A Heat Loss Is Not Rocket Science

Chris Maderia

This is a follow-up Post to a previous post. The author of that post was looking for a rule of thumb. That to me means a way to ESTIMATE not a hard concrete number to run with. I received numerous e-mails on this subject and I feel that some look at a heat loss as being this fancy, down to a science, technical, mathematical equation that takes a Harvard graduate to figure out. Well, I'm sorry to disappoint you, it's not rocket science and you don't need to be a professor to figure it out. There are really only two equations that you need so I figured I'd post them.

Formula for Heat Loss through an OUTSIDE Structural Panel:
(T1-To)/R-Value = Btu per square foot

T1= Temperature Indoor
To= Temperature Outdoor
R-Value= The resistence value of the structural panel

Example = 70-0/19= 3.68 btus per sq ft

Now take your sqft of wall and multiple it and boom there is you loss for that wall. Now what if their is a 3x5 window on that wall? Take 15 sqft off your walls sq footage and calculate the window with the same formula then add the 2 together and there is your loss. Now continue using the same formula for the whole room. Remember that a ceiling is a outside wall if their is no heat above.

Now the other formula that you will need is,

Formula for heat loss due to air exchanges:

(T1-To)x (AC/HR x Volume) x 0.0183 = BTUH

T1= Temperature Indoor
To= Temperature Outdoor
AC/HR= Air Exchanges per hour
Volume= Rooms cubic feet
0.0183 is the amount of energy (BTU) needed to raise 1 cubic foot of air 1 degree farenhite.

An 8 foot wall, 200 sqft room that has an air change of .5 per hour with a indoor temp of 70 and an outside temp of 0 will have a heat loss due to air exchange of 1025 btuh.

Now add this to the loss through your walls, windows, doors, etc and bingo- there's your loss for that particular room. Like I said it's not rocket science and for those of you that think it is, how come your not complaining that you have to put in those 3 section boilers when all you need is a 2 section.

Bill Wright_2

Chris, Well put ...

I don't disagree (sorry if I gave the impression that this is complicated!! -- it isn't.)

The previous post concerned the differences between the heat loss for a Radiantly heated structure, vs a structure heated by some other emmitter.

Heat loss is heat loss (IMHO). My point was that there are various opinons as to what effect various emmitters have on heat loss ... but the bottom line is ALL heat loss calculations are just estimates based on a set of assumptions -- and are no more valid than those assumptions.

Thanks for your post & explaining it in an "easy" way

Bill Wright

Duncan_2

Sounds good...

The math looks good to me, Chris, you're on the money (put a time factor in that 3.68 btu per sq ft).

How about heat loss in a basement with an uninsulated concrete floor that is half below grade and half on-grade?

How do you figure your air changes per hour? It's guesswork if you don't have a blower door, isn't it?

I use the same method by the way. How do you feel about rules of thumb like 25 btus per square foot of floor area?

Bill Wright_2

I hate rules of tumb, but ...

I use 25 Bth's / sq ft. Over the last 5 years, as I finish a heat loss calculation, I tabulate the basic info in a database (btu/sq ft, ft of tube/sq ft, $/sq ft, etc.). So the average load comes out to around 23 Bth/Sq ft (averaged over 2,752 jobs). Obviously the load is higher in the North than in the South ... but 25 seems to be a reasonable number w/o doing any math.

Unknown

Bill

How far from the average do you find some of these jobs working out to. How many do you find that fall far outside the average? I'm just curious.

Noel

Duncan_2

Wow!

"...averaged over 2,752 jobs"

Somebody's been doing their homework !

;-)

Chris Maderia

AIr Changes per hour

Started out using ASHREA but I would have to say EXPERIENCE is the major factor. As far as your basement, soil has an R-value added it to your structure R-Value. The thing about all this is that common sense rules.

PJO

I know it's not exact, but

what about internal heat gains...especially solar? How about the discussion earlier of the lower heatloss calcs for radiant vs. scorched air? I also feel that the wall's "R" value is a misnomer sometimes...what if you have 2x6 studs with 24" centers vs. 16" centers? The former would obviously have better insulating properties (less thermal bridging), but that would not be accounted for either. I believe some calcs. consider this while others don't.

Chris, you make a very good point and I'm not arguing with you as much as I'm looking for your help on these. I can say this only from my own experience...built a house almost five years ago & did the HVAC...heatloss calcs where given to me based on HVAC Calc (I think), and it was WAY too high. I ended up putting tube in the garage slab as well (14,000 BTUH), and still am running my boiler about 10% lower (downsized the nozzle). With the recent cold weather I still don't run very long cycles...although my garage stays a bit cooler.

