Sizing a boiler

Todd S_13

Condensing actually occurs at a return temp of about 130 (at 10% CO2). If you run a boiler at a 20 delta thats 150 supply, go to a 30 delta and its 160f supply. Thats at design conditions remember. A condenser would be great here, fully modulating and delivering the exact heatload while raining condensate. This something to consider when looking at a boiler, can the burner maintain a high CO2 across full modulation? You would be surprised at how few can actually document and deliver this. Do they preheat incoming combustion air? If efficiency and savings are truly the objective then this is crucial (at least in my opinion). Depending upon the dhw needs, I would go with as small as possible and keep the wood stove for beyond design days.

[Deleted User]

Thanks and Question

First I want to thank all those that helped with advice on my son's house. It is now up and running, heating with wood, or gas if the wood burns out.

The next project is my house. I want to install a new more efficient hot water furnace and need to select a size.

The house is 1100 sq. ft. and the heat load (calculated at -17°outside and 70° inside) with the H-Mac spreadsheet is about 37,000 Btu/Hr.

It is presently heated with a Midco natural gas Economite conversion unit in a Crane-line Midget boiler.

The Midco has an input of about 143,000 BTU/Hr determined by measuring ccf/hr.

The heat emitters are cast baseboard in a single pipe single zone configuation that works well. I don't plan to change them.

On days when the low is -20° and the high is -5° the burner runs 6-6.5 hours out of 24 hours. I measured
this, it's not a guess.

Questions are:

1. Is anybody familiar with the H-mac calculator?

2. Is it a fair guide for selection of a new boiler?

3. What is the efficiency of boilers similar to my existing boiler?

4. If I plan to expand the house by 30% (someday) should I increase the boiler output a corresponding amount or will that cost me a lot in efficiency if I never get around to the expansion.

[Deleted User]

Thanks and Question

First I want to thank all those that helped with advice on my son's house. It is now up and running, heating with wood, or gas if the wood burns out.

The next project is my house. I want to install a new more efficient hot water furnace and need to select a size.

The house is 1100 sq. ft. and the heat load (calculated at -17°outside and 70° inside) with the H-Mac spreadsheet is about 37,000 Btu/Hr.

It is presently heated with a Midco natural gas Economite conversion unit in a Crane-line Midget boiler.

The Midco has an input of about 143,000 BTU/Hr determined by measuring ccf/hr.

The heat emitters are cast baseboard in a single pipe single zone configuation that works well. I don't plan to change them.

On days when the low is -20° and the high is -5° the burner runs 6-6.5 hours out of 24 hours. I measured
this, it's not a guess.

Questions are:

1. Is anybody familiar with the H-mac calculator?

2. Is it a fair guide for selection of a new boiler?

3. What is the efficiency of boilers similar to my existing boiler?

4. If I plan to expand the house by 30% (someday) should I increase the boiler output a corresponding amount or will that cost me a lot in efficiency if I never get around to the expansion.

Steamhead (in transit)

That boiler

is oversized. No doubt about it.

Haven't used H-mac, but I've found the Slant/Fin program works well. I think they're coming out with an updated version, don't know when though.

I think that Crane Midget boiler is a steel multi-pass tubular boiler, not unlike today's Crown Freeport. Most Freeports are rated 86% AFUE. So are many cast-iron 3-pass boilers.

If you tighten up the existing house before adding on, you may find you don't need a bigger boiler when the addition goes on.

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scrook_3

Sounds about right

6.25hrs/24hrs = 26% duty cycle at ~ -12° average, 70°-(-17°)/70°-(-12°) = 1.06 so we'll assume you need perhaps another 6% or so at -17°F design.

Let's assume ~82% for a gas firetube boiler, so 143,000 BTU/hr * 0.82 = 117,000 BTU/hr output, and you need something around 0.26 * 1.06 * 117,300 = 32,200 BTU/hr. Your heatloss calc comes in at 37,000 BTU/hr about 14% higher (and all the numbers are a little rough, say +/- 10% anyway), so I'd say your H-mac calc is reasonable.

You might look at modulating/condensing gas boilers and outdoor reset.

scrook_3

but...

"Most Freeports are rated 86% AFUE"

But that's #2 oil not natural gas, which would be slightly less due to more water vapor in the flue gas than with oil

Conversely, this is what makes a condensing gas boiler so attractive, lots of water vapor to condense (if you can run low water temperatures) and recover additional heat from.

Steamhead (in transit)

I don't know

if Crown has even rated them on gas. But the Freeport was the closest match I could find to the type of boiler Fred now has.

If there's enough CIBB that the water temp would stay below 140 degrees most of the time, a condenser would make sense- IF it could be vented safely and the model chosen can be expected to last more than five years.

Otherwise, a VERY small cast-iron boiler would be my choice.


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bill_116

37,000 & 143,000?

Just how many houses are being heated with it?

Todd S_13

Condensing

Condensing occurs at a return temp of about 130 (at 10% CO2). If you run a boiler at a 20 delta thats 150 supply, go to a 30 delta and its 160f supply. Thats at design conditions remember. A condenser would be great here, fully modulating and delivering the exact heatload while raining condensate (and not disolving the pressure vessel). This is something to consider when looking at a boiler, can the burner maintain a high CO2 across full modulation? You would be surprised at how few can actually document and deliver this. Do they preheat incoming combustion air? If efficiency and savings are truly the objective then this is crucial (at least in my opinion). Depending upon the dhw needs, I would go with as small as possible and keep the wood stove for beyond design days.

[Deleted User]

Payback

Thanks to all for the inputs. I have a bit to think about now.

If the existing boiler is 82% and the best I can reasonably get with a new one is 95% It seems like a payback would be pretty long.

My total Nat. gas usage is 8-900 ccf/yr at about $1/ccf so maybe I'd save $100-125/year.

Do I have that figured right? Maybe I'm better to stick with what I have until it fails.

Can you suggest a source for output v. temp data for CIBB? It appears I need 330 BTU/ft at design temp.

The temp in the boiler is mostly in the 120-140° range except in very cold weather when it can get up to 160° or so. That's if I believe that temp guage.:-)

And finally, if I bought a mod/con boiler, how long will it last.