New Boiler Sizing Help

njb52

Hello,
Our existing Utica PGB200A, gas fired boiler lists the following on its nameplate:

AGA Ratings:
Input Water: 200,000 BTUh
Output Water: 160,000 BTUh

Net I-B-R Ratings
Input Water: 139,100
Output Water: blank

I made a rough sizing calculation based upon :
Linear Fin Tube Feet x 650 BTUh = BTUh required
I came up with 180 linear feet x 650 BTUH = 117,000 BTUh
Is there a more accurate way, that a layman can use to size as well? I was thinking of measuring the lineal feet of my exterior walls and total window square footage, if that helps fine tune the sizing.

Thanks in advance

Aaron_in_Maine

Slant-fin has a good app where you can calculate your heat loss to size the boiler. Unless your house is 3000+ square ft your boiler is way way oversized.

Chester

You can also get a reasonably accurate heat loss by using actual weather-adjusted fuel use data. Here's how you do it:

1. Look up the amount of fuel you used between two exact dates during a heating season period (at least a month or two). For example, if you're on natural gas and they read your meter on Dec. 1, Jan. 1 and Feb 1 you can look at your bills to see how much gas you used from Dec 1 through Jan 31.

2. Convert your fuel use (gallons, ccf, etc.) to total BTU.

3. Multiply your total BTU by 0.8. This adjusts for the fact that, at best, 80% of the energy that came through the gas meter was actually delivered to your baseboard. In reality, at least 20% went up the chimney and you may have used some for domestic hot water or cooking. Using an 80% efficiency factor will ensure that your calculated heat loss is on conservative side (highest possible end of the range).

4. Then find the total number of heating degree days (HDD) for your location for exactly the same period used for your fuel use. You can get this from degreedays.net. Use the weather station nearest your location and the "Base 65" column of data.

5. Divide the the number calculated in #4 by the total HDD for that period. That will give you BTU per HDD.

6. Divide by 24, which is your BTU per degree-hour.

7. Find the outdoor design temp for your location at: https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

8. Subtract your outdoor design temp from 65 (since you used Base 65 for HDD).

9. Multiply that number by the BTU/degree-hour you calculated in #6 above.

10. Voila! You now have an implied design day heat load of XX BTU/hr.

Aaron_in_Maine

I size the boiler for the heat loss and size the indirect based on potential load(number of people or bathrooms). The boiler won't be the size the indirect will call for. Run the indirect on priority run it at 140-150 and mix it down to desired temperature. I have never had an issue as far as not enough hot water. Other guys in my area add another section to the boiler for the hot water that wastes money on upfront cost and on fuel down the road. If you have a McMansion with a lot of bathrooms then I would do separate systems one for heat and one for hot water.

Chester

Hatterasguy said:

You're neglecting DHW usage which can be a significant part of the total consumption if the months utilized are not December, January, or February.

The DHW usage, garnered from July and August need to be subtracted from the above.

It would certainly be more accurate if you backed out DHW by looking at a couple summer months of fuel use. Easy enough to do if the gas bills are handy.

Not netting out DHW will definitely overstate the heat loss to some degree, as will the likelihood that the existing boiler may not be hitting 80% overall efficiency in real-world operations. But the fact that this method may err on the high side isn't necessarily a problem considering the size increments that are currently available from residential boiler manufacturers. The question is whether the OP needs 60,000, 80,000, 110,000, etc.

In my case this method produced virtually the same result as a professional Manual J analysis.

Chester

What do you consider to be a 'proper' heat loss? I thought Manual J was the gold standard.

Chester

I think you're right. In my case the Manual J and the energy use calc were equally wrong. Each overestimated what I've found to be my actual heat loss by around 15%. Slantfin was about 20% higher than either of those.

Maybe netting out DHW (if the data is available) and using a lower efficiency factor is the way to go?

Kjmass1

Chester said:

You can also get a reasonably accurate heat loss by using actual weather-adjusted fuel use data. Here's how you do it:

1. Look up the amount of fuel you used between two exact dates during a heating season period (at least a month or two). For example, if you're on natural gas and they read your meter on Dec. 1, Jan. 1 and Feb 1 you can look at your bills to see how much gas you used from Dec 1 through Jan 31.

2. Convert your fuel use (gallons, ccf, etc.) to total BTU.

3. Multiply your total BTU by 0.8. This adjusts for the fact that, at best, 80% of the energy that came through the gas meter was actually delivered to your baseboard. In reality, at least 20% went up the chimney and you may have used some for domestic hot water or cooking. Using an 80% efficiency factor will ensure that your calculated heat loss is on conservative side (highest possible end of the range).

4. Then find the total number of heating degree days (HDD) for your location for exactly the same period used for your fuel use. You can get this from degreedays.net. Use the weather station nearest your location and the "Base 65" column of data.

