heat loss calculation

Constantin

In my mind, the best time to take a reading is when the house is straining hardest to stay warm.

In our house (since we don't set back), the best time to clock a boiler for an hour or more would be the early morning before the sun rises, ideally with a 15MPH wind to stimulate infiltration. At that hour, there is no insolation, no IDWH recirculation, etc. going on. Just a boiler vs. the cold outside. By early morning, the house will have used up the "bank" of stored flywheel BTUs as much as possible, etc.

If you were to compare results from two complete days with the same number of HDD in the middle of winter, you might find that the house loses a lot more heat on an overcast & windy day than on a clear, sunny day. Insolation heats our south-facing rooms so much that the heating system comes on little to not at all in those zones during sunny daylight hours, regardless of how cold it is outdoors.

In the summertime, heat gain via windows accounts for 8 BTUH/ft², more than any other factor. Granted, the wintertime insolation load is not as great (sun is lower, outdoors is not hotter than the indoors), yet even if I only assume that 25% of the heat load is radiation, those 2 BTUH/ft² go a long way to help heating the house. It's 40°F out there and those south-facing zones only ran for 2 hours or less today.

Interestingly, the heat loss and the boiler on-time in our house coincided more than the usual 25% overage reported here. Perhaps that's because I manually put in infiltration numbers that I felt were more appropriate considering our use of foam insulation, etc. As Mark correctly points out, there is no way to "know" the infiltration unless you know exactly how the house was put together, how well it was sealed, etc.

Thus, I would consider it a gamble to assume 25% overage re: heat loss of manual J vs. reality as a rule of thumb.

Sure, you can count on using mass as a flywheel, allow wider swings in temperature, etc. However, such gambling may be best relegated to one's own home (if you and your spouse are OK with the potential for cold conditions). The contractors here don't have that luxury, however. Call-backs are the most expensive kind of call a contractor can get.

Lastly, it may also be a matter of degrees. For example, my energy auditor told me today that the usual oversizing factor they encounter in the field is 2-3x for heating and 2x for cooling equipment. If one were to shave the margin (even if it exists) from 200% down to just 25%, the equipment savings, runtime savings, etc. would already be largely realized. Modulating equipment makes it just that much easier to consistently get great results.

ddenny

heat loss calculation

hello
I used the slantfin hydronic explorer software to help me size a furnace. I was very carful and took the time to measure every room. I know they only have an insullation value of R7 in the walls because my house is in the same 35 year old developement. I was very supprised at the low the heat loss calculation total of 32,000 btu. does that sound right for a 1,300 square foot house? the current furnace which is the original furnace is 100,000 in 80,000 out. I'd be scared to replace it with one half the size. what would you do.

Joe Brix

Trust....

....the software!!!!!! If you are that uncomfortable, re-check your data, and make sure you have entered the correct dimensions, window sizes, r-values, ceiling heights, etc., just to be certain.

I'm not surprised at your results, though, as I see this on practically every heat loss I perform. I can't remember the last time I found a boiler or furnace that WASN'T oversized!!!!!!!

Starch

Brad White_9

Just make sure

as John said, to double-check everything. Is R-7 the wall insulation or the entire wall assembly?

Infiltration is the wild card. Can vary greatly. Also, when you did your take-offs, did you remember to include gross wall areas beyond the room inside dimensions? IOW: If you have 8 foot ceilings, you did take your wall area as 9 feet to account for the structure, right? And to the centerline at least of the partitions? Otherwise you would have "cold bands" not taken into account.

Uni R_2

Reality Check

If you have an idea of total run time for the furnace on an exceptionally cold day, then that will give you a reality check. Ideally, it should have to run almost all day long. If it only ran for 10 hours or less, then you can be pretty confident that 50K is enough.

Brad White_9

That only works

if the boiler or furnace is approximately sized to the load and only if taken over an hourly basis. Otherwise it is like comparing a big rock to a different size rock

Uni R_2

Explain?

Brad White_9

'Splainin'

"If you have an idea of total run time for the furnace on an exceptionally cold day, then that will give you a reality check. Ideally, it should have to run almost all day long. If it only ran for 10 hours or less, then you can be pretty confident that 50K is enough."

