Combustion Air Intake Efficiency

Jack

You will find equipment that is either single stage or variable speed for combustion air.

john johanson

Combustion Air Intake Efficiency / Heat Recovery Ventilators

Question:

1 - What is the most efficient way to bring combustion air from the exterior?

2 - Can/should heat recovery ventilators be used to bring in exterior combustion air?

Background:

I am converting an old church building to a two family residence. The building has a small boiler room in the basement which has no intake of exterior air, except what comes through the interior. For the conversion, I need to fire separate the boiler room, and based on this volume, there is not enough interior combustion air to meet code; my code officer requests that I pull exterior combustion air.

The most straightforward solution is to send the code required ducts to the exterior, which are fairly large as the total BTUH are in the 650k range. Another option is to use an expensive forced ventilator. I am wary of EITHER of these cases, as they would bring in cold exterior air to the boiler room; it seems that some of the heat from the boilers would be lost to this colder air temp - isn't this the case?

Another possible option: I've seen many different types of heat recovery ventilators (HRV). These are typically used to Exhaust stale air, but can you use these as combustion air intakes if you just exhaust both the "stale" and heated fresh air into the boiler room (rather than exhausting the "stale" air to the exterior). Based on the energy consumption of the HRV, would it even be worthwhile in this application?

I'm looking for any ideas or perspective on the situation.
Thanks!

ChasMan

I just had a FIC put in and it sure cools yer toes on the boiler room floor. It dropped my basement temps 10 degrees on average. I have it on low with the restrictor plate too. So yes, it's a problem looking for a solution. Not a 500.00 HRV solution though yikes.

john johanson

Chas,
Thanks for the reply - what is a FIC?

ChasMan

Oh Sorry.

Its a "Fan In A Can". Its made by Field Controls. There are others on the market but the FIC is readily available here and relatively cheap. Tjernlund is another maker but it sits up in the floor joists. I preferd the noisemaker being as far from the ceiling as possible.

Charlie from wmass

John some boilers do have hoods or covers for their burners to bring in fresh air. What are you firing, Natural gas or Oil? WHt is your heat plant? Have you thought of individual heat sources for each unit? 650K would be a very large system for a residence but you know your square footage I do not. using the heat exchanger as you described would not gain you anything for energy losses as it would just be pumping in the cold air and tempering it but the same mass of air is entering and the same change in temp will occur unless you exhaust the stall air. The FIC is a good option as opposed to the large gravity air intake.

Mike D_7

Boiler room air


A HRV (heat recovery ventilator) would be of no help in this application. Since you are bringing outside air into the boiler room to supply the boiler with combustion air and air for the draft regulator, all the air brought in goes up the chimney.
The purpose of a HRV is to remove stale air from an occupied area and replace it with fresh outside air. In the process it attempts to recover the energy in the outgoing conditioned air. In heating season it transfers heat from heated INSIDE air to the colds outside air coming in. In the Summer it would transfer heat from the OUTSIDE air to the exiting stale inside air.
You would be bringing air inside only, so there is no exiting air to recover heat from. It all goes up the chimney.
In addition, a HRV is much more expensive than a fan in a box (3-4 times more).
A fan in a box equiped with a motorized damper that closes when the burner isn't running might be a better alternative if your concerned about cold air causing heat loss.
Another way to go would be to consider the new Field motorized flue damper. It’s approved for oil, and most of the heat loss would be caused by cold air passing through the boiler and up the chimney or being drawn into the boiler room and then up the chimney by the draft regulator.
Heat loss to heated areas outside of the boiler room are easily addressed by insulating the walls of the boiler room.

The bigest advantage of the Field motorized flue damper is that you won't be fooling with UBC and NFPA code requirements for boiler room air. It will make you liability insurer much happier.

Unknown

Is it steam?

If it is steam the fan in a can mentioned would work well. I have a couple of boiler rooms with intake air blowers installed. I have the unit interlocked with the burners w/proving switch.

If it is hot water then I would use a sealed combustion unit. HTP Mod/Con, Knight boiler, Burnham Alpine all can do the job.

john johanson

Charlie,

The BTUs are high because of the large volume; the footprint is 3500sqft, and the are two floors, the top floor with a pitched ceiling height of 24ft at the peak.

All units are natural gas. We have a ~500k BTUH steam boiler, a ~100k BTUH air furnace for a small kitchen addition which is heated separately, and a ~50k BTU water heater.

It sounds like the consensus re the HRV, just a misunderstanding I made on how it could be applied.

Agreed on the motorized options being better than the gravity intake, it is a square footage issue for us, the required area of ducting for the gravity intake is fairly large.

john johanson

Mike,
I am not familiar with the Field Motorized Flue Damper, I'll have to research a bit and get back.
Thanks for the lead...