Perimeter Heat 2

[philgr]

I am trying to remember amount of exterior wall heat loss per foot (btu/ft) when perimeter heat (finned tube, radiant panels, etc.) should be used to supplement overhead heat. I know it is somewhere in ASHRAE but I cannot find it. Can you help me? This is for a health care application.

 

[EnergyProfessional]

I don't have the textbook answer. I assume you have VAV ceiling supply and your concern is that VAV does not heat well from above.

You could get away without perimeter heat if:
- you are in moderate climate (not less than 20°F in winter (I'm just makign up these numbers)
- and if yoru envelope is very good (R25, no windows or really really small and good ones.

Anything else I would recommend perimeter heat. Besides being more comfortable, it also prevents you from running AHU during off-time.

Once you have perimeter heat, you probably want to size it to cover most if not all of the perimeter losses.

Regardless of application (healthcare, office...), you need to make people comfortable. Ceiling based VAV with slot diffusers really has a hard time heating, is noisy, drafty, and inefficient.

 

[MisterE2]

I have a ashrae hvac 2001 ebook, I couldn't find anything specifically dealing with perimeter heating thresholds, do you know the specific wording?

 

[philgr]

After starting this thread I found the following in Trane's VAV system design manual. If the heat loss of perimeter wall space exceeds 450 btu/ft of outside wall, an under-the-window, or baseboard, heating system is typically used. If the heat loss is between 250 and 450 btu/ft of outside wall diffusers should discharge direct directly downward and blanket the perimeter wall with heated air. If the heat loss is less than 250 btu/ft diffusers can be located in the center of the room and still provide adequate blanketing to handle the heat loss.

 

[willard3]

Trane has a horse in this race (they make air systems) and I would not accept their analysis without further investigation.

In cold climates, ie, temps <30F, counteracting downdraft is best done by a heat source under the wall......heat rises and blowing sir down from the ceiling does not do the trick.

 

[Drazen]

you need to calculate heat losses through exterior walls to get input data for your recommendations.

as willard mentioned, there is high probability of local drafts if temperature difference between outdoor and indoor space is too high.

this depends both on your climate and level of insulation.

there is currently a new "school" of green design which advocates avoiding perimeter heating wherever possible, but i'm not fan of it.

you should also tell us about type of spaces you have at perimeter.

in general, though, hospitals deserve more conservative approach than other types of buildings.

 

[willard3]

A good way to counteract downdraft with no perimeter units or air systems is radiant heating in the floor.

 

[EnergyProfessional]

I think Trane ha a good approach to take into account the heatloss. Using OAT only does not take into account the insulation. You should also use infiltration and wall losses, since infiltration is by perimeter and make people uncomfortable.
Also near entrance doors you have more infiltration and perimeter heat is needed.

I think roomheight is important. An 8ft ceiling may work with ceiling diffusers, a10' won't. If in doubt, use perimeter heat.

We have many buildings in WI. Some w/o perimeter heat, some with. Guess which ones have the comfort complaints in winter?

Even whne Trane has a certain repertoire of products, their advice is very neutral. i never had the impression they lie to sell me something.

 

[willard3]

I don't think Trane is lying, they are spinning the subject to suit their equipment.

 

[EnergyProfessional]

willard3: agree, you always have to consider the source.

but I found compared to all other manufacturers their advice is most neutral. All their technical papers, seminars etc. are very professional and sophisticated. They are applicable to Trane equipment, but also to Carrier etc.

in the end the engineer still needs to use his own brain