All appliances are rated in BTU's per Hour. If a given appliance output is rated at twice the requirements of a design day (your "exceptionally cold day") it will never run "all day long", much as you describe. Only if accurately sized will it do that and we know that is not the case here. Now if it ran 10 out of 24 hours, that is meaningless because there is no longer a per-hour basis. Yes, you may have 10/24ths but without knowing the hourly outdoor temperature during each hour of that 24 hour period, there is no reference point for heat loss.

Take the same appliance and clock the on-time (assuming non-modulating fire here, for we cannot readily observe the "how much" as we can the "on-off")... If the cycle times are equal and it runs 50% of the time for that hour and the outdoor temperature represents a design condition, you have a shot at it. That is how I would apply the method.

Over a day? Anyone's guess.

Uni R_2

Brad

Yes Brad, that's a far easier reality check than someone merely knowing that their furnace only runs 7 hours a day when it's really cold out so maybe it really isn't that risky to get a furnace half the size.

I wonder if a PEX expanding tool might help loosen your pinky ring a bit - design day is technical jargon to most folks asking questions here.

Uni R_2

Brad, that pinky ring needs a Wheezbo expanding tool

My post was meant as a quick reality check. The poster states "I'd be scared to replace it with one half the size. what would you do."

I offered a quick reality check.

Your 'Spainin' of waiting for design day condition and monitoring the run time over a one hour period doesn't seem like a reality check to me. At least not a simple one.

I would in fact argue that the 24 hour method is far more accurate. Who really cares what the exact design day temperatures are? We would all agree that Manual J calculations are overstated.

Furthermore, testing as you state above could be extremely inaccurate because of it only being done for an hour. If you were to do that test late in the afternoon of a cold clear day on a brick home, the lag on the external mass would really skew the results.

If the poster's furnace only runs 7 hours on a really cold day (and a few of those periods might even be design day temps), then they should be able to feel very confident that they'll be warm and comfortable with a boiler half the size.

Constantin

Seems plausible...

Your calculated heat loss sounds pretty reasonable, depending on what the outdoor conditions are on a design day. After renovating the house I'm sitting in, the place has more usable space and a boiler about ½ the size of the furnace that it replaced. What I'd do in your shoes is two things:

1) Hire an energy auditor from energystar.gov to look over your house, do a blower door test, etc. Then use his/her suggestions to tighten up the place. Furthermore, the blower-door data will help you calculate the heat loss.

2) Next, find the best contractor you can. Get him or her to install a furnace that is close in size to the heat loss on a design day. Condensing furnaces can help save you money, as do models that incorporate multiple firing levels. The payback/benefit depend largely on the marginal cost of going with better equipment vs. the cost of your fuel.

When the time finally comes to change out equipment, I'd also recheck everything attached to it: ductwork, chimney's, flues, etc. You never know if everything is in tip-top shape, unless you investigate. A quality contractor (like many of the luminaries found here on the wall) will get you on the right path...

Lastly, malfunctioning heating equipment is not only typically more expensive to run, it can also kill you. So don't forget to get an annual maintenance contract with a conscientious contractor; they are vital to the health of the equipment, as well as for yours and that of your family.

Weezbo

*~/:)

i read what Brad just said as, if you have a boiler that runs all day long and has done so for months and it might not be looking like winter is over...Your boiler is running oki doki it just aint going anywhere because it Must be Undersized:)

And the Over size sign on the back of the semi trailor is either a statement of a philosopical nature or a particular viewpoint ,as related too posibly large rocks that roll down the sides of mountains onto the road.

