Circulator sizing - return temp...

rsilvers

If my water is leaving the boiler at 197 degrees F, and returning at 163 - and if I am replacing my system - would I use the same capacity circulator, or change it smaller or larger?

EdTheHeaterMan

Does your building have comfortable rooms? any cold rooms? any overheated rooms? Why are you replacing the system? Do you have a Load Calculation for the building?

Based on the answers to these questions and many others, the answer is yes, or no, or maybe?

rsilvers

I am replacing the boiler because it is corroded and leaking.

I need 155,000 DOE BTU assuming 85% efficiency to be at equilibrium on a 0 degree day when all zones except the garage are set to 70, and so will get a G215/5 as that is about 1.4x larger - which is a guideline I read. Currently I have a 1.9 GPM but it is firing at 1.78 and runs 35 minutes per hour on a 15 degree day and is about 80% efficient.

Not really cold rooms - but there are three window areas that are cold near the window. The walls below the windows appear to be missing insulation according to my thermal camera.

No overheated rooms.

Zman

A 34 degree delta is pretty high and would generally indicate inadequate flow.
A boiler output of 197 is smokin high. Why are you running so hot?
Where did you get the 155,000 number?
The boiler should be sized based on the heat loss of the space.

rsilvers

Running it hot because it is cold out. Normally it would be at 180, but I know that outdoor reset controllers use high set points for cold days, so I turned it up when it was under 20 degrees out.

I got 155 from my boiler having a 1.5 nozzle and running at 140 psi and that is 1.78 GPH. At the tested 81% efficiency, the oil being burned started with a gross BTU of 144 (and after losses was 117KBTU) since it ran for an average of 35.1 minutes per hour on a 15.7 degree day. If I use degree-day calculations, I can see that on a 0 degree day it would need a 154KBTU DOE unit to maintain 70 degrees at equilibrium if the new boiler was 84% efficient. All zones were at 70 for this test, except the basement was at 62 and the garage was at 50. Then I read a great blog post that said that after finding what you need to balance heat loss, multiply by 1.4 to pick the boiler size. This new boiler will be 1.7 and my old one is a 1.9, so it is down-sizing a little.

EBEBRATT-Ed

@rsilvers
Normally at full load you want a temp difference of 20 deg. Your circulator should be sized using the net boiler heating capacity. Example a boiler that puts out 100,000 btu should have a circ that will do 10 gpm

10,000 btu/gpm is standard for a normal system

rsilvers

If my drop is over 30, would that imply that going one size up would be better?

Zman

What circulator is presently installed?
What type of heat system is this? What are the pipe sizes and length of runs?
Circulator sizing is more than just up or down a size.

rsilvers

It's a huge 12,000 square foot house and it would be hard to estimate the loop sizes. It's part radiant, part baseboard, and part hydroair.

EBEBRATT-Ed

@rsilvers

Nobody has x ray vision. Post a sketch of the piping and the length and pipe sizes and we can figure out what pump you need. Need to know the btus of whatever radiation you have.

or

There is another way. Install pressure gauges on the pump inlet and outlet. Run the boiler full boat witth all the heat calling at once and measure the boiler supply and return temp accurately and post what circulator you have now

or

guess.

rsilvers

Ok on the pressure gauges. I really like measuring from what is there now rather than estimating from estimated parameters.

hot_rod

A single pump supplying a 12,000 sq ft home with 3 different type of heat emitters? Sounds unusual.

rsilvers

There are more pumps but only one was running for this test, and I really just wanted to know if more than 20 degree drop meant that a larger or smaller pump was more optimal.