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Heating Load - Rules of Thumb

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Member Posts: 27

I am not a "rule of thumb" kinda guy, and always perform heating and cooling load calculations when sizing an HVAC system, but I am looking for information on ballpark estimates of btu/sq ft of heating required for a single family residence. I know that there are a lot of variables, but any assistance would be appreciated. The winter design temperature for the project that I am working on is 30 degrees F.

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There are no rules of thumb when it comes to an accurate heat loss calc. You gotta do the math.

I've got houses in my area that range from as low as 10 btus per square foot to as high as 35 btus per square foot.

From experience, the age and the type of construction, I've got a pretty good idea what a given house needs, but there's no accurate shortcut.

I've also been doing load calcs for over forty years, back to the days of doing them by hand. So, I've got a little bit of experience that others may not.

Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 8,153
edited March 2023
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I lived in an area where the design was 14° but I always used Zero (0°) for my heat loss calculationd and always came up with a heating system much smaller that the one that was there. Each time I woukd do a BTU/SqFt analisys and found that the numbers were as high as 68 BTU.SqFt on old uninsulated farm houses and as low as 27 BTU/SqFt on some townhouse middle unit condos. You got to use common sense when using the rule of thumb number for the sales estimate to ballpark a size to rule out the shoppers that are not going to buy. Once you get them to agree that the price might be over \$X thousands of dollars for something … is that in your budget? then you can move on the do the work of the real load calculation and real job quote.

If that is all you are looking for then I would go over 10 of the last heat losses you did and pic a 40 BTU SqFt or 45 BTU/SqFt based on YOUR experience. I did work as a salesman for one HVAC dealer between owing my own business. There was a policy that every new equipment proposal MUST have a load calculation in the file. I agree with that policy.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

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Yes Ed.  You have to have some basic rules of thumb and after doing this 40s years you can get VERY close "guess-timating"  pipe size, boiler size, gas meter size, baseboard amount, Radiator size.  You know what the right size or amount looks like.  Once the client is a go and serious, only then am I running the numbers...you'd be amazed at how close you can get...Mad Dog
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Ed, you must of had some real leakers with those loads! Even my church's sanctuary with a 35 to 50 foot peak is only about 50 btu/sqft at 0F. with exposed roof deck, no wall insulation and leaded windows. I've nearly always sized for hot water or steam, so loads will tend to be lower than forced air, but our bad ones are more like 35BTu/sqft for early 1900's homes down to around 17 to 20 btu/sqft for early 1900's homes that have seen basic upgrades like storm windows and attic insulation. Big 1920's brick multi-unit buildings usually work out to 17 to 20 btu/sq ft, since they are inherently more efficient ( less exposed surfaces per square foot of living space). This is all 0F design.
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I tell this story a lot. Here in NYC, the electric utility was running radio ads telling people to multiply their room area by 30 BTUs/sq ft to get their proper air conditioner size. It's not the first time I heard that number. I believe that's a pretty standard rule of thumb around here. But we also have a lot of attached houses, row houses, Brownstones, townhouses, and apartment buildings here. The heat loss is all over the map, but typically kind of low for our climate. I ran into a guy from Con Edison and asked him what he thought of that 30 BTUs and he told me it's a great CYA multiplier because it always works. His team did energy audits and most homes they were testing came in at the 12-19 BTU per square foot range.
Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
Consulting & Troubleshooting
Heating in NYC or NJ.
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Here is a rule of thumb or hand actually. Walk across the street from the house close one eye and hold up two fingers. If you can’t see the house its 60 K btu. Add 30K for each finger until the house disappears.
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100 year old house.
Both floors 1000 Sq Ft
1st floor renovated, spray foam walls and Floor.
2nd floor original

how do you apply rule of Thumb to sizing?
• Member Posts: 6,505
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For oil, I use my own calculator, based on an article written by @Robert O'Brien . Works great. Works also for gas/propane, but you would need to isolate usage for heat only.
You can use my Heat Loss db created for IOS devices, or the Excel SS version.

