Boiler Size Calculation based on therms, HDD and 99%

Users who are viewing this thread

regrebleo

New Member
Messages
6
Reaction score
0
Points
1
Location
Minnesota
Hello, my family and I recently bought a 1800 sq. ft. house in Minneapolis that was built in 1900. The house is currently heated by a Weil Mclain HE-6 cast iron gas boiler but it has not been well maintained and needs replacing. The plan is to replace it with a mod-con boiler with an indirect water heater this summer. The radiation in our house is via cast iron radiators.

In order to properly size the new boiler it seems to me that the most accurate way to do so is to use the equation mentioned many times on this forum that uses therms, HDD and the 99% outside design temp. If I have understood the previous posts correctly the formula is as follows:

BTU/hr = (therms used over a certain period * 100,000 / HDD over the same period / 24 hrs. * Estimated efficiency of current boiler) * (65 - 99% outside design temp)

If I plug in my numbers:
Therms (Sept '14 - Feb '15): 1183
HDD (Sept '14 - Feb '15): 5958
Estimated boiler efficiency: 75% (which is probably high)
99% outdoor design temp for Minneapolis: -11

47,157 BTU/Hr
= (1183 * 100,000 / 5958 / 24 * .75) * (65 - (-11))

With this in mind, my questions are as follows:

1. Is my math above correct?

2. If so, can someone confirm that even if an indirect water heater is added to the mix, there is no need to increase the BTU/hr figure. If I understand correctly, unless it is a 99% outdoor day and someone needs a hot shower during the coldest part of the day, the boiler will be more than adequate. And, even in this extreme circumstance the indirect will be on a priority loop so it will ensure we have hot water even on the coldest days.

3. We are also hoping to insulate and add heating to our mud room/entry porch this summer which is 90 sq. ft. If so, how much would this addition affect our BTU/hr number. Assuming the space is insulated to a level that is at least on par with the rest of the heated space would the equation below be an approximation of the additional load?

Updated BTU/hr = (current sq. ft. + additional sq. ft.) / current sq. ft. * current BTU/Hr

49,515 BTU/hr = (1800 + 90) / 1800 * 47,157

If not, are there any recommendations as to how to calculate the additional load?

4. When searching for a new boiler, I should be looking at the DOE rating rather than the Max input or IBR rating? (assuming the boiler will be in a basement not exposed to outside air)

I think that sums all of my queries for now. Thank you in advance for any and all assistance.
 

BadgerBoilerMN

Hydronic Heating Designer
Messages
485
Reaction score
10
Points
18
Location
Minneapolis
Website
www.badgerboilerservice.com
We do this for a living here in Minneapolis.

There are a few that can back out the heat load and DD days to get close, but they have to consider the boiler efficiency, unknown without combustion analysis. Dana can.

Regardless, one of the best ways to chose a condensing boiler is to find a contractor that knows how to do an ACCA Manual 'J' heat load. This method gives the most consistent results with documented data to refer to. Since the contractor is more important than the equipment--we have installed 12 different condensing boilers in as many years--this a good first step. (Additional loads for additions etc are easily modeled on good software.) They will not do it for free but should show you a sample of how it's done.

You can compare your own numbers, likely low with there's before you sign the contract.

An indirect water heater is first sized to the DHW load, considering bath size, shower flow, number of bedrooms, then the heat load results for boiler output and the zone loads for possible micro-loads that will effect boiler cycling, DHW load time etc. If is very unusual to up-size boiler output for the additional DHW load. Anyone that does so routinely should be suspect.

You will want to make sure that the new MN plumbing code, adopting UPC, is in force so that you don't get stuck with a inefficient indirect water heater with a sub-standard double-wall heat exchanger.

Boiler efficiency is complicated since the old cast iron boiler will cycle more than a properly sized condensing boiler so operating cost will not follow combustion efficiency.

Cast iron radiators normally operate in a perfect condensing boiler range 85-150F if programmed correctly and not over-sized.

http://www.badgerboilerservice.com/modcons.html
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
The 47,157BTU/hr math based on 75% efficiency is correct.

Basing the change in heat load with the 90' addition based on a BTU/hr per square foot average is completely off base. Heat loss scales with exterior surface area and the U-factors of the exterior surface type, not by floor area.

