Rule of thumb for heating system selection based upon wall heat loss?

[sabotage]

I have a copy of "HVAC Equations Data and Rules of Thumb" and one of the sections has the following:

A. If heat loss exceeds 450 Btu/hr. per lineal feet of wall, heat should be provided from under the window or from the base of the wall to prevent downdrafts.
B. If heat loss is between 250 and 450 Btu/hr. per lineal feet of wall, heat should be provided from under the window or from the base of the wall, or it may be provided from overhead diffusers, located adjacent to the perimeter wall, discharg-­
ing air directly downward, blanketing the exposed wall and window areas.
C. If heat loss is less than 250 Btu/hr. per lineal feet of wall, heat should be provided from under the window or from the base of the wall, or it may be provided from overhead diffusers, located adjacent to or slightly away from the perimeter wall, discharging air directed at, or both directed at and directed away from, the exposed wall and window areas.

I've also seen the 250/450 BTUH limits in other publications, namely manufacturer publications. I have tried to look for supporting evidence in more 'prestigious' publications, such as ASHRAE, ARI or even AMCA, but have had no luck.

Does anyone know where this rule of thumb originated, or if its even accurate/applicable?

 

[imok2]

Lewis Research Center,
National Aeronautics and Space Administration,
Cleveland, Ohio, July 6, 1965.

The longitudinal variation of wall temperature in finite aspect ratio ducts can be predicted with engineering accuracy (10-percent deviation) from the analytical results for the parallel-plate channel when the half length of the short side of the rectangular duct is used as the characteristic dimension

 

[317069]

I think these values 250 and 450 btu/hr are the limits to define the rgions( cold, very cold or similar) the same idea of the degree day deffinition.
based on regular home construction in certain region.
ASHRAE handbook has a good chapter about air desribution and say the same idea that " in cold weather the heater has to be under windows"
I don't know about accuracy but I guess if we have some procedures why should we go with rule of thumbs, we could use them for roughly estimation only.

 

[sabotage]

imok2:

I'm not sure what your reference to the Lewis Research Center is for, I did a quick google search on all those terms and came up with nothing produced on July 6, 1965. Is that supposed to point me to a research article?

Also, can you rephrase your accompanying sentence? I'm not sure what you're trying to say.

317069:

I did look through ASHRAE Fundamentals Chp. 20 on Space Air Diffusion and saw no mention or reference to heat loss per linear feet of wall.

You mentioned procedures for selecting air outlets and types, would you mind explaining these procedures or pointing me in the right direction?

 

[willard3]

Rules of thumb are for cost estimating purposes only and not for design.

If you design to rules of thumb, you will spend a lot of time defending yourself in court.

 

[sabotage]

willard3:

Which is the primary reason I was looking for research data or the like in ASHRAE, ARI or AMCA. At least with these publications I can find supporting evidence to defend my design decisions, although hopefully not in court!

With that being said, do you have any experience or knowledge concerning air outlet selections for various wall heat loss loads? This issue arises because I am currently designing the HVAC system for a small office building, with one wall consisting of full height glass windows. Typically I would provide electric baseboard heat below the windows, but due to client cost issues I was wondering if I could use linear slot diffusers instead. Any help you can provide on this would be appreciated.

 

[willard3]

It is important to note here that hot air rises and trying to get hot air to fall through colder air is a problem.

I know that Architects don't want radiation because rooms look better without it, but physics is not overcome by opinion. Cold climates require different approaches than warmer climates, but heating from overhead is an insurmountable problem, especially with full-height windows. I practice in the Northeast and put heat sources at the floor.

Perhaps you should investigate radiant floors which don't have the aesthetic problems finned radiation does.

 

[sabotage]

I too practice in the Northeast and that has also been my experience, with locating heat sources for exterior walls at the floors.

However, as the Architect or Client will inevitably ask, can I definitely demonstrate that floor heat is more appropriate than overhead heat? This is when I would like ASHRAE or the sort to back me up on my assertions. If I could show that overhead linear diffusers is not recommended and we should have baseboard heat, costs be damned, it would make for a much easier conversation.

As for radiant floors, it is a bit late in the design process to switch but I will be sure to keep it in mind for my future projects.

Thanks for the insight and please let me know if you see any reliable information on this subject.

 

[ChrisConley]

We use ceiling mounted radiant panels adjacent the exterior wall with great success in our climate (Canada... cold). Not as effective with tall spaces, but very effective in standard height applications.

 

[317069]

I didn't say that ASHRAE has mentioned anything related to a linear feet of wall as a procedure for a heating application,
I said ASHRAE has good chapter about air defusion types and you have read it, I think it is a good refrence.
Again, 250 and 450 btu/foot of wall are not accurate design way, but they just give idea about the location.

 

[cdxx139]

I feel there may be some confusion. Even though the book is "Rule of Thumbs..." (which is a great reference), the comment was not on a rule of thumb for loads, but rather a rule of thumb on when to use fin tube radiation, and when overhead heating alone is acceptable from a human comfort standpoint.

Just becasue you have huge windows, doesnt mean you need fin tube, especially if triple pane and installed correctly.

I have also been confused of when to use fin tube and when it is ok for the architect to win, and not provide it.


knowledge is power

 

[marcoh]

"I know that Architects don't want radiation because rooms look better without it, but physics is not overcome by opinion. "

Not according to some (many) architects I work with!

 

[willard3]

Eventually, your client/owner is going to have to trust your Engineering judgement and, if he doesn't, you need a different client.

Hot air rises, cold air falls and you don't have to prove it.

 

[317069]

what about cooling in summer