Discharge temperature of diffuser during heating

[jvithayapun]

I have tried looking and found a range b/w 80-130F as a good heating discharge temperature. I have also read that aiming for 105F is a good spot.

When adjusting the heating cfm, do you adjust it up and down until the discharge temperature reaches a certain temperature you are aiming for? And what is a good discharge temperature? Or is it in fact a range that is ok to use as stated above?

Thanks.

 

[mauricestoker]

I like using 95* F as a maximum for comfort.

 

[willard3]

The higher the discharge temperature of ceiling-mounted diffusers, the worse the mixing/heating.

 

[ChasBean1]

Your question can’t be answered very thoroughly as presented… The heating criteria could vary quite a bit depending on load and other heat sources such as perimeter radiators. Hot water heating coils might have for example a 20°F rise for a one row model and a 40°F rise for a two row model, so selection matters.

Also for a typical VAV with reheat application, cool airflow first reduces to try to allow the space to warm, then reheat kicks on if cooling flow reduction alone isn’t enough. The diffusers are usually sized for a good air throw at the max (cooling) airflow; performance diminishes at reduced VAV airflow during heating. This could make for stratification (hot head, cold feet) So it’s often a good philosophy to use VAV with reheat but to increase the airflow again during heating mode if necessary (i.e., max/min/max versus the typical max/min control)…

 

[mauricestoker]

If for human comfort, then take a look at ASHRAE 55.

SMACNA handles throw pretty well if you're concerned about ceiling height. That's an area where I run into difficulty with architects. Fortunately, architects don't get involved in box control. Like Chasbean states above, if using a VAV, from an energy standpoint it is better to use a dual maximum approach, increase air flow then activate the reheat.

 

[ChrisConley]

Any discharge temperature above 90F (or so) will result in stratification when supplying from above. A high throw diffuser can help, typically if the same diffuser is used for heating and cooling you don't want cold air 'dumping' on people though.

Heating problems intensify in this situation as people try to further increase discharge temperatures to 'fix' the heating problem, which increases stratification.

Typically I keep discharge temperatures below 95F on air heating systems.