Kitchen Fan Affect Boiler Flue?

Big Ed

Forget about the boiler venting problem for the moment..

You still need to vent a gas stove.If house is too tight the fan will not vent.Stove venting is more important now since the fad of installing commercial grade stoves in the home.....

PS...Confuses say ..........The bigger the kitchen the more you order take-out...

Dan_15

As part of my kitchen remodel, Im putting in a state of the art 36" venting hood over my cooktop that has the capacity to move 625 cfm of air. I have a Buderus G215 boiler in the basement that is vented into a masonry chimney. Also, I have a gas laundry dryer in the basement that is vented directly outside. My house is very old (1921) and I would not consider it to be air-tight by any standards. Im wondering if there is anything I should look out for in terms of the hood fan creating negative pressure and sucking CO into my house. I would like to make sure I do this correctly. Thanks.

Big Ed

Fan Tester

Close all the doors and windows and turn on the fan...Now open up a window a crack.If you feel a incoming breeze you have a negitive pressure problem.... Add more intake vents to balance pressure..

Constantin

One approach...

... could be to put the boiler into a air-tight room, supply it with combustion air, and call it a day. That is the approach I am taking in my house.

I also had my contractor install a make-up air duct that leads into the basement AH return duct. Whenever the kitchen fan lights off, the damper opens and hopefully allows a fairly balanced system.

Constantin

I'm confused...

... but as some might say, this is not a unusual condition! :-P

It is precisely for the reasons cited in your post that I installed a makeup-air entry into the home that is damper-controlled. Whenever the kitchen hood goes on, the damper opens and makeup air is let into the home via a 6" duct that terminates in the return of the AC system.

Between that and the HRVs, I'd like to think that our home will remain reasonably stable (pressure-wise) no matter what the fan in the kitchen is doing. However, if there is something I overlooked, I'd love to hear some further insights.

Kevin Jahn

hood make up

You will be hard pressed to get much over 200 cfm thru a 6" intake pipe.

Steve Ebels_3

You'll need makeup air

> As part of my kitchen remodel, Im putting in a

> state of the art 36" venting hood over my cooktop

> that has the capacity to move 625 cfm of air. I

> have a Buderus G215 boiler in the basement that

> is vented into a masonry chimney. Also, I have a

> gas laundry dryer in the basement that is vented

> directly outside. My house is very old (1921)

> and I would not consider it to be air-tight by

> any standards. Im wondering if there is anything

> I should look out for in terms of the hood fan

> creating negative pressure and sucking CO into my

> house. I would like to make sure I do this

> correctly. Thanks.



FOR SURE!!

Steve Ebels_3

You'll need makeup air

FOR SURE!!

To put some perspective on the amount of air you are exhausting from your house think of it this way. A typical furnace for a 1,500 to 1,800 sq ft home will have a blower that produces about 1,200 cfm for heating and cooling the whole house. Your kitchen hood is capable of exhausting nearly half of that. If you are going to re-supply that amount of air to the home without some sort of fan assist you should be looking for a spot to cut an 8x14" hole in the side of your house. After that you need to contemplate how to bring that amount of incoming air to the house ambient air temp. Direct blasts of 10* air in the winter or 90* air in the summer kinda ruin the comfort level in your home.

I would dare bet that your kitchen designer didn't bring this up even given the remote chance that he/she knew about it in the first place.

You will definitely cause some backdraft problems in the chimney regardless of how old your house is. At 625 cfm you need to replace 37,500 cfm per hour and that my friend is a LOT of makeup air for any normally sized house.

joel_19

Kitchen designers

They NEVER bring this up . You are an astute home owner to ask . Yes it very well could backdraft a gas boiler . I recently saw a natural draft gas boiler back draft with just two small bath fans running. It was a newer house but that doesn't mean it's tight. We performed a blower door test and found 23 air changes per day . That's not tight at all. Your 1921 house might actually be better than that!
Out here we have people buying over 1200 CFM and then the kitchen guy leaves room for only an 8" duct to vent it . it's really funny because you'll never move that much CFM through there.

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Constantin

Interesting...

Is the capacity of a 6" pipe that low? After all, a Lifebreath DCS or TRV unit will make it up to 300CFM with their small PSC fan motors. Or, is the real issue that we'll have some whistling when the kitchen fan revs to high speed?

Also, considering that the kitchen fan is variable-speed (i.e. heat dependent), the likelyhood that we'll hit very high fan speeds is pretty low since few people actually use all of their burners at once and likely rarely at that. So, adopting commercial codes for residential use is a bit of overkill, no?

Plus, there are the two HRVs that can also add some CFM via their 6" intakes. I'll try the window test and see what happens.

Steve Ebels_3

BIg difference

300CFM in a 6" pipe is possible when fan forced. When you are talking about natural flow of make up air however, it's just not going to happen unless the house is in a severe negative presure situation. You'd have to actuate a fan in your 6" pipe along with the hood to acheive the desired result.