All the info is on my youtube page or send me a private message for a copy:

There's also an HVAC-Calcs file for IOS devices of helpful hvac calcs, with most of those calcs also in a spreadsheet version.

There was an error rendering this rich post.

• Member Posts: 1,218
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JohnNY said:

I tell this story a lot. Here in NYC, the electric utility was running radio ads telling people to multiply their room area by 30 BTUs/sq ft to get their proper air conditioner size. It's not the first time I heard that number. I believe that's a pretty standard rule of thumb around here. But we also have a lot of attached houses, row houses, Brownstones, townhouses, and apartment buildings here. The heat loss is all over the map, but typically kind of low for our climate. I ran into a guy from Con Edison and asked him what he thought of that 30 BTUs and he told me it's a great CYA multiplier because it always works. His team did energy audits and most homes they were testing came in at the 12-19 BTU per square foot range.

Well, if you want to sell more electricity this is one way to do it. We run a one ton window AC and have very good circulation through our 1 story home and it keeps us cool in the hottest, wettest Chicago weather... that about 7.5 btu/sq ft. 1900 construction brick veneer construction, but with R-13 walls, R-25 ceilings, fairly airtight, and storms ( low e storms on the west)
• Member Posts: 531
edited April 2023
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For oil, I use my own calculator, based on an article written by @Robert O'Brien . Works great. Works also for gas/propane, but you would need to isolate usage for heat only.

A heat loss calculation is only as good as the data we put in it. On old houses, the level of insulation and air changes per hour is often our best guess. In those situations the fuel-consumption heat loss calculator takes a lot of the guess work out.

I used your calculator this winter to size a boiler for a friend's house. The house is large, has some electric baseboard in rooms seldom used, and the infiltration rate was a big question mark. The new boiler is HALF the size of the old one and the first week it was in service we got a blast of -20F with strong wind, which is actually colder than our design conditions. The house maintained indoor setpoint with a LOT less boiler cycling.
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By way of cross-checking your rule of thumb guesstimation, could you not use the BTU input ?
If you had a known amount of btu input , could you not guesstimate the eff% high/low and thus the btu required high/low window ?

30+ yrs in telecom outside plant.
Currently in building maintenance.
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I think I may be the only contractor in a 50 mile radius that knows what a heat loss calc is. Several times a week I'll get an inquiry telling me they need a 199k boiler for their new 2500 sq ft house because (insert other guy here) said so. I've found that most new construction homes tend to land around 15-20 BTU/sq ft unless they've got high ceilings or a ton of glass, but design temp is -25F here. For a quick and dirty when estimating, I typically use 30 for new construction and if I get the bid, then I'll do an accurate heat loss. Very seldom does it go over 30 and that's usually for shop type buildings with 16+ft sidewalls and several overhead doors. Existing construction, I will sometimes guess at 30-45 (depending on age and build quality) for estimating purposes, then again perform a heat loss later on or at the bare minimum, measure radiation. Never have I seen even a crusty old 1800s farmhouse come in over 48 and that one you could literally see through the walls. I bought a 1100 piece of junk 1905 two story with newspaper insulation and single pane windows a few years ago, that's heated with a 30k Empire wall heater. Never done a calc on it but it's low-mid 20s somewhere at -25F based on the heater size
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By way of cross-checking your rule of thumb guesstimation, could you not use the BTU input ?
If you had a known amount of btu input , could you not guesstimate the eff% high/low and thus the btu required high/low window ?

