How do you calculate heating savings with reduced CFM

[stellabella]

Problem: Bldg has toilet room exhaust fans operating 24 hrs per day year round. We will be time clocking the fans to run only 8 hours per day instead of 24 hours per day, and to have the fans run only 5 days per week instead of 7 days per week. What is the energy savings of not having to heat-up the exhausted air during the heating season?

Basic equation for the annual 24 x 7 operation heat loss would be BTU/Yr = CFM x DD (heating season only) x 24 x 1.08

How do you adjust the DD component for the reduced fan operation?

 

[MintJulep]

You'll need to work with hourly data rather than DD data.

 

[stellabella]

There must be away of making some basic assumptions to revise/adjust the DD component, unless there is a software calculator that can compute the degree days for just the period of operation. If there is such a calculator, does anyone know where to find it?

 

[Compositepro]

I would suggest that you parallel the fan with the lights and put the lights on a motion sensor.

 

[mauricestoker]

The hourly would not be too difficult as long as you are only talking heating, know how much CFM you are exhausting, and what the design heating temperature is set for.

Download the hourly bin data into spreadsheet. Delete all rows for hours of operation. Delete all rows where outside air is greater than or equal to design heating temperature. Subtract design temperaure (copy and paste) for all remaining rows. Muliply each row by 1.08. Multiply each row by CFM of exhuast avoided. Sum up the column and that will give you the yearly Btu savings.

I'd recommend first making sure that the exhaust isn't necessary for any air balance, and that supply is rolled back commensurate with exhaust. If you are not cutting back on supply you are not saving money by reducing exhaust.

As an alternative, you could close the bathrooms, turn off the fans and lights, and have everyone wear astronaut pants.

If you are going to include an occupancy sensor for lighting, I would recommend putting it on the outside of the stall. That way anyone staying too long has to wave their hand under the stall to get the lights back on, or use a "wide stance". After a bit of time, walking in and seeing the hands and feet waving and stomping might be enough to make people wait until they go home (or wear astronaut pants) and you can save on water and sewer, also.

 

[nuhvac]

HA, i like your style maurice!

 

[ChrisConley]

We (being people who do this for a living) stopped using Degree Days for energy modelling a long time ago. I understand the appeal: it looks easy! The issue was more importanly... it doesn't work.

 

[stellabella]

Thanks, MauriceStoker and ChrisConley. Where do I find the hourly Bin Date? is it used in the same manner as DD?

 

[MintJulep]

Assuming that you are in the US, one source of hourly weather data is

http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/

 

[RossABQ]

I strongly suggest running the fans ocassionally (for an hour or so) over the weekends. My office building's system turns them off on weekends and it is really putrid on Monday mornings, and this is a building only a year old. I suspect even during the week they have reduced the CFM so much that the rooms never really are adequately flushed. Also don't forget that the janitorial services wash down the toilet rooms at night, and need ventilation to dry out. I think your plan is a little too agressive.

 

[Waramanga]

the savings are likely to be much less than you might imagine. I have done some research on outside air volumes, economy cycles and heat recovery and the benefits or otherwise of reducing outside air intake are very dependant on location. If your building is needs cooling in cool weather (ie large commercial) then the extra fresh air is free cooling which can complete counterbalance any losses when in heating.
They only way to do it is with a weather file for your location.