How does one figure the size needed for an air conditioner in a church keeping in mind that the church is full of people at once for a couple of hours and then empty for several hours later considering the heat emisson per person while church is full and then offsetting the times when the church is empty.

I thought you were hvac1000. Lol. Anyway, size for maximum occupancy. The thermostat will take care of the rest. Better to have enough cooling to take care of a full building than to not have enough. Like I said, size unit for maximum demand. Then install a programmable thermostat.

Thanks for responding so quickly! How do I size for maximum occupancy? Do you know how many BTU's of heat per person one puts out?

Actuall sizing is a little out of my league at this time. I will learn eventually. I would wait for the other experts to come online. HVAC1000 will most likely be here soon. He will need more info. Not just people but size of building, windows, insulation etc etc. I believe there will be folks here that will be able to help you if you can give details as required. Good luck and hope you get the help you are looking for here.

Thanks I will be looking forward to talking to him

He's a new member, give him time and he'll get there... :D

To size a unit properly you will need to do an ACCA Manual J. It will take into account all the variables involved in a building such as a church which usually have very high ceilings. Keep in mind that the churches I have worked on in the past have all had multiple units which conditioned the different areas (i.e. offices, nurseries etc etc)

When you contact the profession installers they should do the manuel J to determine what size is needed,, if they don`t,I would move on to the next professional company,, and I would get at least 2 or three estimates

Wmproop is right, get multiple quotes and don't just take the cheapest.

A manual J is really needed to size equipment properly.

On trick that is used for churches and other occupancys is the early cool down. Many times you can get by with smaller equipment (save money on both equipment and utilitys) by doing early cool downs. If service is at 9:00 AM Sunday then use a program thermostat to turn on the cooling at 6 or 7 AM to precool the building. When the church members show up adding door opening and closings (air infiltration) and these facts below need to be added to the final calculation.


The human body maintains a basic minimum rate of heat production at about 250 Btu/hr during sleep, the heat equivalent of about 75 watts, and about 400 Btu/hr (120 watts) when awake but sedentary. As bodily activity increases, the rate of oxidation of food, with its attendant release of energy, must increase. The level of heat production for light work will be about 650 Btu/hr (190 watts), the extreme value for heavy work, about 2400 Btu/hr (700 watts).

Many times especially with churches the local patrons try to help out. One of the big failures of this is that the helpers have no idea what they are doing. That is OK on some subjects but not with HVAC work. Errors can be very costly and I suggest you get at least 2 estimates for the work and while they are estimating ask them to do a manual J so you know a bit more about this job before you try to do your own calculations.

In many years of experience churches run into more trouble on HVAC work than any other item of the construction. It is wonderful to help your church out just make sure what you are doing is absolutely correct and the preliminary calculations need to be exact.

If this is a Catholic church, you'll be closer to that 650 btuh... aerobimass! Forgive me Lord...

It would also help that you get multi stage or 2 or 3 units that equal to the max load

What the hell... KC opened the door. Church members will always put out twice as many BTU's because of the non-stop HOT AIR!!

Is this an existing building that you wish to cool, or new construction?

Amen! All rise...

One church I go to has a nice system. They have the package units along the wall with two VisionPro thermostats on the oppposite walls.

There seems to be a couple of grills at the back of the church.

Remember to size for an overcrowded condition, which means standing room at Christmas services not just a totally full church sitting down.

There is always Space Pak... I've seen it in a 1800's church remodel and it works absolutely fine.