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# load calc question

Member Posts: 316
edited January 17
the manual J load calculation... is whatever formula, you gather numbers of various things, add, multiply, come out with an analytical heat loss value...

in BTU's per time ?

so my question is, because like anything of that nature is easier to f up than get it right plugging in numbers into equations, having not measured something correctly or not taking something into account,

is there a way (while probably not convenient) such as turning the heating system off in the house and taking an accurate temperature measurements over time of indoors vs outdoors (like with 1 minute resolution) and then determine heat loss, or heat gain? Which would be a real world measurement you could be confident in? Do this multiple times, having different outdoor temps, to get a real world BTU/time measurement?

It's 30°F outside in CT, I turn the heat off in my house overnight and then have outside temp overnight at 25° - 35°F and I have my first floor temp go from 70°F at 5pm to 59°F by sunrise for example, and the second floor temp from 70°F to say 56°F. How would you calculate BTU heat loss to size a boiler? Likewise during the afternoon for AC tonnage ?

• Member Posts: 20,524
With Manual J, you do come out with power -- BTUh (BTU per hour). There are a lot of assumptions in the numbers you plug in, so unless you have a lucky guess on all of them, the answer is actually an approximation -- certainly not better than 10 percent either way, and could easily be farther off.

Your suggestion of delta T per unit time has some merit -- but to make it work you'd have to know the total specific heat (BTU per degree F of temperature change) for the structure as a whole, including its contents, and correct for any internal heat sources. The best that can be said for that is that at least it involves a very different set of assumptions which there is no simple way to make. Would it work? Yes, it would. I very much doubt that its accuracy in terms of producing a power figure would be any greater, and very likely considerably worse, than a Manual J.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 13,169
No. That will not work. You have equipment in the house like a refrigerator that gives off heat.

You can do a Manual J heat loss, or you can have the most sophisticated software calculate the heat loss but it makes no difference in the end.

Someone still has to do the measurements, make some educated guesses and plug in the right #s.

Plus, the heating system has to be designed to heat the house on a design day when it is empty. No heating credit for lights, refrigerator or people in the house.

There are ways to calculate the heat loss based on the past amount of fuel used but something this is questionable.
• Member Posts: 1,027
is there a way (while probably not convenient) such as turning the heating system off in the house and taking an accurate temperature measurements over time of indoors vs outdoors (like with 1 minute resolution) and then determine heat loss, or heat gain? Which would be a real world measurement you could be confident in? Do this multiple times, having different outdoor temps, to get a real world BTU/time measurement?

Similarly, you could keep the system on and track fuel usage vs. outdoor temp. You can use a gas meter or fill receipts to do it. It's quick and accurate.
• Member Posts: 107

is there a way (while probably not convenient) such as turning the heating system off in the house and taking an accurate temperature measurements over time of indoors vs outdoors (like with 1 minute resolution) and then determine heat loss, or heat gain?

If you have a record of fuel bills, I found success with the method detailed here: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

It essentially allows you to measure the heat load by using the measured fuel input to your heater and the measured outdoor temperatures (with some wrinkles).

I don't have evidence, but I personally think this measurement is likely to be better than a manual J estimate. It certainly was in my case.

Dana does a great job of explaining the "why" of every step in the computation.

I don't like the fact that he suggests you can do this with one fuel bill. I looked at a few years of bills.
• Member Posts: 18,279
To get the numbers more exact a blower door test could be performed. That eliminates the guessing for infiltration. Which can be a big number in older construction.

I like the room by room load calc, as it can identify high load area that could maybe upgraded to lower to loss.
Certainly no harm in doing multiple methods, as described above.

With todays variable speed equipment both forced ai and hydronics, getting the number spot on to select a boiler or furnace is a bit more flexible.

With small energy efficient homes, the DHW load could be the largest load. So with a boiler that will also supply DHW, you need to size to that load, regardless of the heat load.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 316
what are typical values for heat loss of a home in BTU's per hour?

Take a modulating boiler/furnace for argment's sake, has a 10:1 turn down ratio so does that mean if its max output is 100k BTU/hr then the 10:1 turndown means it can operate at 10k BTU/hr? Again for arguments sake to wrap my head around real world BTUs/hr, at what BTU/hr value would such a modulating boiler operate at to run constantly to maintain temperature in the house at a constant level of say 70°F when it is 30°F assuming modern construction and insulation?
• Member Posts: 18,279
First off, the heatload on a building is constantly changing, it is a dynamic load. As such having a dynamic, instead of fixed output heat input would be desirable.
The change in load can be second by second. A momentary gust of cold wind changes the load, for example.
The heatload number is just a point in time.

So if you could calculated the load of your home at 70F indoor, 30F outdoor, then see if the modulation curve hits that number. If the partial load is within the modulating rate, 10- 100,000 for example, its possible the heater could attempt to run non stop matching the heatloss exactly.

Outdoor reset controls can do some of the work for you. They change the furnace or boiler output as outdoor temperature changes. But still not perfect as you also have internal gains. It could in fact be that outdoor temperature is dropping and the boiler or furnace may not need to ramp up. Cooking, clothes washing, people, indoor activities add to the heat energy within the space.

A step up in control is outdoor reset with indoor feedback, see below.

I liken reset control to cruise control on your vehicle. You will feel the gas pedal constantly moving trying to match engine output to the vehicles desired "cruise" speed.

Your thermostat control has a say in all this, here are some examples of different thermostat logics available. P. PI. PID. The more "logic" you add the better the thermostat can control the whole process. PID is most accurate and also the quickest to the win
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 20,524
ron said:

what are typical values for heat loss of a home in BTU's per hour?

Take a modulating boiler/furnace for argment's sake, has a 10:1 turn down ratio so does that mean if its max output is 100k BTU/hr then the 10:1 turndown means it can operate at 10k BTU/hr? Again for arguments sake to wrap my head around real world BTUs/hr, at what BTU/hr value would such a modulating boiler operate at to run constantly to maintain temperature in the house at a constant level of say 70°F when it is 30°F assuming modern construction and insulation?

How big is a box? There is no such thing as a typical value for heat loss of a home! How big a home? Where is it located? How well insulated is it? How many floors? How much glass? How is it oriented?

Some sources will try to give you a heat loss per square foot of floor area. Really? Even in a small area with more or less similar weather, I've seen real world values varying by a factor of 5. Now throw in the quesstion of where is it -- coastal Virginia vs. high elevation Montana -- and you run the range up to around 30 to 1.

Then define modern construction a mid-range spec built house in a subdivision? A high end custom designed and built house?

Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 13,911

ron said:

what are typical values for heat loss of a home in BTU's per hour?

Take a modulating boiler/furnace for argment's sake, has a 10:1 turn down ratio so does that mean if its max output is 100k BTU/hr then the 10:1 turndown means it can operate at 10k BTU/hr? Again for arguments sake to wrap my head around real world BTUs/hr, at what BTU/hr value would such a modulating boiler operate at to run constantly to maintain temperature in the house at a constant level of say 70°F when it is 30°F assuming modern construction and insulation?

How big is a box? There is no such thing as a typical value for heat loss of a home! How big a home? Where is it located? How well insulated is it? How many floors? How much glass? How is it oriented?

Some sources will try to give you a heat loss per square foot of floor area. Really? Even in a small area with more or less similar weather, I've seen real world values varying by a factor of 5. Now throw in the quesstion of where is it -- coastal Virginia vs. high elevation Montana -- and you run the range up to around 30 to 1.

Then define modern construction a mid-range spec built house in a subdivision? A high end custom designed and built house?