Boiler sizing

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Old 02-25-08, 01:56 PM
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Boiler sizing

I currently have an older boiler that I am going to replace. The boiler is approximately 30 years old and has an output of 280,000 BTU's.

The home was built in 1929 and has 3 inch cast iron pipe that supplies hot water to the radiators. Estimated square footage, excluding basement, is 3,500 sq ft. Currently this is a single zone heating system which will be converted to two zones. One zone will be 3,000 sq ft and the other will be 500sq ft.

I used heat loss models that I found on web sites for Crown Boiler and Weil-McLain to determine boiler size. In each case, I came up with a number of approximately 140,000 therms.

The two heating contractors that are bidding on the job have come up with boiler sizes of 160,900 (Buderus GB 145) and 227,500 to 232,00 BTU's (Peerless MI-08 and Burnham 208). The IBR rating for the Peerless and Burnham are approximately 160,000 while the Burderus IBR rating is unknown.

It seems to me that the Buderus boiler is more properly sized but the IBR ratings cause me to question that.

So I am looking from some insight has to which contract has a better bead on the boiler size.

Thanks in advance for any help that you give me.

Bruce
 
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Old 02-25-08, 03:04 PM
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Did the contractors also do a heat loss ?

It seems as though if you came up with 140K and they are all recommending something bigger, they didn't ...

so maybe you answered your own question ?

Whatever the case, make sure that the contractors install a BOILER BYPASS line when they install the new one. With that size of piping, you will need some boiler protection from cool return water temperatures.
 
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Old 02-25-08, 04:47 PM
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If your heat loss is 140k than stay as close to that as possible. As Trooper said if you use a standard boiler you must do boiler protection such as a boiler bypass. Cast iron radiation I presume. If so I don't do this too often but this is an ideal job for a mod/con boiler. 95% efficient, condensing all winter, outdoor reset Ahhhhh a boilers dream.
Is your home ready for a new boiler though. Are the insulation values, doors and windows all good? If this is not an emergency they should come first
 
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Old 02-25-08, 05:02 PM
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Both contractors did a walk thru and counted and measured the size of the radiators which I assume allowed them to do a heat loss calcualtion. What does the bypass valve do?
 
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Old 02-25-08, 06:05 PM
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I second what rbeck said, so to amplify and expand:

1) if it's not an emergency, get the house tightened up first to reduce the heat loss, then size the boiler off that. Insulation is fuel that you pay for only once.

2) the contractors should have done a real heat loss, not a "rad count." Chances are with cast iron rads and 3" piping, you have a converted gravity system which would have way more radiation capability than you need with forced circulation. That means: SMALLER BOILER, less money for fuel for the rest of the boiler's life. Do an accurate heat loss, or find a contractor who will. Crown and W-M sites are good. The free software from Slantfin is a bit more user friendly and complete.

3) Don't size on IBR. Size on DOE output, or possibly even gross input.

4) a modcon for this application would be well worth it.

5) 500 sf is a pretty small zone. Why? If it is the only zone calling for significant periods, it will short cycle the boiler, even a modcon.
 
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Old 02-26-08, 05:21 AM
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Boilers

The 500 sq ft is the family room addition which I have difficulty in heating. When the temperatures are below freezing, the room rarely will get above 62 degrees when the main house is set at 70 degrees.The hot water to heat the registers comes from the main house and the 3 inch pipes. Both contractors would pipe the addition directly to the boiler using smaller pipe. The actual heating area is closer to 800 sq ft due to catherdal celing and the kitchen area.

What is modcon?
 
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Old 02-26-08, 02:16 PM
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Insulate and air seal the addition as best you can, first.

Modcon is short for "modulating-condensing boiler." In the right situation (and your oversized radiation and large piping water volume is likely very "right"), they can achieve efficiencies well into the 90s%. The modulating part basically turns the flame up or down depending on the heat load ("simmer" on warm days; "boil" on cold days). The condensing part allows the boiler to extract a few extra percent efficiency by allowing cool return water to promote condensation in the boiler heat exchanger. The latent heat of condensation released in the heat exchanger makes the most of the combustion before it gets vented outside.

Some brands/models in no particular order:

Buderus GB142
Burnham CHG
Burnham Freedom CM
Triangle Tube Prestige
Lochinvar Knight
Viessmann Vitodens
HTP Munchkin

These are pretty high-tech pieces of metal (cast aluminum or stainless steel) with a bunch of pretty sophisticated electronics. Annual maintenance is *required*.
 
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Old 02-26-08, 05:08 PM
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Boiler sizing

Thanks guys. The information I have recieved has made a confusing project into a more understandable one.