Daniel Silver

I appreciate all of your thoughts on this. Let me just clarify, I have an oil boiler in the basement that is vented into a masonry chimney, not a gas furnace. The boiler is a typical setup with draft damper and thimble. So will there be a possibility of creating backdraft through the flue damper? Through the fireplace opening? I also have a gas dryer in the basement. Will there be a possibility for creating backdraft in the dryer too? The ventilation hood of German design claims to push up to 625 cfm through a 6" duct, and I am inclined to believe these facts, but I doubt I will use the max blower speed on a constant basis. More likely will be using a low blower speed for regular cooking. Do I need serious makeup air in my situation? Thanks.

Constantin

Yes you can create backdrafts...

... that fan is certainly strong enough, and the nature of the heating appliance does not matter... only sealed-combustion (a.k.a. "direct-vent") appliances are immune from pressure fluctuations within the home envelope. Oil, gas, anything that burns has drafting requirements, and if they're off the results may vary from as little as bad combustion to backdrafting, etc.

In your case, preventing the dryer and the boiler flue from backdrafting may be as simple as keeping the door to the basement shut and having adequate combustion air intake into the basement to allow the dryer and the boiler to function normally. In my case, the OEM boiler manufacturer recommended/required a 4" intake, which is why I am treating the boiler room as if it was outside the house.

The real question is whether the fan will be able to pull 600 CFM with your house as is. For if the resistance inside the home is too great (i.e. you have a tight, well-insulated/weatherstripped home), then you can have the most powerful fan and it won't do much for you.

Thus, the suggestion to have a duct come into the kitchen to supply the kitchen with make-up air is a good one. In an ideal world, I suppose one would install two ducts and two fans. The fans would always run at the same speed but in opoosite directions...

Kevin Jahn

air flow

You are dealing with more than pipe size here, the grill or bug screen, 90° turns all have an effect on the flow. A 6" round at 500 fpm is only 100 cfm as read on my SMACNA duct calculator. When pushed at 3000 cfm it shows only 600 cfm.

Mark Hunt_2

You won't know


unless someone tests, but I have seen cases where exhaust fans cause depressurization enough to cause atmospheric vented appliances spill into the home.

This will depend on how tight your home is and how connected the combustion appliance zone is to the rest of the house. Any time air is exhausted from the home, replacement air must come back in and the chimney is usually the largest "intake" in the home.

Find a good contractor in your area that knows how to test for depressurization and have them make the determination as to whether you will require an additional source for make-up air when you are running ANY of the exhausting appliances. Running all of them at once will give you the "worst case" secnario, but a test should be performed to see what effect each has on the combustion process and draft in your boiler.

Mark H

Big Ed

ERV

What would be a good choice would be a ERV energy recovery venterlator. I beleve its code for new construction in Canada...

Harold

One approach to the problem is to install a range hood designed to provide balanced make up air. The only one I found, and the one I have installed, is from captive aire (yes - spelled that way).

It is not for everyone. It is a commercial unit. It seems to be machined from a block of stainless (just kidding) but it is solid. It is big (has many sizes) and can provide makeup air below the back of the stove and over the work position. This has the effect of making a very efficient confinement of cooking emmissions. There is a large fan unit in the attic, and a large mushroom on the roof for the exhaust fan. Small mushroom for intake. Can be had with a direct fired heater for intake air.

I put in a water to air coil and a temperature sensing pump instead. Added a couple of interlocked and time delayed powered dampers to close off the system when it is shut down.

I have not run it yet except for a component test.

If you use one you may have to be creative. The design target is units mounted on an accessable roof. The speed controls are designed to set and leave alone. One is in the roof exhaust unit and the other on the attic (roof normally) mounted fan unit. These need to move and be made more user friendly.

If you are real sensitive to cooking emissions (I am) this appears to be the way to go. And when set up properly it should not create a negative inside pressure yet move 1200 CFM off the stove. I have thought about adding motor speed controllers that have an external control input and tie in a differential pressure sensor to automatically balance the input/output. Maybe later.

If you are trying for the "professional look" in a kitchen this thing may be the "ultimate" accessory. It grabs peoples attention.

jim sokolovic

Sounds like it would...

grab peoples wigs, too! {<-(:0

Steve Ebels_3

Make up air hoods

What provides the heat to warm 1,200CFM of incoming air in the winter?

Constantin

Jerry pointed this one out to me:

The Stirling system from ultimate air. Probably the best ERV on the market, and it uses standard parts, not $$$ cores made by only one manufacturer.

Harold

We are a bit concerned about flipping pancakes at the highest speed. There is a possibility that the basic rule of "what goes up, must come down" may experience a local variance.

The heating coil is a custom add-on which is a high temperature loop from a Weil Mclean condensing boiler. The unit can also be purchased with a in-line gas heater.

We are in the Pacific NW and the temperatures here do not usally get very extreme. But I believe (I don't remember the numbers) that the coil could represent pretty much the full capacity of the boiler at full speed and real cold. The cook claims she won't use the heat - it is always too hot in the kitchen when she is cooking. It wil be interesting to see how that works out. The range will be 4 burners and a grill. Two of the burners are 18K BTU so a reasonable air flow is necessary. I installed the heater now because trying to retrofit the installation if it turned out to be necessary would be a real horror. The stuff in the attic is seriously difficult to access.