Sizing circulators in gravity to hydronic conversion

DMassey

I am a homeowner with a gravity hot water system and an early sixties Weil McLain boiler. I want to convert to two pipe hydronic, change the 2" main lines to 3/4" while keeping most of the 3/4" and 1" feeds that connect the main line to the radiators, and replace a couple of the existing cast iron radiators with the thin European style radiators(those cast iron ones are huge). I don't have a budget right now for a new boiler, and will be doing all of the work myself with the help of a plumber friend. The house is an old farmhouse with insulation in the walls and ceiling but still has the old windows with triple tracks. The guy that sold me the European panel radiators said I could use 40 Btu per square foot as a rule of thumb to figure Btus of heating. I ended up with four zones-2 upstairs and 2 down. 1st zone up is 22,400, 2nd zone up is 16,800, 1st zone down is 23,600 and 2nd zone down is 19,500. This works out to 39.4 Btu/Sq. Ft. Based on these zones, how many GPM should I be circulating at? How do I calculate head so I can be sure I am getting a circulator with the correct GPM? I read on the wall about Grundfos 3 speed pumps. Would this be a good choice for me? Also, do I need any other hardware to make this thing fly? My plumber friend knows installation and a little design but didn't want to tackle the design on this one. Can you help me?

Boilerpro_3

Move back to square one

The 40 btu/sq ft rule is not the proper way to size equipment. In my 2 story, 1400 sq ft per floor,1906 home in Northern Illinois with 700 sq ft of original windows with aluminum storms and insulated wall and ceilings my heat load is only 18.5 btu/sq ft. The heat load was calculated using standard engineering methods using the R-values of the materials and their areas, and the infiltration rate for the home. This heat loss has been confirmed by monitoring fuel usage under real conditions. Do yourself a great favor and order the heat loss software available at this site. I am forever replacing grossly oversized boilers, in both commercial and residential applications, sized by previous installers using those types of sizing methods.

Boilerpro

c-rex

back to square one

is a good idea! The "corn crib calculation" that has always been my favorite has been one foot of basaboard(3/4fin tube) per two feet of running wall, unless it was under a window in which case you would go foot for foot. Now nuts as this is, I personally came from a job where in terms of actual footage used, that was a good calc! I'd still recomend the IBR method, you'll be happy you spent the time,and I know for a fact you will not over spend on your mains & run outs. Good luck.

tim smith_2

sizing rads & pumps

If you are going to install the panel radiators, do not try to put them on a zone with other cast iron rads. The heat and cool down rate is much faster on the new panel rads than the cast iron. 2nd, if you are trying to use the existing runouts to rads you will have to bell reduce those pipes near the main to get some pressure drop to help equalize the flow to all radiators on that zone. There is a really good software program from Elite software called hvac calc. you drag and drop a boiler and heat loads, ie radiators, and a pump. Then you connect the piping with there connector icon. Then you stipulate heat load of each radiator at the icon and stipulate segment lengths of pipe and type of pipe. Then it will will transfer loads to boiler sizing automatically and give you gpm and head based upon the size of rads and pipe length and sizing. Quite a good program. You can try their fully active program on a demo to see how you like it. Good luck Tim

Mike T., Swampeast MO

CAREFUL! CAREFUL! CAREFUL!

"I want to convert to two pipe hydronic..."

Most gravity systems ARE two-pipe hydronic. If you have a one-pipe gravity system is reasonable working order please don't let anyone but an expert touch the piping!

"...change the 2" main lines to 3/4" while keeping most of the 3/4" and 1" feeds that connect the main line to the radiators, and replace a couple of the existing cast iron radiators with the thin European style radiators(those cast iron ones are huge)."

Not many guarantees in life, but I can nearly give one in this regard. Do this and the heating balance in the system will be shot and you'll be writing asking "why aren't some of my radiators heating?" Of course I can't see the system but ¾" is quite small and MAXIMUM safe flow velocity in such will only deliver about 40,000 btu/hr.

Don't be dismayed or think I'm "beating up on you". I'm primarily "just" a homeowner as well and I'm trying to save you TERRIBLE aggravation.

Study, study, study or bite the bullet and find a good HYDRONIC HEATING firm!

Allied

Circulator Sizing

This info might be useful:

Siegenthaler on selecting a Primary Loop Circulator (3/03)

Siegenthaler on Smart Circulators and Pump Curves (10/03)

Steamhead on Circulator Sizing with EDR


Some "Rules of thumb"

Flow rate - can be solved by remembering:
BTUH = GPM * 490 * DeltaT

Pump head -
3.5 ft in a gravity conversion.
OR, ((Longest run in feet) + 50%) * .04

Pump curves can be found at the Mfg sites. For example here are two (PDFs):

Grundfos UPS15-58FC

Taco 007

Steamhead

A simpler zoning solution

would be to install Thermostatic Radiator Valves in any room you want to keep cooler than the main living area of the house. This would avoid re-piping the system, which as Mike says is a very bad idea. I've attached a pic of one of my TRVs.

Your existing radiators are so big because the water temperature they run at is lower than on a newly designed system. If you replace some of these monsters with smaller ones, the rooms they're in will be too cold.

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