New construction: venting and make-up air for 48'' range

aeliallJuly 4, 2012

Hello,

We are designing a new home, and I'd like to have a 48" range with probably a grill section in addition to 6 burners (CC or BS most likely). The house will have an ERV and no forced air heat - we are installing hot water cast iron radiator baseboards.

Unfortunately it seems that none of the professionals who are working on this project with us - the architect, the mechanical engineers who are designing the HVAC system, the kitchen designer, or the builder - have any experience with such a setup. Literally none of them have ever stuck a 48" range into an air-tight residence. I spent some effort just to convince everyone that we really do need 1200 CFM, and not 600. Nobody knows what the best way to design a make-up air system is. Nobody knows how to select a hood and external blower. I feel like we have to make engineering decisions in the dark, and it's very frustrating.

Has anyone had the new residence-48" range combination, so they could maybe share what their solution for venting and make-up air was?

The details are that the rangetop is against the wall; there are no wall cabinets to work around (open shelving that we can design to be any width and height); we don't care what the hood looks like in the least - a rough-looking commercial restaurant hood would be acceptable; we are in the mid-atlantic region, so the climate goes from fairly cold to rather hot; we need to be able to use the hood at a very low setting for stuff like cooking stocks overnight; and I'd like to have an automated make-up air setup because I am concerned that a child or my elderly MIL will forget to crack a window while using the hood and backdraft the boiler. The ERV exchanges air at a rate of 200CFM, so it apparently is not sufficient when the fan is going at 1200CFM.

1. what kind of hood + blower setup have people used? has anyone used commercial equipment? it seems that residential hoods are something like 22" deep - is this true? it seems like it would leave half a burner's worth of rangetop uncovered. commercial hoods are more like 48" deep, which seems pretty huge, but they are usually hung at a height of 6'8". do people usually get 54" hoods when they have the opportunity?

2. how have you set up make-up air for a 1200CFM hood? do you use a heated system, or just a damper? where do you located the grille in the kitchen?

Thanks for all of your replies!

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BentZero

Piggybacking this one because I'm in the same boat.

    Bookmark   July 5, 2012 at 1:02AM
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kaseki

Wow! A rare case where the aesthetics part of the performance-aesthetics-economy: pick-any-two limitation is not a factor in hood selection.

But first, given a sealed household, I would strongly recommend obtaining a furnace kit to use outside air. This will avoid the possibility of the MUA system not working perfectly and the hood causing negative house pressure and hence backdrafting of the combustion appliance. I know that such devices exist for Beckett burners, and would guess that there are adapters for other burners.

As the OP correctly notes, a 200 cfm ERV is not going to cope with a 1200 cfm vent hood. (Actually, what would happen with a sealed house is that at full power, the hood would pull the cfm the ERV would let through into the now minus a couple of inches w.c. house pressure.)

So MUA is essential, and in the mid-Atlantic region would be pretty uncomfortable in winter if unheated. Depending on one's tolerance for heat and humidity, cooling in summer might be optional or not. We are starting to get into the realm where performance-economy, pick any two is applicable.

MUA can be roughly characterized as passive or active. By passive is meant no fan in the circuit. Active uses a booster fan to keep the house pressure stable. No-fan only works for simple ducting with a vent hood air flow controlled damper. (A recent post here provided some sources for these.)

If a filter is used at 1200 cfm (or whatever flow actually could be pulled with all the doors and windows open, then the house pressure will drop. One can see on websites selling filter packs that the pressure drop at such flow rates is too large for combustion appliance safety, which require negative pressures not be greater than 0.03 to 0.06 inches depending on appliance type.

Active is needed to overcome the filter restriction. It may be needed to overcome heating heat exchanger restriction and/or air conditioning heat exchanger (expander) restriction. The difficulty with active is balancing the MUA air flow rate against the variable hood flow rate such that the house pressure is near zero. This usually requires a control system of some sort, and can be a "project."

What to do? What to do? Only one free-of-processing-controllers active MUA has occurred to me. In the simplest embodiment, the hood control is connected to two parallel wired identical fans, one in the hood exhaust path and one in the MUA path. Then the two will (try to) operate at nearly the same flow rate at every control setting, and one only needs to adjust the MUA duct restriction until it matches that of the hood path. More exactly, adjust the MUA path so that house pressure falls very little as the hood is turned on. Note that this doesn't account for bathroom fans, fireplaces, or other exhaust flows not due to the hood. In such cases, PID control of the MUA fan would be needed.

In my case, due to a 1500 cfm hood, 1000 cfm over-oven vent, fireplace, and three bathroom fan household, I am building an active MUA that will use an axial blower in the 2000 cfm regime. It has to overcome the pressure loss of its intake vent, its filter, and its heat exchanger (which is hot water pumped from my oil burner). Control is via a Fuji PID controller operating from a BAPF differential pressure sensor. The motor power control is not yet selected, and a lot of sheet metal action is still needed, along with further attic revisions that I seem to have trouble getting to at the needed rate.

I recommend the OP and/or his HVAC person read the "Kitchen Ventilation Systems Application & Design Guide" that may be found at Greenheck's web site. It periodically moves around, URL-wise, so a Google search by name may be fastest.

Greenheck or one of its competitors may be able to provide what you need for considerably less agony than a do-it-yourself HVAC project would.

kas

    Bookmark   July 5, 2012 at 9:37AM
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gluzd

Thank you kaseki! Questions:

1. The synchronized fan solution sounds good, however it may affect air temperature in the kitchen too much. Heated MUA can add at least $1k or more to the project and is more expensive to run. Do you know if there's a way to set up controls for the makeup fan to kick in only past say 100 CFM? You can probably get the first 100 CFM from the ERV, etc.

2. Do you know of electronics/controls that can be purchased to control these things, i.e. when the damper opens/fan kicks in, manual override, etc.?

3. What can be done to lower the CFM required for proper ventilation? Lower the hood? Perhaps there are more obscure but also more efficient designs you would recommend looking into?

4. Do you know how to choose / where to find a fan that allows for low CFM, such as say 50? Or perhaps they all do?

5. Are there instruments/ways to measure the actual CFM upon installation?

Thanks,

Nick

    Bookmark   July 5, 2012 at 1:49PM
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kaseki

1) Since the goal is keeping the house pressure from going too low, better would be a differential pressure sensor and a relay to inhibit MUA below some pressure, such as 0.03 in.

2) I am building my own. A real HVAC person who works in commercial buildings would be a possible source of knowledge on this.

3) I don't understand the question unless you mean how small a cooktop would it take only boiling water to avoid MUA altogether. Restaurants, by the way, have to use MUA, and they spend most of their electricity cost on venting, air replacement, and air heating/cooling. Once you select the 48-inch range (with the implication of using it) and a grill (massive effluent), you are into the realm of approximating commercial cooking, if only for shorter periods, and the whole cooking package requires a big, powerful hood and MUA.

4) Most induction motors run at low speeds when controlled by a suitable controller, even the simple diac/triac configurations (such as my Wolf hood has). Just be sure you have a continuous control, and not a "n" position control. Keep in mind that effluent rises at about 3 ft/second, so if the air velocity at the hood aperture is significantly less, containment is unlikely.

5) I think yes. Well, of course, but I'll interpret the question to mean easily accessible means. There are people who measure how tight a house is by replacing a door with a panel having a calibrated fan and differential pressure sensor. Such a setup could be used to compare negative pressure at known flow rate to negative pressure you get when the door is sealed and you run the hood.

I suspect your HVAC person needs to consult with one who has more experience for your particular situation. That isn't me.

kas

    Bookmark   July 5, 2012 at 7:29PM
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