correct pipe size for your system. @EBEBRATT-Ed has the info in the previous post but the book gives you the reason why those pipe sizes are what they are. It will have more info to answer some of your other queries that you have been discussing with hot_rod Bob Rhor From Califfe

hot_rod

When pipes are oversized flow velocity can be too slow. If flow drops below.2 feet per second it can be hard to purge air out. The air-will not move along with the water. You end up with noise, lack of heat and frequent purging

So spend some time with pipe sizing