sizing boiler?

[jrc2905]

I have HVAC-calc 4.0 that I use for heat loads. So if I do not want to oversize a replacement boiler isn't the only way to do it is by the btuh output of the baseboard? Not to beat a dead horse but I am not sure of the consensus here?

Any suggestions for a new boiler conditoning product and does anyone do scale removal as PM on existing HW boilers?
Thanks

[Noel Murdough]

on hot water, no.

If you know the load of the room, and the number of feet of baseboard, you can calculate what water temperature that baseboard needs to do the job.

If you just choose an arbitrary water temperature, figure how much heat you CAN move into the room, the zone will short cycle because the room doesn't USE the heat.

....and the thermostat notices that you picked too high a water temperature, and shuts the zone off early.

Too much baseboard allows cooler water temperatures, not more boiler.

If your building has 100 feet of baseboard installed, does that mean the building looses exactly 60,000 BTUH?

If you add 50 feet of baseboard, does the building lose 90,000 BTUH now?

Would you put in a 90,000 BTUH output boiler in this house if you got there and it had 150 feet of baseboard in it?

Would you not recalculate the water temperature so that the baseboard output and then the boiler output matched the building heat loss?

Or is there a reason that you would put a much larger boiler in and have it short cycle on the thermostats?

Noel

[hydronicsman]

Listen to Noel. 95% of the year an oil boiler is oversized. Size it for the load. DOE # is what to look at.

And..existing baseboard amount is irrelevant to boiler sizing. Whole 'nother issue if there is not enough. Too much and you can run lower water temps-though it's not the prettiest thing to look at.

[Edited by hydronicsman on 02-15-2005 at 06:22 PM]

[Paladin]

I must agree with both Noel, and Hydronicsman here the boiler needs to be sized by doing a heat calc first. Also, since the subject of boiler shock was was brought up, is the new boiler slant fin or cast Iron? Do you plan to use mixing valves in the bypass? Short cycle can make a mess of the venting system as well by not keeping it hot and dry enough especially if the draft is induced like several models I have seen in the field.

[Noel Murdough]

All Slant/Fin boilers are currently cast iron.

In the 1940s, Slant/Fin was the finned tube baseboard product that the company was founded on.

Noel

[Paladin]

Thanks for bringing me up to speed Noel, I guess what I meant was the boiler type that is copper finned type which I am told doesn't handle boiler shock as well as the cast Iron type. I believe Teledyne-Larrs makes these?

[Noel Murdough]

Laars, Lochinvar, Raypak, and those little water heaters that hang on the wall that people use to heat with.

Noel

[fat eddy]

I would prefer to undersize a piece of equipment although slightly.

[beenthere]

Do a load calc, and check the output of your baseboard to make sure the baseboard atleat matches your load, then get a boil that has an IBR output to match your load.

Again thats I B R output, not DOE output, a mistake made by many.

[sweet]

3000 sq ft @ 30btu/= 90000 btu heatloss devided by 580 x .8= 124 IBR rating thats gonna be quite a bit larger than 100000 btu boiler. Do a heat loss calc. and size it accordingly for design temp. This should give ample wiggle room and efficiency. I also think 580 is a closer number for output of basesboards ( you are talking about fin tube convectors, correct??)

[casturbo]

Originally posted by Noel Murdough
AND I stand by what I said.

I spend a major portion of my day explaining to homeowners of contractor installed equipment why the thing short cycles so badly, why condensation is running out of the chimney cleanout, and why controls die so young.

You do what you want to, in your new construction jobs.

Noel

Too bad I do ZERO new construction jobs. I do mostly repair work and replacement jobs where teeny boilers in large homes are thrown out. It's routine, sorry bud.

A SLIGHTLY oversized boiler will not short cycle badly, will not condensate, and will not shorten controls. In reality, controls last longer, circ pumps last longer, and the boiler itself lasts ALOT longer. LET ME REPEAT...SLIGHTLY OVERSIZED I SAY AGAIN, NOT GROSSLY OVERSIZED. Remember, I can always SLIGHTLY drop nozzle size on a larger boiler to compensate, you CAN'T put in larger nozzle in a small boiler. Think long run and you'll see what I see everyday.

[jrc2905]

I guess I will have to do a heat loss calc, I was hoping for an easy way out in using the baseboard method.
Is there a formula to use to adjust baseboard water temp to match heat loss?
I might try to get homeowner to go for one of the new products that use outside temp sensors and mixing valves to adjust the water temp of the baseboard

[hydronicsman]

I detest math so I play with the load calc baseboard output numbers to see what water temps match the existing linear feet-if you know you have too much bb for the load at 580 BTU per foot, drop the "output" a hundred degrees and see how it works. Clear as mud?

I use IBR net w/ steam sizing. Modern hot water baseboard really doesn't see the 15% pickup loss. I daresay IBR may be even a little antiquated . DOE is better suited here, especially when these days most jobs get an indirect water heater placed right next to the boiler-pickup loss is nil.

By all means check out outdoor reset. With oil prices, why run the boiler wide open on those 40-50 degree days?