Whole House Fan Boiler Safety

dreens · June 14

Hi all, I have a whole house fan that I absolutely love and use frequently and will not give up on. But I also have an oil fired burner with domestic coil. Just the other day I had a close call- I was starving the WHF to try and rapid-cool my kid's bedroom so they could go to sleep, and when the boiler came on to heat the water, yucky smells from the basement. It didn't smell like oil, more like burning plastic. I was wandering around my basement looking for something like close to catching fire like a melting wire, and then I put my hand near the damper and I could feel hot air coming the wrong way. Ran upstairs and shut off the red switch, smells abated rapidly. I was short of breath for a few hours after (partial CO poisoning? Or just stress?). I have an alarm but I was closer to the boiler than the alarm is.

I'm seeking feedback on what to do next to prevent any such close calls again. I'd like to add an air intake, and Carlin sells a kind-of case you can put over the oil burner that adapts to a 3" PVC tube that runs outside. But even with easy access to outside air, the backdraft damper could still let plenty of fumes through if the whole house fan is pulling the house negative, and I imagine my chimney needs air to come through the back-draft damper in the forward direction for favorable exhaust temperatures and faster venting rate.

Could I just also run the air intake to the backdraft damper? Is this allowed or has anyone seen this done? This way there'd be easy access to outside air for both the burner and the chimney, independent of the pressure inside the house. There's still the possibility of small gaps in the masonry of the chimney or flue pipe, but the chimney's lined and the flue is well sealed where it meets the chimney. It'd be 100x better than the fumes coming right out the backdraft damper even with a few tiny cracks.

Of course I could also have the fan interlocked to the domestic loop, so the boiler doesn't run with the fan on. What would you do? Both interlock and add an air intake? Would you add an air intake for the chimney via the backdraft damper too or only for the burner? Something else I'm not thinking of?

EBEBRATT-Ed · June 14

II would interlock the fan with the burner. You should probably check to make sure the chimney is drafting properly even when the fan is off.

I assume your fan is 120 volt. You will have to get a 120 volt wire from the fan or from the fan switch possibly and run it down near the boiler. Then use a relay with normally closed contacts to control the boiler. When you turn the fan on the relay gets powered and the boiler circuit running through the NC contacts will open to shut down the boiler. We can help with a wiring diagram once we know some more details.

Jamie Hall · June 14

Must be quite a fan. And quite a tight house (probably too tight).

What @EBEBRATT-Ed said above. But — I would also check draught conditions for the boiler when any other exhaust fans are running — or even when they are not but when the house is closed. You may indeed need an outside air source for the boiler (NOT the damper! The boiler!)

WMno57 · June 14

Several solutions to this:

Open some first floor windows when you run the fan.

Get an electric water heater.

Leave a basement window open with a screen.

Oil boilers should be cleaned and have a combustion analysis annually.

You may get some additional advice if you post some pictures of your boiler and flue piping.

EBEBRATT-Ed · June 14

Two things running together that affect the operation of one of the others in a negative way needs to be electrically interlocked.

All you need in this case is to have someone (a house guest, a relative, a visitor) inadvertently turn on the exhaust switch while looking for a light switch to cause a problem.

Maybe not code required,

At a minimum I would make the exhaust fan a pad lockable switch. Then post at the fan switch and at the boiler that they cannot be run at the same time.

Murphy's law if it can happen it will happen.

Zman · June 14

Having a powerful whole-house fan with no means of makeup air is problematic. As you are discovering, air will find its way in. I have a similar setup with a ~1,200 cfm evap cooler that positively pressures the house. At each corner, there is a barometric relief damper that lets the exhaust out through the attic.