I use actual usage history for nearly all my heat load calculations......I agree it tends to be much more accurate than calculations. For heating load, I have yet to see any calculations that take into account thermal mass coupling into the ground ( like in old brick buildings) and thermal mass lag. Both of these seem to have a big impact on heat loads, but are ignored in heating load calculations. However, in cooling load calculations, thermal mass and length of occupancy are accounted for.....but I believe engineers typically ignore these factors and just try for the biggest possible load they can get i.e. recovery of spaces at design conditions and full ventilation load of an empty building.
• Member Posts: 3,244
edited April 2023
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Heat loss calculations matter less when using a modulating condensing boiler, the whole point of which is to load match using integral logic and a regulating gas valve.
And while I'm at it, I absolutely believe there is something to be said for adding a significant fudge factor. A building that can't heat because someone likes to open a window at night, some exhaust fan is left on too long, or some abnormally cold and windy weather sticks around becomes a problem far beyond overshooting your heat load by 20% and being prepared for unforeseen conditions. I've never collected a final payment because someone admired my heat loss calculations. I've had payments withheld because I can't maintain 74° on a weeklong stretch of single-degree weather. Try fixing that once the system is installed.
Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
Consulting & Troubleshooting
Heating in NYC or NJ.
Classes
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You're not kidding JohnNY, and that goes for cold spots on a radiant floor and big patches of unmelted snow on a driveway...ha ha..I Have Always agonized over load sizing, spacing, heat spread and performance, just to never have these things occur.  You get ONLY one  chance sometimes....Mad Dog
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JohnNY said:

Heat loss calculations matter less when using a modulating condensing boiler, the whole point of which is to load match using integral logic and a regulating gas valve.
And while I'm at it, I absolutely believe there is something to be said for adding a significant fudge factor. A building that can't heat because someone likes to open a window at night, some exhaust fan is left on too long, or some abnormally cold and windy weather sticks around becomes a problem far beyond overshooting your heat load by 20% and being prepared for unforeseen conditions. I've never gotten a final payment because my calculations were dead on. I've had payments withheld because I can't maintain 74° on a weeklong stretch of single-degree weather. Try fixing that once the system is in place.

That is "street smarts" speaking reality. However, there are more and more governing bodies that are requiring heat load calcs be followed. IIRC, Canada has been there for several decades and so has Minnesota. There are increasing number of suburbs in Chicago that are requiring the same. From my perspective and understanding, probably about half the efficiency gains attributed to mod cons is due to previous equipment being oversized. Even when compared to properly sized conventional equipment about 2/3 of the real efficiency gains are due to the modulating ability and the connected reductions in heat loss from a building and only about 1/3 is from the "condensing" aspect. We see this with steam boiler replacements all the time... moving from typically 60% oversized atmospherics to properly sized atmospherics yield about 10 to 15% reductions in fuel usage, moving from atmospherics to power burners about 5 to10 % and then moving to power burners sized to the current building heat loss with 3 to 1 downturn that modulate the firing rate on outdoor reset for two pipe steam systems another 10 to 15 %. Since nearly all of the hot water systems we deal with are single zone high mass gravity conversions, the savings realized by mod cons also tend to be reduced....Properly sized conventional boilers run very long and efficient burn times with very long off times in between and the system, due to the mass, naturally resets its operating temperature, so heat losses from the structure tend to be reduced just like a lower mass outdoor reset controlled system.
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JohnNY said:

Heat loss calculations matter less when using a modulating condensing boiler, the whole point of which is to load match using integral logic and a regulating gas valve.
And while I'm at it, I absolutely believe there is something to be said for adding a significant fudge factor. A building that can't heat because someone likes to open a window at night, some exhaust fan is left on too long, or some abnormally cold and windy weather sticks around becomes a problem far beyond overshooting your heat load by 20% and being prepared for unforeseen conditions. I've never gotten a final payment because my calculations were dead on. I've had payments withheld because I can't maintain 74° on a weeklong stretch of single-degree weather. Try fixing that once the system is in place.

I can agree with some of that, but managing expectations during the bid and explaining what the system can and can not do is important too. I'd explain what the system should do at design temp. Just like I would explain for AC that if the design temp is 95°, that means your house is expected to maintain 75° @ 50% RH

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• Member Posts: 22,353
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JohnNY said:

Heat loss calculations matter less when using a modulating condensing boiler, the whole point of which is to load match using integral logic and a regulating gas valve.
And while I'm at it, I absolutely believe there is something to be said for adding a significant fudge factor. A building that can't heat because someone likes to open a window at night, some exhaust fan is left on too long, or some abnormally cold and windy weather sticks around becomes a problem far beyond overshooting your heat load by 20% and being prepared for unforeseen conditions. I've never gotten a final payment because my calculations were dead on. I've had payments withheld because I can't maintain 74° on a weeklong stretch of single-degree weather. Try fixing that once the system is in place.