*~/:)

Weezbo

I would insulate the Bejezeezue outta the place,

and seal everything and anything that allowed infiltration except possibly the man door on the home. Then i would have a blower door test done and seriously consider keeping a slightly positive pressure with the use of an HRV or ERV ..if you have an oil fired boiler you might not find one with an out put of half that new number on the heat loss calc.

with a Furnace you might already have sealed ventilation with secondary heating and cooling..to me insulation is always the place to Start.

amhplumb_2

Sizing

I'm staying neutral with regards to your numbers! However, that existing 100,000 Btu unit was probably never properly sized in the first place. Many years ago, too many! I apprenticed for a guy that wouldn't install anything under 100,000! He didn't do cheap and shabby work, 100,000 Btu for a house was bragging rights of sorts! After taking my first IBR class way back in 1980, I was surprised at how much smaller the boilers could have been. Fortunately, he trusted me and left me size the jobs after that!

Supply House Rick

Mr Constantin

If you are attempting a battle of wits with Brad (Dan's favorite Engineer)you are coming in empty handed. This is what he does for a living and he is the best! Please stick to the mere homeowner posts. Thank you very much

Constantin

help me please...

... how/where do Brad and I disagree on this topic? Moreover, how about posting with a real name and a real e-mail address instead of playing troll?

Uni R_3

SpecEng

Why not insult me instead of Constantin?

This thread went off on a tangent when Brad chose to refute what I have said. I have no qualms with that. While Brad truly is a professional HVAC engineer and is one of the most informative and helpful posters here and I'm but a lowly high school dropout that doesn't even have a trade, there is nothing wrong with disagreeing with people.

Instead of resorting to insults, I asked Brad to explain. So far, I'm not convinced that his argument with my first post holds merit. Nevertheless, I'm still trying to see what he is trying to explain. Discourse in this manner is constructive.

Does Constantin intimidate you? Is that the reason for this post? I would consider Constantin along with most people here to be at the top end of the wits scale. That's what I like about this place. Where do you think most people would rate you on the guts scale after a post like this? BTW, no forum is as anonymous as you think.

Brad White_9

Gentlemen!

I think Constantin agrees with me; I took his "I disagree" posting as a disagreement with UniR and a mild one at that.

I do not have a real disagreement with UniR, just that my means and methods for "clocking a boiler" would have a basis on an "hourly rate" not so many hours out of the day's total because a boiler or furnaces input/output is on an hourly basis. Constantin stepped up to the plate and far better than I explained the "pre-dawn test" which explains my point in a way I wish that I had.

UniR and I both agree that this method is inherently sloppy anyway; not to mention, we both agree that disagreement is constructive. It keeps me on my toes for one thing, not only in probing my convictions, logic and experience, but my ability to express it concisely. I still have work to do in that regard!

I learn here every day from people with abundant common sense and real experience as much as the more "book-learned". I do not see tiers of "some better than others". Rather I see a loose team of folks with broad yet different experience. We all learn from each other. Not just the technical but the articulation of it all.

Thanks for the compliments by the way. It is appreciated and I hope to be worthy of it. It makes me want to be better.

Thanks all!

Brad

Supply House Rick

Whoops, I am #2

Troll

1. A large, brutish creature of European myth, often lacking in intelligence. Sometimes compared to the Japanese oni.

2. A **** who makes idiotic posts in message boards newsgroups for the sole purpose of pissing people off, often lacking in intelligence.

Sorry

D107

By comparison

LI NY 1950s split level moderately insulated house 2100 sq ft. Two detailed heat losses done, each came out 47K BTU, 80K convector emitters. FWIW.

Mitch_6

I think we can simplify things

Radiation rated at 500 btu per hour per foot output at 180 with a delta T of 20.

Boiler rated at 500 btu per hour net IBR

System design 500 btu per hour heat loss at 70 degree rise.
based on proper heat loss.

System calling for 0 outside for 70 inside.

In the system will run almost constant for the full hour.

Double the radiation 1000 but keep the boiler the same 500 the result will be the same run time. Lets assume a radiant panel or other radiation that does not require minimum temperatures to create output (move air).

Double the boiler output to 1000 and it should shut down in half an hour at "peek design temperature". even with the 500 output of the radiation

Double the radiation 1000 with the doubled boiler 1000 and the thermostat will satisfy in half in hour.

all if 0 outside looking for 70 inside starting at a steady state were ever that is.

All total theory real world unknowns will get you.

Mitch S.



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