I would suggest giving the air a better path using barametric or intergrated mechanical dampers.This solution will give you the added benefit of better distribution.

mattmia2 · June 14

Looks like the code exempts whole house/attic fans but any other fan over 400 cfm requires a makeup air source:

EBEBRATT-Ed · June 14

@Zman

There is a "means of MU air" it's called opening the windows. But this ia a manual thing. The HO has to remember to open the windows, start the fan and check the boiler. What happens if the phone rings or he has to take someone to the hospital for an unrelated problem and forgets? You can't protect against everything but with guests or other family members in the house, they may not know of this issue. And when he sells the house the problem exists.

It may not be code required as @mattmia2 posted but common sense (which is also not code required) tells me the fan should be interlocked or as a minimum a lockable fan switch and some posting on the fans operation.

The simple fact that he had this issue caused him to have a potentially serious problem.

dreens · June 22

Thanks so much all for your thoughts and suggestions! Yeah the fan is a 3000 cfm from quiet cool, and I’m only 1500 sqft so it really cranks. I recently had my rim joist sealed, there’s no central air ducting or baseboard heat, so it really is sealed up very tight.

I can handle the fan side of the relay no problem, but I’m less familiar with what to do on the boiler side. I’d like it to be so that the fan takes precedence, because I can almost always afford to wait a couple hours before the water heater kicks in. The boiler must already have a suite of safeties like a high temp sensor near the ceiling and so on. Would you integrate the new fan relay with the boiler’s interlock system, or just power the whole boiler circuit through the NC relay, so that turning on the fan causes all power to cut to the boiler as if one had flipped the red switch?

JakeCK · June 22

I personally hate the idea of whole house fans. You're negatively pressurizing the house so it sucks in makeup air from where ever it can, as you just found out. This air will be unfiltered air that got sucked through whatever little openings there are. Imagine if a mouse had at one time made his home in one of your walls, peeing and pooping all over the place. Now that nastiness is getting sucked out, and into the room with the makeup air. What if you have asbestos somewhere, perhaps on some pipes buried in the wall for the boiler?

Then there is the outside air quality concerns. Smog, smoke, pollen, mold spores, humidity... All of that getting sucked into every crevice of the house. I have box fans with merv 13 filters taped to them when I want fresh air. Works amazing during allergy season, except it can cause my house to be real humid if I'm not paying attention to the dew point when I run them. But atleast its positively pressurizing the house instead so all the nastiness that might be in the walls never gets drawn in. And if my boiler did turn on for some reason, such as me falling asleep with them on and the house gets a bit too cold, it doesnt suck the boilers exhaust gas back into the house to kill me.

Personally I'd get rid of the thing, but thats just my opinion.

SuperTech · June 22

I have a huge 30×30 whole house fan that I use primarily in the spring, my windows are huge too and the home is an older drafty home built in the early 60s. Unless I have 85% of the windows open the flue will backdraft. I've confirmed this with my Testo 300LL combustion analyzer.

I don't have an outdoor air intake on the burner but I don't think that's the best solution. Interlocking the fan like @EBEBRATT-Ed mentioned is the best idea, especially considering the boiler needs to maintain temperature. I have an indirect tank, I'm cautious about using hot water while running the fan and have low level CO monitors throughout the home.

Jamie Hall · June 22

By far the simplest solution — and in some ways safest — is to interrupt the boiler power supply with a relay when the fan is running. Fan energizes relay, relay opens, boiler power is shut off. Some wiring to do, but pretty straightforward.

JakeCK · June 22

https://forum.heatinghelp.com/discussion/comment/1804614#Comment_1804614

Three box fans with restrictive merv13 filters can pressurize my 100 year old house with ease. It really doesn't take much.

chief · June 22

hello dreens,

master electrician here. If you’re going to interlock with a relay I would suggest to wire it fail safe. Meaning the a failure of the relay coil will not allow the fan to run. Wire the fan to the normally open contact. Rib relays have different coil voltages for line or low voltage control. I would keep the boiler as priority. Supply house has them in stock.

DCContrarian · June 22

Second the recommendation of RIB relays.

If the whole house fan is multi-speed this gets tricky, you have to power the relay whichever speed is on.

If you want to go down this path, and you have a multi-speed fan I can post a diagram for how to do it using relays.

