Heat Loss Calc for School Building with Coil in Fresh Air Intake?

Steamhead

Haven't run into this one before...............

One of our customers, a 1950s-era church and school building, is going to need a new boiler. It's a hot-water system using Baseray, and there's also an air handler that brings in fresh outside air, tempers it and distributes it to some of the classrooms.

The heat-loss came out to roughly 852,000 BTUH at design. The air handler coil has 1-1/4" pipe connections, and according to the B&G Handbook, 1-1/4" can handle up to 180,000 BTUH. My question is, do we really need to add that much to the boiler sizing?

ratio

You've got to figure the load of the air handler via airflow, outside temp at design, & required ΔT. I'd start out by measuring current conditions, but see if you can find the original design, & maybe look at current code requirements. I'd only use the size of the piping as a not-to-exceed/sanity check for the sizing.

I think there's going to be a strong drive for more outside air, esp. in places like schools.

dopey27177

The air handler that brings in fresh outside air most probably has or had a set of dampers that operated in accordance to the outside air temperature.

Let say the system is designed at 0 degrees outside the dampers would only be open at 5 to 10% fresh air in.

As the temperature outside rose the dampers would allow more outside air into the system. Usually the ventilating system heats public spaces, If some class rooms are included there probably are radiators in the class rooms as well.

To be sure what you are doing you must get the old drawings for the building, Also you cannot cut the BTUH that was originally designed for the building.

When you make your proposal you need to include a disclaimer,

That disclaimer must spell out that you are installing boilers not fixing anything that does not work correctly or work at all in the buildings heating and ventilating system.

Jake

EBEBRATT-Ed

OA is usually required by code to prevent excess Co2 in classrooms filled with kids. The coil capacity is different depending on the building design.

Sometimes OA is brought in @OA temp and heated to diischarge at 55 deg and then an allowance is made in the baseboard or elswhare to bring that air to 70 deg.

Or they may be using the coil to bring OA to 70 deg which is a stretch for one coil. In that situation it is common to have a preheat coil OA temp to 55 deg and then a reheat coil from 55 to 70 deg if they are using the ahu for replacement air. If they use it for heating the building the discharge from the reheat could go to 110-120

What you could do if the old boiler runs is get a balancer to measure the air flow with everything wide open. Put the coil into full heat and measure the inlet and outlet temp.

Then CFM of air X 1.08 x (Temp of discharge air-Temp of Outside air)

Example: 1000cfm x 1.08 x 20 deg temp rise=21,600 btu/hr

Sometimes the MFG puts a tag on the coil you could get info from.

Depends on the original design. If the info is not availabal you can only reverse engineer it.

Outside air is not required to be brought in when the building is unoccupied. Usually controlled by building automation or a time clock

The Steam Whisperer

Something which I don't think I have ever seen taken into account with boiler sizing calculations is that the only time the ventilation air should be running is when the space is occupied. Well if the space is occupied, there are internal gains offsetting the heat loss, so you don't need as much capacity to maintain temperature. Also, if you need extra capacity to recover setback spaces, the ventilation should not be running until the space is occupied. That outdoor air damper should be controlled by a CO2 meter, so it stays closed when there is light or no occunpancy.

Zman

If you cannot find the specs for the coil, you can usually reverse engineer it by comparing the duct size cfm at 0.1" static and the size of the supply pipe at 4 ft/sec and 20 degree delta. (which sounds pretty close to what the B&G manual is telling you).
Ed's formula above will help you convert CFM to BTU/hrs.
If you post the duct size, I have a ductilator around here somewhere

Solid_Fuel_Man

Going along with what Jake said, be sure to check the damper actuators. 1950s would suggest pneumatic if there hasn't been any updating. Water coils in ductwork with outside air always make me nervous!

CO2 us absolutely the best way to control outside air. So much energy is wasted otherwise.

EBEBRATT-Ed

The coil should have a freeze stat if the system does not have antifreeze.

If the freeze stat trips the sequence should be:

Fan off
OA air damper closes
control valve goes to full heat.

Freeze stat can be a manual reset or auto reset

Zman

If you assume they designed the piping for 4 ft/sec, that would put you at 16.3 GPM for 1 1/4" pipe. http://s3.supplyhouse.com/product_category_files/11448-Flow-Rate-Chart.pdf
Most engineers design for a delta t of 20 so, 16.3 x 20 x 500 = 163,000 Btu/Hr.

I usually then take a look at the duct size. 163,000 / 1.08 (assumes sea level) / 60 (figuring a 10 degree design day and a DAT of 70) gives you 2,515 CFM. At 0.1" WC the duct size would be 18" x 18" or equivalent.

If those comparisons look similar, I figure I am close on the reverse engineering. Sometimes you can find more clues in the fan or coil sizing as well.

Back to your original question. When tempered outside air is being added to a space, I usually scrutinize the ACH assumptions in my heat loss. To some extent the outside air being added is offsetting some of the ACH assumed for to leakage so there is some double dipping in the math.

Is the outside air system set up to make the building pressure negative, positive or neutral? If it is positive, I would deduct the coil from the heat loss ACH number.

I agree that the best way to go is to modulate outside air based on the measured CO2 of the space.

Steamhead

Thanks all- obviously I need to research this further.

luketheplumber

Is @Gordo going to film this one?

nicholas bonham-carter

Is this an opportunity to use two “tag-team” boilers, or a hi-lo gas valve for better matching the needed firing-rate/heat-loss between occupied, and unoccupied periods?
Peerless boiler has the Mod-U-Pack valve system accessory for the 211-A.—NBC

EdTheHeaterMan

You Guys Rock!
Great discussion.

tim smith

Steamhead, here is some historical info for what cfm that age of system would have been designed for. https://mosaicscience.com/story/school-design-through-decades/
Currently I think it's at 15 cfm/person minimum
You can find model of old fan unit/coil which you might be able to find someone who has books on it.

Steamhead

luketheplumber said:

Is @Gordo going to film this one?

Wouldn't put it past him..............

tim smith said:

Steamhead, here is some historical info for what cfm that age of system would have been designed for. https://mosaicscience.com/story/school-design-through-decades/
Currently I think it's at 15 cfm/person minimum
You can find model of old fan unit/coil which you might be able to find someone who has books on it.

Thanks! I'll take a look.

EdTheHeaterMan said:

You Guys Rock!
Great discussion.

Glad to help..............

tim smith

If no luck on blower model # research, get the blower wheel diameter & width and whether forward or backward curve blades, the pulley sizes and motor rpm and hp, we should be able to come pretty close on cfm then we can calc the btu do provide 70 degree air at design. Kind of my take on it, maybe some others will be able to add to this.
Good luck

tim smith

Probably the easiest way to figure this is to find out what their allowed occupancy # is and multiply by 15. Just for area served by this air handler and coil. That will give you the fresh air requirement then multiply that by 1.08 x 70 (if design is 0 for your area. That gets you to what would probably required today. Back then they may have only counted on 10 cfm per person.