I believe this was due to the fact I have radiant in a large portion of my home, and it is also sealed better than they calculated...Tyvek wrap plus seals in all penetrations. I also have very good passive solar, but this only accounts for a portion of the day.

Chris, if you could state your opinion on these I would appreciate it. I don't do this for a living, but am interested.

Take Care, PJO

Chris Maderia

Radiant

While Solar gain plays a role especially in radiant it has no bearing on the heat loss of the structure itself. Also, as posted above the formula never changes either. It seems that we all forget that a heat loss is NOT and I repeat NOT 100 percent accurate. We do the best we can will the information that we have in front of us. If I have a print and I doing my loss or radiant design and the print says R-19 for the walls and I do my loss and give it to the contractor/home owner and then down the road there is a change because of budget and they decided to go with R-11 but never come back for a new heat loss who's to blame. If you use the formulas above you should be able to get an accurate heat loss. Infiltration factors vary, so you have to get a manual but experience tells me in a radiant design I use a .35/ a tight baseboard new construction a .5/ any other I use a .75. I have NEVER overheated or underheated a residence or commercial bldg, never oversized or undersized a boiler and take pride in my losses and radiant designs. I get yelled at everyday by contractors who tell me my heat losses are to low. So, I ask them what's low, see here in my arear for years a supply house was using a 0 degree outside temp and a 80 degree inside temp so there heat losses have been out of this world for years. Now to your problem.

The greatest thing about radiant is that it's very forgiving and as long as overheating is your problem it can be resolved.

Step 1 - Get a new heat loss done.

Step 2 - Have a new radiant design done. I tell you this beause if your original heat loss was wrong you can bet your radiant design water temps are wrong.

Step 3- After reviewing steps 1 and 2 find a solution.

You really can't get any answers until you get steps 1 and 2 done. If I can help you e-mail me.

Chris Maderia

Radiant

While Solar gain plays a role especially in radiant it has no bearing on the heat loss of the structure itself. Also, as posted above the formula never changes either. It seems that we all forget that a heat loss is NOT and I repeat NOT 100 percent accurate. We do the best we can will the information that we have in front of us. If I have a print and I doing my loss or radiant design and the print says R-19 for the walls and I do my loss and give it to the contractor/home owner and then down the road there is a change because of budget and they decided to go with R-11 but never come back for a new heat loss who's to blame. If you use the formulas above you should be able to get an accurate heat loss. Infiltration factors vary, so you have to get a manual but experience tells me in a radiant design I use a .35/ a tight baseboard new construction a .5/ any other I use a .75. I have NEVER overheated or underheated a residence or commercial bldg, never oversized or undersized a boiler and take pride in my losses and radiant designs. I get yelled at everyday by contractors who tell me my heat losses are to low. So, I ask them what's low, see here in my arear for years a supply house was using a 0 degree outside temp and a 80 degree inside temp so there heat losses have been out of this world for years. Now to your problem.

The greatest thing about radiant is that it's very forgiving and as long as overheating is your problem it can be resolved.

Step 1 - Get a new heat loss done.

Step 2 - Have a new radiant design done. I tell you this beause if your original heat loss was wrong you can bet your radiant design water temps are wrong.

Step 3- After reviewing steps 1 and 2 find a solution.

You really can't get any answers until you get steps 1 and 2 done. If I can help you e-mail me.

Bill Wright_2

Noel,

Well that is hard to answer. I just crunched the database & for radiant structures (not snowmelt) came up w/ Average 22.6 bth/sq ft and a Standard Deviation of 15 -- but that includes some weird stuff (>60 bth/sq ft Orangatang house for instance). Most seem to be in the 11 to 30 range.

By the way -- thanks for the book tip ("Warmth disperses, Time passes - the history of heat") my copy arrived yesterday ($11.47 @ Amazon). Dan H. (if you are reading this post) -- here's another one to add to your reading list!! I also picked up "Absolute Zero, the search for cold" from Amazon.

Thanks Again,

Bill

PJO

Good points, Chris...

and thanks for the offer to help. Raincheck, please! :-)

I keep a pretty low water temp...about 10F lower than I originally planned. I did re-do the heatloss calc using the Slant/Fin at the left (thanks Dan and SlantFin)...the final numbers where still a bit high but I may have went a little conservative on the infiltration...it was a while ago and me feeble mind can't remember that far back!

I also am tweeking the system here and there. A bit of re-piping, adding a little here and there, and I'm going to try something different in the spring. I don't want to spill the beans just yet, but will post it on The Wall when it's done...won't hold a candle to the jobs here, but there will be a story behind it (after all, I am Irish - got to be a story for everything!).

Again, thanks for the offer to help. God Bless The Wall! Take Care, PJO