EBEBRATT-Ed · June 23

Rib relay may not be stout enough for the fan. I would take the boiler circuit and run it through the NC contacts on a rib RIBU1C. Wire the rib coil in parallel with the fan.

That is assuming the fan is 120 volt.

dreens · June 24

All, thanks again so much. I really appreciate the recommendations and the experience behind them. I'll have to ponder this a bit. Thanks again, and I'll be sure to write back with the resolution.

PRR · June 26

Depending on the routing: simplest could be a double-throw AC switch. Fan to one throw, boiler to the other, return to single circuit. You see this in motels to prevent running heat and cool at the same time; with a note trying to explain how to get what you want.

https://www.homedepot.com/p/Leviton-20-Amp-Industrial-Grade-Heavy-Duty-Double-Pole-Double-Throw-Center-Off-Maintained-Contact-Toggle-Switch-White-1286-W/301447135

https://www.homedepot.com/p/Leviton-20-Amp-Industrial-Grade-Heavy-Duty-Single-Pole-Double-Throw-Center-Off-Momentary-Contact-Toggle-Switch-White-1257-W/301447152

Double throw ; double pole is not enough (but can be combined with DT). Sadly this is uncommon in house wiring so you pay through the nose and may need an experienced electrician. And it will have to be explained to house-(or child-) sitters and future buyers.

WMno57 · June 26

Might also be a good idea to tie it into the smoke alarms. You wouldn't want a whole house fan to accelerate a small fire into a large one.

ChrisJ · June 27

To me, since your boiler is also doing your domestic hot water that puts the whole house fan completely out of commission.

Unless you can make sure plenty of windows upstairs are open. They should always be open before turning on the fan regardless.

But disabling DHW whenever a fan is on seems unacceptable in my world.

Bob Harper · June 27

It is incompatible to combine a 3000CFM exhaust fan with atmospherically vented appliances- period. You're never going to be able to provide enough MUA that, a) It works, b) you don't freeze in winter/ swelter in summer from unconditioned MUA or c) wake up dead.
You need one CFM of MUA for every CFM you suck or exhaust from the house. If that fan is running concurrently with the boiler, you'd need a door wide open. You can perform a Worst Case Depressurization Test to prove the House Depressurization Limit and that of the vented appliances. No, you cannot duct MUA directly to a barometric damper. It must reference the Combustion Appliance Zone- not the outdoors. If the house is depressurized, you are sucking outdoor air in from every crevice, which is not healthy. When firing just for DHW, you may be able to orchestrate the dance between fan and hot water but not concurrently. Even when the fan is off, it will leak like a sieve. If the upper level is positive, room air will exfiltrate by Stack Effect and exacerbate lower level depressurization. If the upstairs is negative, it will draw warm moist air into the house. When the boiler is firing, the leakage around the fan will provide some of the MUA for the boiler.
You indicated the chimney was lined. Do you mean a listed ss liner or some visible terra cotta flue tile? Masonry chimney leak like a sieve. If you have leakage in the flue adjacent to a negative pressure With Respect To the flue, it will exfiltrate into the room. That's flue gases, heat, water vapor, odors- all of it. If the flue draft pressure is greater WRT the room, ambient room air will infiltrate the flue thereby cooling the stack resulting in a weakened draft pressure.

The safest, most reliable approach would be to choose between the Suck-O-Matic or chimney venting. You can't have both and think you're living in a healthy, energy efficient structure.

If you wire the appliances to take turns, the fan will suck all the flue gasses and odors into the building easily. You might as well just disconnect the chimney then use the house and fan as your power venter. Breathe deeply.

Note that UL listed CO alarms, when they work, are merely death alarms. They respond very slowly by design and provide no protection against CO poisoning. Rather, they are intended to wake you once you've inhaled enough CO to meet the medical definition of CO poisoning- 10% COHb. Get an unlisted low level CO monitor.

Whole House Fan Boiler Safety

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