Assuming that 90' is a 9'deep x 10' wide porch 10' tall, the previous gross exterior surface area was 100 square feet. About 25' of that was probably U0.5 door (solid 2" door), and you may have had another 20 square feet of U0.5 window (a coupla wood sash double-hungs with clear storm windows). If the rest is well insulated 2x4 framed wall that leaves 55 square feet of U0.08 wall, with 45 square feet of U0.5 window & door. The old heat loss for that section, assuming an interior temp of 68F and a 99% design temp of -11F (a 79F delta)

Window & door: U0.5 x 79F x 45'= 1778 BTU/hr

Wall: U0.08 x 79F x 55'= 348 BTU/hr

Total: 2026 BTU/hr. (plus some infiltration factor- call it 2.5K)

You'd have to use the real surface areas and U-factors to get it close to reality- this is just an example.

But the porch now has 90 square feet of ceiling loss, 90 square feet of floor loss, 180 square feet of side-wall losses, plus the original 100 square feet of wall parallel to what was covered up. That's 360 square feet of new exterior surface area, plus the 100 square feet of (hopefully higher performance) exterior surface area that replaced the old 100 square feet.

The R-values/U-factors of the new construction are under your control. But the difference between the heat loss of all that new construction surface area and the heat loss of the amount of old exterior wall that is now an interior partition wall. In rare instances the heat load can even go down with a new high performance addition, but it usually increases. But applying the average BTU/ft of the "before addition" house to the new square footage is almost certainly going to be wrong, and in the case of an enclosed porch with lots of windows that sort of estimate can be qwite a bid rong.

Unless the boiler is isolated from the conditioned space by a highly insulated wall or floor, use the DOE output of the boiler for sizing purposes. In MN I'd be surprised if the old boiler is out in the garage, and most older homes would not have significant thermal isolation between a basement and first floor. The I=B=R output is just a crude estimate of how much of the heat was jacket & distribution losses would be to completely unconditioned space, isolated from the fully conditioned space. But if the boiler is inside (say, in an unheated basement with less than R10 between the basement ceiling and the first floor), most of those jacket & distribution losses still accrue to the whole house heat load, and are not actually lost. (If the foundation and band joist are insulated even less is lost.)
 

BadgerBoilerMN

Hydronic Heating Designer
Messages
485
Reaction score
10
Points
18
Location
Minneapolis
Website
www.badgerboilerservice.com
We do use DOE on most applications, but in these century old houses one has to be careful. With 2.5 to 3" gravity mains, some of which terminate "in" the stone foundation before rising to the radiator above, DOE may be a little short for "pickup", which is what the old IBR was originally about. Most of the new ModCons have a boost feature to cover such an event provided the boiler has the required capacity.

As Dana suggests, the asymmetrical thermal loads of new additions and old houses makes for challenges in radiation compatibility, controls and potential comfort issues.

Built-in ODR will diminish distribution losses and the warm floor effect created by the old boiler's jacket losses. Cold just above the boiler if that is currently your favorite part of the house...

In practice we most often decrease the boiler output i.e. use a smaller boiler, and still meet the loads with room to spare. More often we are using a condensing combi boiler such as the new Viessmann 222F that may slightly exceed the heating load but perfectly match the DHW load. With the considerable mass of the old radiators and the advantage of ODR we find run times more than acceptable and overall performance exceptional.

Our last two kitchen remodels included the 222F driving the whole house--old CI rads--and the new radiant floor in the kitchen addition with perfect results.
 

Tom Sawyer

In the Trades
Messages
3,625
Reaction score
34
Points
48
Location
Maine
If you go from 75% to 95% it will save you a whopping $ 21.05 for every $100.00 spent in fuel, so the payback period should be, and calculating in the average 10 year or less life for mod-con boilers...........never.
 

BadgerBoilerMN

Hydronic Heating Designer
Messages
485
Reaction score
10
Points
18
Location
Minneapolis
Website
www.badgerboilerservice.com
Poppycock!

It doesn't work that way.

Most of our old-home retrofits yield 50% plus savings. Since most of these older homes are not small, the ROI can be considerable and the payback well within the 20 year life we expect them to have with any professional care.

I have been installing condensing boilers since the 80's and even the first generation--no modulation, no ODR, no night time SWT setback, recovered the latent heat of vaporization and properly sized, saved the operating penalty incurred by standby stack losses, over-temperature operation and cycling off limits.
Even Dana can't tighten up a turn-of-the-century Queen Ann and get a better ROI than we can with a properly sized and installed ModCon.