I agree with the mod con point. The load calc can be useful for a radiator or heat emitter survey. It can help determine how low of a SWT can be use, which could add to the mod con efficiency in over radiated buildings.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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One of the things I do after doing the heat loss/ gain is to look at the existing circulator and pipe size. I see whether the existing piping /pump can handle the actual load. I also ask the customer what areas are hardest to heat so I can pay particular attention to the area.As you know, you own the system after you install a boiler.
Ray Wohlfarth
Boiler Lessons
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JohnNY said:

Heat loss calculations matter less when using a modulating condensing boiler, the whole point of which is to load match using integral logic and a regulating gas valve.
And while I'm at it, I absolutely believe there is something to be said for adding a significant fudge factor. A building that can't heat because someone likes to open a window at night, some exhaust fan is left on too long, or some abnormally cold and windy weather sticks around becomes a problem far beyond overshooting your heat load by 20% and being prepared for unforeseen conditions. I've never gotten a final payment because my calculations were dead on. I've had payments withheld because I can't maintain 74° on a weeklong stretch of single-degree weather. Try fixing that once the system is in place.

I can agree with some of that, but managing expectations during the bid and explaining what the system can and can not do is important too. I'd explain what the system should do at design temp. Just like I would explain for AC that if the design temp is 95°, that means your house is expected to maintain 75° @ 50% RH
So what is that conversation like?
"Mr. Client, I'm going to install this system for you and it's very close to meeting the needs of what we expect to see in terms of winter weather. However, you should be prepared for it not to meet your expectations on some very cold days or some other times that we're not thinking of at the moment and that I can't explain to you in this conversation. Sign here."

Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
Consulting & Troubleshooting
Heating in NYC or NJ.
Classes
• Member Posts: 1,529
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@STEVEusaPA Love the calculators you wrote. I plan on trying them
Ray Wohlfarth
Boiler Lessons
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JohnNY said:

So what is that conversation like?
"Mr. Client, I'm going to install this system for you and it's very close to meeting the needs of what we expect to see in terms of winter weather. However, you should be prepared for it not to meet your expectations on some very cold days or some other times that we're not thinking of at the moment and that I can't explain to you in this conversation. Sign here."

No need to over-react.
But a conversation at time of replacement regarding what the customer thinks should happen and what is real world is important. Simply explaining what a heat loss is, and how it relates to equipment sizing and the downside of oversizing, and what will happen on the coldest days of the year, as well as why AC isn't oversized, and what happens when it is, poor humidification, etc. And that the AC system is designed for a 20° temperature drop.

Now if their 'expectation' is 75° inside when it's 5° outside, then you can design for that and explain to them why it's a bad idea.
Same as people wanting their house 65° when it's 105° outside.
Don't know what is so hard about that, putting it in writing, and having them sign it.

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• Member Posts: 6,505
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@STEVEusaPA Love the calculators you wrote. I plan on trying them

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JohnNY said:

So what is that conversation like?
"Mr. Client, I'm going to install this system for you and it's very close to meeting the needs of what we expect to see in terms of winter weather. However, you should be prepared for it not to meet your expectations on some very cold days or some other times that we're not thinking of at the moment and that I can't explain to you in this conversation. Sign here."

No need to over-react.
But a conversation at time of replacement regarding what the customer thinks should happen and what is real world is important. Simply explaining what a heat loss is, and how it relates to equipment sizing and the downside of oversizing, and what will happen on the coldest days of the year, as well as why AC isn't oversized, and what happens when it is, poor humidification, etc. And that the AC system is designed for a 20° temperature drop.

Now if their 'expectation' is 75° inside when it's 5° outside, then you can design for that and explain to them why it's a bad idea.
Same as people wanting their house 65° when it's 105° outside.
Don't know what is so hard about that, putting it in writing, and having them sign it.