The last question is - that I have used 3 sizing templates (Slatfin (excellent sizing tool {I did room by room} Crown Boiler and Weil Mclain). All have yieled a heat loss of 135,000to 140,000 Btu's. How does that convert to DOE or Gross Input? Right now I plan on going with a Buderus GB142-45 that produces 160,900 rated BTU input.

By the way, my insulation and doors are up to snuff, except for the crawl space which I will be upgrading.

Bruce
 
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Old 02-26-08, 05:33 PM
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140k heat loss for 3500 sf is about 40 BTU/hr/sf. That's not terrible for a house of that vintage, but it could be better. Keep looking for ways to improve the envelope.

If the heat loss is 135-140k, then in the Buderus line the GB142-45 would be the typical choice. The next size down might be a bit small, but with your likely oversized cast iron radiation, I'd be very tempted to go with the GB142-30. Especially if you are going to improve the envelope in the future. Plus you probably only need the extra output of the -45 for a few hours a year.

Your heat loss might also be overstated by ~30% (not unusual for Manual J calcs), in which case the -30 would be just right.

Installation is everything. Make sure the contractor knows what he's doing. Ask to see pictures of his other jobs. Check references. Read up on the install manual. Anything doesn't look right during the install, ask questions.
 
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Old 02-26-08, 07:16 PM
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I'd be strongly tempted to keep it one zone but put TRVs (thermostatic radiator valves) all of the rads except for the ones in a particular reference room and that's where I'd place a t-stat (it could even be that hard to heat space).

For a boiler, I'd go with a Prestige because of the open heat exchanger passages and all the older piping. I'd say it based on best match of flow.

The 175 if the flow is higher. You could also go high tech and use a smart pump on it. If the flow requirements permit, a 110 would probably also be a good match and the purchase cost is lower. It has an internal 3 speed circ but might need a differential pressure valve somewhere for when it is supplying heat to just the a few rooms unless the calcs show the velocity isn't excessive. Most people tend to find that the true heat load is about 70% of their Manual J. The Manual J allows for fairly good recovery at design temps when instead it should be the minimum amount of BTUs it takes to maintain temps. That said 70% 140 MBH is 100 MBH.

Modcons make it very easy to know what the true heat load is, but only once they are installed. A 110 using just its internal circ, keeps your electrical load under 100 watts total during most heating cycles.
 
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Old 05-15-08, 05:34 AM
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New boiler installed

I went with the buderus mod con. I set the temperature back to 67 degrees at night. In the morning I have the temp set at 70 degrees.

It takes the boiler approximately 3 hours to raise the temp 3 degrees (i.e. to 70). In your opinion is that normal? If not, what would be a normal recovery time?

Last question. How far can I set the thermostat back e.g. 67 65 or even 60 degrees?

Thanks
 
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Old 05-15-08, 12:39 PM
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banovak1, you can set you house back to 40 if you want... but you need to understand that aggressive setbacks work best with oversized furnaces in very leaky houses.

The amount of savings potential with your modcon has been reduced. To recover from the temperature being set back, you'll either more recovery time (which reduces the setback period by hours if you want recovery by a certain time - as you're finding out) or higher recovery water temperatures. Now keep in mind, that the colder the return temperatures, the more latent heat that can be extracted by the boiler. Increasing your water temp will reduce your efficiency.

If your boiler recovers that slowly, it means that your curve is well dialed in. I'd suggest you reduce your setback to a degree or two or drop it entirely. It tries to make the boiler work more intensely and that's the opposite of how our modcons should be working. Slow and steady wins the race for these guys...

When was it installed?
 
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Old 05-15-08, 02:41 PM
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What Who said.

1F per hour is a pretty good indicator that your reset curve is close to optimized. 1F/3 hr would be even better, but that would take some serious fiddling around and lots of patience.
 
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Old 05-15-08, 03:50 PM
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I'm going to somewhat disagree with the others. The whole purpose of the modulating burner is to best utilize the fuel to keeping your house at the desired temperature without having the boiler cycle on and off.

Depending on how much oversized the heat emitters (radiators) are for the upper design water temperature you CAN run higher water temperatures to recover from setback at a faster rate. This would work better if you had baseboard convectors or fan-forced cabinet heaters but it WILL work with cast-iron radiators.

I don't know that much about residential controls but there should be a way to override the outside temperature reset action for quicker load pickup and then go back to the outside temperature modulating curve once the house has attained the desired temperature.

Or to put it another way...if the water temperature in the system is controlled ONLY by the outside temperature then you are not fully utilizing the the capabilities of the modulating boiler.
 
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Old 05-15-08, 10:19 PM
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Guys,
Thanks for the info. I am meeting with the Buderus rep today and your comments will be helpful in challenging what he has to say.

Who,
It was installed March 8th.
 
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