All due respect.
 

Tom Sawyer

In the Trades
Messages
3,625
Reaction score
34
Points
48
Location
Maine
It does work that way. Math is math and numbers don't lie. It works exactly that way unless they do something else to improve the envelope. You know darn well that very few mod-cons are going to make the twenty year mark either. At least not without replacing lots of their innards, up to and including the exchanger.

Honestly, we've all but stopped selling them for retrofit applications. Between the high cost of the equipment and the almost always need to add radiation and or a buffer tank, the ROI is nowhere near as good as it is just replacing the old unit with something non-condensing, in the 85% range with reset and other service tweaks. Now, if you're willing to do the install for under 7 grand or so, we're talking a whole different animal but around here a typical retrofit is gonna set the customer back between ten and twenty grand or so. Hard to pay that back.
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
The idling, distribution & jacket losses of a mod con are lower than with cast iron, which lowers the overall actual load of the house, since the boiler room is no longer being overheated.

Given that the current boiler at ~137K of D.O.E. output is more than 2x oversized (almost 3x oversized!) for the apparent load based on fuel use, it is guaranteed to have NEVER actually it's AFUE numbers, and may be averaging well under 75% for as-used efficiency (which means it probably is over 3x oversized.)

A properly sized & tweaked-in mod con would leave the boiler room colder (lowering the heat loss out of the basement), and would have an order of magnitude fewer ignition cycles, and wouldn't have a parasitic flue depressurizing the basement and driving infiltration rates (and the heat load with it) sky high while idling the way the old high-mass beast does. A 50% reduction in fuel use isn't at all out of the question, and at least a 35% reduction would be almost guaranteed given the oversizing factor and age of the existing beast.
 

Reach4

Well-Known Member
Messages
38,749
Reaction score
4,400
Points
113
Location
IL
Even Dana can't tighten up a turn-of-the-century Queen Ann and get a better ROI than we can with a properly sized and installed ModCon.
If you expect average 10 year or less life for mod-con boilers, there can hardly be a positive ROI.
 

Tom Sawyer

In the Trades
Messages
3,625
Reaction score
34
Points
48
Location
Maine
I wasn't recommending doing a input for input swap. Obviously the old equipment is oversized but that alone isn't going to save 50% or even 35%. 15 % if he's lucky.



I think you need to look at the specs on equipment capable of 85% or higher, cast iron, with automatic flue damper and combustion air from the outside. For that matter 85% direct vented. Let's take a look at say this product from Burnham
https://file.ac/lFlKAogAIi0/ESC Product Data Sheet.pdf
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
If you expect average 10 year or less life for mod-con boilers, there can hardly be a positive ROI.
:)

I suppose if you take a ridiculously short lifecycle on anything as an axiom you can prove that nothing will have a positive ROI.

Boiler designs with defective heat exchanger designs or owner/operator maintenance neglect notwithstanding, where's the data that indicate a 10 year max lifecycle for mod-cons?

But it's true that right sizing a direct-vented mid-efficiency cast iron boiler would not have the same inefficiency problems as the pre-existing oversized hunk o'scrap.

As far as new cast iron goes, I too like the Burnham ESC series for a number of reasons (heat purging boiler control, 110F return water tolerance, direct venting etc.) From the fuel use numbers it sounds like the load on this house would find a right-sized boiler in that lineup- probably the ESC-3, which is smallest in the line with 52K of DOE output. (I'd sanity check the fuel use numbers against a Manual-J or I=B=R spreadsheet load calc first, including the addition- it might actually need the ESC-4.) But for an 1800-2400' IRC 2012 code-min house done right, even the ESC-3 would be more than 2x oversized, and would fall slightly short of 85% as-operated AFUE.

An ancient gas boiler past it's prime that was rated 78% AFUE on day-1 and is 3x oversized for the load is probably doing about 65% as-used efficiency (70% best-case) at age 40-50 or however old that thing really is. Just on that factor alone a right-sized 95% mod con would be saving about 25-30%. The fact that the old boiler is also raising the heat load of the house by greater than 5% with it's idle-time infiltration drive & boiler room overheating brings the fairly assured savings to about 35%. Depending on just how egregious the infiltration drive factor really is, it could be as high as 50% or maybe even a bit more, but I wouldn't promise anyone that type of performance without knowing more about the condition of the house & existing boiler. Even replacing it with a right sized direct vented 85% cast iron boiler should deliver at least 15% fuel savings, and it may be as high as 30% depending on the other factors.