5. Divide the the number calculated in #4 by the total HDD for that period. That will give you BTU per HDD.

6. Divide by 24, which is your BTU per degree-hour.

7. Find the outdoor design temp for your location at: https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf

8. Subtract your outdoor design temp from 65 (since you used Base 65 for HDD).

9. Multiply that number by the BTU/degree-hour you calculated in #6 above.

10. Voila! You now have an implied design day heat load of XX BTU/hr.

I'm trying to get a rough number before doing a Manual J and I can't seem to come up with a decent number.

3 month's of bills (minus 20 therms/mo for DHW) Oct-Jan
407 therms=407,000 BTUs
407,000* .8 = 326,000
HDD 10/13/15-1/13/16 = 2029 HDD
326,000/2029 = 160 BTU/HDD
160/24= 6.7 BTU/degree hour
0 degree design temp for Boston
65-0=65
65*6.7=435 BTU/hr

What am I doing wrong?

Chester

You just missed a couple zeroes....

1 Therm = 100,000 BTU, so start with 40,700,00 BTU. Then you end up with an implied heat loss of 43,461BTU/hr assuming you matched up the energy use and HDD data correctly.

Depending on the boiler you're replacing the 80% efficiency factor could be on the high side, which probably means this is an upper band.

Let us know if the Manual J confirms that as a reasonable estimate.

njb52

Thanks everyone, but this is way over my head.

Robert O'Brien

Chester said:

Hatterasguy said:

You're neglecting DHW usage which can be a significant part of the total consumption if the months utilized are not December, January, or February.

The DHW usage, garnered from July and August need to be subtracted from the above.

It would certainly be more accurate if you backed out DHW by looking at a couple summer months of fuel use. Easy enough to do if the gas bills are handy.

Not netting out DHW will definitely overstate the heat loss to some degree, as will the likelihood that the existing boiler may not be hitting 80% overall efficiency in real-world operations. But the fact that this method may err on the high side isn't necessarily a problem considering the size increments that are currently available from residential boiler manufacturers. The question is whether the OP needs 60,000, 80,000, 110,000, etc.

In my case this method produced virtually the same result as a professional Manual J analysis.

Impossible! I never heard of such heresy!

Rich_49

Question for the fuel usage boiler sizers .
Is it possible to determine a proper reset curve , SWTs without doing a room by room heat loss and an installed radiation survey ?
Since I think these are required as does the majority of the industry to provide the above mentioned , did you really save any time since they will need to be performed anyway ?

Rich_49

How do you determine the proper SWT w/o doing these exercises?

Rich_49

That would be correct in old leaky , multiple story buildings . Not everyone lives in those Hat . Maybe we should only heat the lower floors in those buildings since the cold air enters and is heated on the lower floor then allows stack effect to move the heated air upward where it leaves the building . In a well built tighter building does that stack effect take place as well ?

Hot always goes to cold , even on a first floor . Infiltration is kinda weird , is hot air leaving or is cold air entering ? Is it infiltration or exfiltration ?

If you place a plastic sheet over a 5 story building under construction the bottom of the plastic will be sucked in while the top is bulging out , this does not happen when the building is finished .

It was a POS when we got here so we don't have to worry about it , is that the goal ? Everyone should try the building or house as a system approach and don't just take the money for a new boiler and call it a day . More should actually try to improve the comfort levels and the environment just a bit .

How's that for a nonsensical rant , or is it ?

doughess

Chester has really got it right. Using real data from actual operation is more accurate than textbook calculations.

Very helpful data comes from using an elapsed time clock wired into the burner firing circuit (solenoid on oil) not the motor.

Clock hours, degree days and gallons for period are the basis for all kinds of data; actual firing rate, dd/gallon, run time dd. To figure boiler size btu use efficiency either measured or some guestimate like 80%.

One source says average US burners runs 20% to 25% of the time. In my case, at design temp of 0F it runs 8 hours/day or 33%.

New boiler sizes start around 80,000 but/hour. For many homes that boiler is going to more than twice the btu capacity needed

Actually cutting the firing rate of a new boiler from rated btu will increase efficiency. Using clock run time It is easy to calculate a smaller nozzle size that will still provide plenty of heat.


.

SWEI

doughess said:

Actually cutting the firing rate of a new boiler from rated btu will increase efficiency. Using clock run time It is easy to calculate a smaller nozzle size that will still provide plenty of heat.

...and may significantly reduce both the safety of the occupants and the lifespan of the boiler. This is not a DIY project. Proper training and tools, including a digital combustion analyzer, are required.

Lance

Not meant to rustle any feathers but suppose the sizing is over a little. No big deal. Just replace it with a condensing boiler using outdoor air for combustion, make it two staged or multi staged burner and control it with an outdoor temp reset control. This way you can maximize savings and better control the results from poor sizing of radiation, or changes to the dwelling that occur over time by the users. The best way is to get the J done. Its not too expensive, and get the features I mentioned for low cost comfort operation. If you add a DHW feature, make it another zone with storage. Hot water at 90+% AFUE is much better than the old 70+% Afue. Outdoor combustion air can save 17% heat load by keeping the infiltration lower during combustion. Lots of options and lots to learn, oh when will it ever end? DONOT mess with the boiler manufacturers engineering! Build with making sure the boiler works as designed to insure its long life!