Whenever looking at HVAC replacements it's also useful to look at load reductions by improving the building envelope. Reducing the heat loss delivers more creature comfort than pumping more heat in with the heating system. On a century-old building there is almost always cost effective air-sealing to be done, and if the foundation & rim joist are not air-sealed & insulated, fixing that could easily knock 8-12,000BTU/hr off the heat load. If building envelope improvements are anticipated, sizing the equipment for the "after" picture is the right thing to do.

The fuel-use calculated heat load of the house as-is current has a ratio of about 26 BTU/hr per square foot of conditioned space. With air sealing, foundation insulation, and spot insulating where it's easy (or even more complicated, if it has a lot of uninsulated stud bays) most 1800' wood framed houses can be brought well under 25 BTU/ft-hr @ -11F, even under 20 BTU/ft-hr.
 
Last edited:

regrebleo

New Member
Messages
6
Reaction score
0
Points
1
Location
Minnesota
Thank you very much for all of the replies, I greatly appreciate all of your perspectives. Badger, thanks for the clarification about the DHW needs; Dana, thanks for confirming my previous calculation was correct and pointing out my math for the porch was too simplistic; and Tom, thanks for bringing the Burnham ESC to my attention.

Since last week I have spent some time measuring our radiators with the hope of determining whether we will be able to run the system with water temperatures that will allow a condensing boiler condense most of the year. If my calculations are correct, our house is perfect since it appears that the radiators are also grossly oversized.

That said, I am hoping someone can confirm that I am measuring my radiators correctly. Almost all of the radiators in our house are of an uncommon style (click here for a photo) so I was unable to use the charts of column and tube models I found online. I have therefore assumed that sq. ft. of radiation is simply the exposed surface area of the radiator. Is that correct? If so, it was pretty easy to calculate the surface area of the horizontal pipes based on their diameter and length and then add the flat surface area of the end caps.

Assuming for now that I have measured the radiators correctly it means that our house is equipped with 589 sq. ft. of radiation. If I then take the 47,157 from my previous post, divide it by the 589, and then convert the BTUs to temperature I arrive at about 135 as a max temp which looks good for a mod-con.

The plot thickens however since at least this winter our radiators didn't ever get close to maintaining a temperature any where near 135.

I am therefore curious wherever or not it is possible to estimate heat load with an equation that uses radiator temp, sq. ft. of radiation, indoor temp and outside temp. I came up with the formula below but please correct me if I am wrong.

BTU/hr. = ( ( ( (av. radiator temp. over time period - 100) * 2 ) + 10 ) * sq. ft. of radiation ) * (indoor temp 99% outdoor design temp) / ( indoor temp over same period outdoor temp over same period)

* I was unable to find an equation to convert water temperature of radiators to BTU heat emission but reverse engineered something that matched the tables in the charts I found for traditional column or tube radiators. The equation does not work for temperatures below 95 deg. but since it matches anything above 100 I thought it was good enough for now. That said if anyone one knows the real formula it would be great to have. The formula I used is: BTU/sq. ft. = ((Water/radiator temp - 100) * 2 ) + 10

Is this sort of calculation valid or am I missing something? I have data from 3 hours on a cold day this winter where the average radiator temp was 113 degrees while the average outside temp was -1. Using my BTU/hr formula above I arrive at:

23,841 BTU/hr. = ( ( ( ( 113 - 100 ) * 2 ) + 10 ) * 589 ) * ( 66 – ( -11 ) ) / ( 66 – ( -1 ))

This is just about half of the number calculated using the therms used equation when I assumed the boiler was 75% efficient. I realize that the 75% was quite ambitious considering the vintage and the over sized nature of the boiler but if I used the new number based on radiator heat output it would suggest our current boiler is only about 38% efficient. Is that possible? Could it be that bad?

I realize that neither of these equations can provide the exact heat load since the data is not perfect. For example there is definitely some radiation surface area unaccounted for since I did not include the uninsulated sections of the large pipes in the basement. Is it fair to say however that the actual heat load is somewhere between these two values? My amateur heat load calcs confirm this idea but it would be great to get some confirmation from the pool of expertise on this forum.

Thanks again.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks