Broan's offer.

Here is a link that might be useful: Broan solution.

It seem to me that someone here installed such a system. He also added an inline thermostatically controlled electric heater and air filter.

All it is is an electrically actuated damper. You can get those in a wide variety of shapes and sizes from a variety of manufacturers, in both 24V and 120V versions (Google will find them for you). You can use a current-sensing relay to actuate the damper - that avoids the need for a special switch in the hood (Google will find that for you, also).

To give you an example: my current plan (subject to checking code) is to put a current-sensing relay in the breaker panel on the dedicated circuit that feeds the hood, and run a 120V circuit via that relay to the MUA damper. Any competent electrician can set this up.

Model #: MD8TU/ MD6TU replaced Broan's MD6T/MD8T models which have been around for quite some time.

Broan was the first company to address economical residential make up air systems for kitchen venting and deserves credit/respect for designing this imo. The 6 and 8 refer to MUA duct sizes which draw from one's house exterior. These unconditioned (not tempered air systems) mua units integrate with the return side of the HVAC system, coming on simultaneously when the vent hood itself does, recognized by a sensor in the vent duct damper just above the hood. The damper sensor communicates with the house central hvac to open the exterior vent duct (covered with critter cage and mandatorily above the snow line as well as set back/down far enough from your furnace chimney. Read the instructions for full understanding.

Caliente-would you give a few more of the specifics to your circuit sensing relay in the breaker panel that feeds the vent hood? Is that a Reed current sensing relay? It sounds like your set up is direct wire based, not low voltage, another alternative. So 120 v wire runs from electrical panel to the mua damper duct joined to the central hvac return? Why are you not considering a low voltage circuit system?

It's these details I get tripped up on. I'd never know how to use Google to search for this information. Still, I'd like to converse with our hvac and electrician with basic knowledge.

Needinfo1: yes, this is a nice solution for many providing unconditioned air is allowed (conditioned air is mua within 10 degrees of house temp air). Some localities, especially in cold areas need conditioned air in residential settings which truly drives the price of MUA. I hope our states remain reasonable on this issue.

@SparklingWater asked "Why are you not considering a low voltage circuit system?"

24V means a transformer somewhere. Transformers generally hum/buzz annoyingly, and they add cost and complexity for no significant benefit.

I should have said that the circuit to the damper can be from the same breaker as the vent hood (tapped off before the current-sensing relay, obviously).

Note that MUA intakes must be 10' from sewer vents, furnace and water heater vents, etc.

Link to a typical current-sensing relay below.

Here is a link that might be useful: Fantech current-sensing switch

Thanks for all of the info from everyone.

"this is a nice solution for many providing unconditioned air is allowed (conditioned air is mua within 10 degrees of house temp air). Some localities, especially in cold areas need conditioned air in residential settings which truly drives the price of MUA."

I live in Minnesota. So, are you saying this would not work out for us? I'm hoping that is not what you are saying because we just do not want to get into the MUA game, especially since ours is a 100 year-old, drafty house heated with radiators.

In most cold areas of the country that are actually enforcing the makup air provision, no, you cannot use just a damper that dumps cold air into the home for your makeup air. It has to be heated or cooled to within 10 degrees of the dwelling's temperature. That means a pretty large sized ERV to handle those lare CFM fans. 8-10K is about average for those systems.

I live in St. Paul and just installed a 36" Bluestar RNB & 600 CFM hood. The inspector allowed a "J-tube" in the basement for MUA (its 8" or 10"). No conditioning was required. He just wanted an air balance calculation from the HVAC installer (flow in = flow out). Running MUA to the furnace return was not allowed. It works fine as the warm air in the basement moderates the incoming air.

needinfo-it's beyond me to comment on if this system will work for you or even should be installed. MUA and cooking btu is a very hot, complex topic even for those in the know, which I clearly am not. I do read on it a lot as I prepare for my kitchen remodel. Your state and local codes must be known re: conditioned or non conditioned.

I have learned that high powered range hoods can theoretically suck out tons of house hold air quickly and this is not easily replaced. This point is a driving factor for me as I pick my vent hood (my code is non conditioned mua for hoods > 400 cfm). Do your own research: don't let appliance dealers decide your venthood cfm purchase for you: often your range or cooktop manufacturer stipulate what cfm hood is needed, while other manufacturers are silent.

There are many hvac discussions suggesting mua damper into the home central duct system is not good enough, but at this point in time, the universal mua damper for residential use seems to be at least a doable system for many, myself included. Good luck on a very complicated issue.

This post was edited by SparklingWater on Sat, Mar 9, 13 at 13:36

We are fortunate - being in coastal CA, we can just dump the MUA into the kitchen through a ceiling vent. Sure, it will be chilly on winter mornings, and toasty on summer evenings, but we can work around that - right now we open a window, which is no different.

In lieu of writing (again) the thousands of words the subject needs, I recommend that you visit My Page and find my Clippings. Review those that address make-up air. Search this forum for MUA (recommend Google pointed at this site's URL).

To set the stage, the following may provide some insight:

o What goes out comes in from somewhere; what doesn't come in won't go out; so MUA limits are also hood effectiveness limits.
o Insufficient MUA may also cause back-drafing of combustion appliances which is dangerous.
o A big hole provides MUA; that is the easy part. Adding a damper is still the easy part. Where it gets complicated is adding MUA with some restriction, and then making up for that restriction. There can be some restrictions.
o A filter is needed in the MUA path; this is the first bit of restriction. Depending on whether it is sized for eagles, mosquitoes, or dust, it will cause some pressure loss. This will reduce the MUA flow, and to make it up for the finer filters one may need another blower.
o In northern climes, a big hole pulling in hundreds of CFM will cool down the house rapidly. The house heating system is not designed too make up that much heat loss rate with room registers, baseboard heating, or whatever is used, even if the boiler itself can supply enough heat. A dedicated heater is required in the MUA airstream, providing some more pressure loss and a thermal control scheme. Electric heating or if there is a suitable boiler hot water heat exchange are possibilities.
o If there are multiple exhaust fans and/or fireplaces in the house, the amount of MUA will vary due to more than just the hood cfm. Compensating for this can be complicated, and may lead to a control system for the MUA blower, or a pressure balance loop around the MUA blower controlled by house pressure. (This is the scheme used by large building HVAC.) So that the MUA blower is not on continuously like in large buildings, the MUA blower would have to be engaged only when the hood blower is above a certain flow rate.

kas

Ha kas, I had your clippings open just today, reading on mua. Good stuff people. Thanks for your effort, kas.

mata--I'm in Minneapolis. We have a 30" Wolf AG so less BTU total than you have. Do you find the 600 sufficient? I'm starting to wonder if we could get by with a smaller cfm vent and avoid this entire mua issue. I don't know what a J tube is and guess we'll have to find out. I guess we'll also have to contact a HVAC professional to help out with this mess. We downloaded the worksheet required by our state and couldn't make heads or tails out of it.

kas and others--thanks once again for all the comments. I was hoping this simple solution would be it for us, but apparently it isn't. We've had out Wolf installed for a month now and haven't really noticed the need for any type of venting at all other than knowing we'll want something to remove heat exuded by the range in the summer.

And, as my husband said the other day (as much as we did not want to) perhaps we should have just replaced the old JennAir with another one rather than putting in a Wolf. Oh well, too late now.......

One might imagine two categories of reasons for using adequate ventilation, the personal and the post-personal. The personal reason relates to odor and one's tolerance for it, and to grease deposition and one's tolerance for it and repainting. The post-personal reason is the next buyer of the home's tolerance for grease deposition and the cost of removing it.

However, the amount of odor and grease will greatly vary with what is cooked and how it is cooked. There a great difference between searing steaks or cooking in a wok, and boiling eggs and vegetables. So, as they say, YMMV. Some things are mandated by codes; otherwise, in the end it is a personal decision.

I tend toward the low odor preference, which also provides for low grease deposition. With controlled ventilation, one can decide which odors to allow and which to exhaust.

A historical approach, not compatible with the open concept, is to separate the kitchen from the house as in rural Chinese house construction. Or, one could tile all six surfaces of the kitchen, use stainless steel cabinets, put a drain in the floor, keep the door to the rest of the house closed while making all the grease vapor and odor one can stand to cook with, and when finished cooking clean everything with a steam lance.

kas

needinfo,

I have found 600 CFM's to be more than adequate. It easily dissipated the heat during the burn-off period with the oven@ 450F and all the burners running. As KAS says, it will depend on what you cook and your tolerance/expectations for odor control.

A "J-tube" is just a passive make-up air duct. A duct from outside which terminates on the basement floor in a J shape.

I would recommend getting several quotes from HVAC contractors. They know the regulations and how inspectors enforce the codes. Stafford home services did my installation. They do both HVAC and electrical and were easy to work with. They actually do most of their work in minneapolis.

There are also dampers that use changes in barometric to activate the damper. This is useful for dealing with bathroom fans and perhaps furnace setups where the pressure can drop too low in the house.

I did a quick search for electric duct heaters and found these. They can be used to heat the incoming outside air before it gets to the furnace.

Here is a link that might be useful: electric duct heaters

Here's another vendor. Not $10k.

Here is a link that might be useful: electric duct heater

Thanks for all of the new info. I wouldn't have even known what to do a Google search for.

And, mata, thanks for the Stafford idea; I'll look into them. If you don't mind my asking, can you give me a somewhat ballpark figure on what this cost you?

needinfo,
The cost to install hood, inline blower, make-up air duct, & vent through roof was $1400. I purchased the hood & blower separately.

Has anyone installed one of these directly underneath a 48 inch Bluestar Range? I live in the Mid Atlantic area, so for the most part it doesn't get too cold. Also, would I be okay with an 8 inch duct for the Broan?

Doba,

8" ducts are recommended for 1000 cfm blowers. Two ducts advised at 1200 cfm I believe.
6" ducts for blowers below that. It's on the Broan mua spec sheet.

Can't answer your other question about where to place, but believe it's not recommended directly beneath the appliance.

mata--

Immense thanks for your comments. Reading what you had to say helped us figure out that we do already have a passive makeup air system that was installed when we put in a new boiler nearly 20 years ago. So, we should probably be able to go to the 600 cfm fan without any problems.

When the question of where to inject the MUA comes up, there is a short answer and a not very short answer. The short answer is wherever you want so long as schlieren photography shows that it doesn't disrupt the flow of effluent into the hood.

The longer answer is that the air should reach the hood area in a way that doesn't cause flow disruption; that is, it should be fairly laminar and low in velocity. This is done in lab settings with an entire perforated wall. In a residence, large, preferably distant diffusers should be good enough.

Under the stove (actually ducted down and released at the floor level) was tested somewhere that I read and it wasn't too bad in flow disruption. The air would have spilled out horizontally for some distance and then flowed up around the stove, making up for the air rising into the hood. This was likely a commercial setup, so the stove was probably not against a wall. In any case, in a Northern residence in winter such a scheme will put very cold air at the cook's feet. I can only imagine the comments I would get from that.

In the context of this thread, there may be a way to supply heated, low pressure loss air under the range at modest cost. (The range manufacturer's installation manual needs to be checked to make sure that such as scheme isn't prohibited.) The cold air could be put into the basement via passive J-tube as described above. The basement could have added heating such as a gas/electric/boiler stand-off heater (examples may be seen near the ceilings in large automotive garages.) With a thermostat this will be self regulating. It is still necessary if there are combustion appliances in the basement that the pressure not drop more negative than 0.03 inches of water column, so the size of the J-tube will be determined by that limit.

A register in the floor under the stove would allow basement heated air to rise up and flow into the kitchen. Note that this will generally preclude the basement, or that part of it, being used for tasks that generate chemical odors.

So that there is minimal pressure loss through the floor register, it might have to be fairly large, depending on hood blower capability.

kas

"we do already have a passive makeup air system that was installed when we put in a new boiler nearly 20 years ago."

Make sure you know the difference between MUA and combustion air. They are not the same.

So, do any of the experts here, kas, weedmeister and more, think the Braun MUA damper installed per specs, is an acceptable method for MUA?

Assuming that:
o The damper MUA duct diameter is the same as your hood duct diameter and MUA duct length is no longer than the hood duct length
o There is no air conditioning or heating being performed to restrict flow in the MUA duct
o There is no filtering in the MUA duct
o There is no other resistance (like a closed basement door) in the path from MUA duct to vent hood
then it should be adequate to avoid too low a house pressure.

This is not the same as asserting that it meets code, which would be a component of "acceptable.".

kas

Thanks kas. Appreciate your words always.

weedmeister--

What is the difference between MUA and combustion air?

We have a big stainless steel duct thing that goes out of the basement wall to the outside.

I have one in our house. I have a 36" Braun hood. I really should have installed 2 of them, as if i leave the fireplace damper open and put the hood on high, we can smell the ashes in the fire place.

The only downside to these MUA units is and 8" duct is pretty f'n big. then again to have to of them.

Combustion air is that which is required for your furnace and/or hot water heater (gas). There are complicated formulas to determine the size of the pipe required. Usually it is simple the same size as the vent pipe on the furnace and/or WH. It goes to the outside. MUA would be in addition to this.

Actually, the mention of combustion air raises another point.

For all the dire warnings of death due to lack of MUA, no one seems to have noticed that modern HE furnaces have their own external source of combustion air. No backdrafting possible. No MUA needed, at least not for those appliances.

Yes, the codes may not be fully conformal to reality in all cases. I have pointed out before that sealing off the combustion appliances and providing them MUA should solve the safety problem, but not the hood flow rate reduction due to gagging the vent hood.

There are other ways for lack of MUA to be an issue. My oil burning furnace has MUA piped directly to the blower. However, it also has a chimney damper connected to the basement, and a chimney clean-out that isn't perfectly sealed. It is possible using my kitchen vent hood and ceiling vent to reduce the house pressure enough that I can set off the smoke alarm (due I think to the basement sensor) even with the furnace not running. It evidently only requires traces of residual effluent pulled through the furnace or down the chimney to cause the alarm. And this pressure drop -- of the order of 0.1 inches w.c. -- is achieved across a 2 x 2 ft 3M pleated air filter and similar-sized heat exchanger effectively between the kitchen and the outside. This is why I tend to recommend a blower in the MUA system when it is restricted.

The fireplace is a similar situation, odor-wise, and will be a problem back-drafting-wise if operated. Fireplaces depend on a very small pressure difference between the hearth and chimney top.

kas

@marcolo said: "modern HE furnaces have their own external source of combustion air."

That's optional, not mandatory. It is "acceptable" (conforms to manufacturer's instructions and code) to install an HE furnace with combustion air drawn directly from the space where it is installed, provided the space is large enough and/or there is enough external air available to the space via vents. Our HE furnace is in our garage; it gets its combustion air from the garage, which has simple vents direct to the outside.

All that said, if I had an HE furnace within the habitable space in our house, I would definitely make sure it got its combustion air directly from outside the envelope. Apart from anything else, I would not want to draw unconditioned air into the house to provide combustion air for the furnace.

"All that said, if I had an HE furnace within the habitable space in our house, I would definitely make sure it got its combustion air directly from outside the envelope. Apart from anything else, I would not want to draw unconditioned air into the house to provide combustion air for the furnace".

Couple of points/questions still on this topic, as I draw closer to actual demo date. First, our local community permit office is now pushing enforcement of mua damper such as above (where as before, they knew, but weren't enforcing).

Broan8T or other universal 8" mua damper: for better or worse, my plan is exterior unconditioned air, with critter cage and down line filter; the exterior duct will be 10' away from exterior vent pipes, chimney, and at 1' above grade. How to ensure the newly made 8" exterior hole through our brick is not a magnet for wetness/other (foam insulation) Is a big concern for me. The 8" duct will run a straight line of about 15' through an open crawl space (but not draw from it), then turn to run in the small furnace room to the return of the 80% efficient furnace. Not wild about this solution but it's feasible and reasonably priced (except for me picking up it's installation/brick cut/return duct cutting/ mua analysis, low voltage interface with hood etc). Low voltage unit in hood will communicate with basement duct/furnace to kick on when hood does.

Regarding our house envelope: I've worked hard to update chimney rain caps/proper pointing and installed a new, properly size chimney liner, as well as new properly sized ducts for both our gas furnace/gas water heater. I also installed for the first time four window well covers which surround exterior crawl space vents. These slightly angled well covers are on raised clips which allow air flow in for mua. I'm watching this situation closely for sufficient mua, as prior to this the exterior crawl space vents were usually open for free access to unconditioned exterior mua.

I'd say the laws pertaining to mandatory make up air (for largely =/ >400 cfm vent blowers) are ahead of reality for many GCs and homeowners and should be given more time and known code meeting types of mua. I understand the need for mua to avoid backdrafting and carbon monoxide poisoning. The industry is just not there yet with feasible solutions. It's the make up air engineers and specialists who benefit the most, as they get hired to check your leaky envelope and help with the universal damper installation plan, if they even agree to come to your house.

This is an important and big investment for most homeowners. Do due diligence and bring your check book. Plan early, no your code and invest yourself. Good luck.

edited for spelling etc.

This post was edited by SparklingWater on Wed, Apr 3, 13 at 16:11

I suspect that it was the pro-style cooktops and the counterpart pro-style hoods that got so far ahead of the understanding that MUA was required for flow effectiveness and in some cases safety, that even the code writers had time to become engaged before demand for residential MUA components developed.

Commercial and industrial MUA has existed for decades.

kas

Yes, guilty as charged with the pro cooktop and hood, kas. Curiously the house came with a 1000 cfm Trade Wind Blower (probably due to long duct run with 90 degree turns). As the blower works, the electrical conduit works and is simple, the wall switch works, we're keeping that 1000 cfm and adding the higher btu range. But even if I had purchased a range needing 400 cfm venting, I would have put needed to put in an 8" universal damper per our new ( by 6 months) code.

Btw, I re-read your kind comments above, and you suggest "no filtering in the make up air duct"? Won't a lot of exterior air borne stuff be able to make their way in? I'm good on all the rest, just thought some type of filter but I've asked you this once before and you said it reduces the mua draw of air and is contra productive.

Thanks for your comments/insight, as always. GW is lucky to have our experts.

I'm all for filters in the MUA duct. But they have a big cost. The cost is that for effective filters, like a 3M pleated furnace filter, the pressure drop across the filter can be higher than the combustion appliance critical back-draft pressure that code compliance is trying to avoid by requiring the MUA in the first place.

This leads to needing a blower in the MUA duct. Then a control for the blower is needed. Complexity compounds until pulled hair goes flying in all directions.

Is there any escape that can be simple and not wasteful of power? Perhaps using a powered baffle to keep the duct closed when not in use (original topic of this thread), and a window screen of sufficient size (TBD, but likely 6 - 10 times the duct area), everything but fine dust will be kept out and the pressure drop will stay under 0.03 inches w.c. at full vent fan power. So this would not be a good idea in a very dusty environment.

So how big does the screen have to be (area presented to the passing air)? In my view this is best determined by experiment because house leakage is entangled in the pressure drop. Get a pressure sensor that is in the 0 to 0.1 or 0.2 inches w.c. range. They aren't too expensive. You will need one length of tubing to the outside. Open a screened window, turn up your kitchen vent flow to max and measure the pressure drop. Adjust the exposed screen area to find pressure vs. opening, or open more than one window until you get a value like 0.02. Of course, during this experiment the power baffle in the MUA duct has to be kept closed.

Now you have to put a duct transition at one end of the MUA duct or a pair in the middle that can transition your duct to that much screen area. If all works well, at full power the vent will not reduce the house pressure too far. A means of cleaning the screen is necessary, so it shouldn't be impossible to remove it.

A refinement would be to order stainless steel screening in a modestly larger mesh pattern if the size of the common house screening needed is larger than can be easily fitted. Of course, less lint will be captured with a more open screen.

kas

Wow - this is a fantastic thread. You guys know your stuff... For Kas primarily, is there a good formula for calculating the losses created by a filter?

Our HVAC guy recommended using the 1100CFM Braun inline blower, with the 8" Braun make-up air, possibly with a second "slave" unit. For reference, we live in a wooded area near Atlanta, GA. Lots of pollen, not lots of cold air. We have lots of shade. Then the options are these:

1. Have the MUA come directly into the kitchen with kickplate grills directly under our 48" 11,2000 BTU DCS range. In this scenario, we'd definitely need filters for pollen and whatnot (right?), but then we need to figure out if even 2 of the 8" MUS units are enough without an inline fan...

2. Have the MUA unit return into our geothermal heat pump system general return to then be filtered and conditioned. The Braun spec sheet seems to recommend this. My only concern here is that if we do that, don't we get the MUA evenly returned throughout the house, still leading to some significant drafts?

We do have two fresh air intakes as part of the geothermal system already planned since we'll have a foam-sealed envelope, but the HVAC folks said that something specific for the range MUA would still be needed.

Any insight would be greatly appreciated.

Adam

Your heating system probably doesn't have the BTU/hr to handle large flow rates of cold air. If this doesn't matter, then the next question is what pressure drop results from approach 2. This may have to be measured to determine adequacy.

Approach 1 in winter will chill the cook's feet unless the MUA is directed toward the rear of the stove and up the back. For many cooks, chilling their feet would prove to be a regrettable idea you will be reminded of often. In any case, decent filters with any filter area you care to imagine will have too much pressure drop for you to get away without an MUA fan, assuming that you actually have combustion appliances with intakes accessible to the interior pressure of the house. If not, then MUA pressure losses only affect the actual flow rate achieved by the hood ventilation. Filtering then affects only the hood performance and not safety.

kas

We decided to go with the Broan tied into our return in the basement and ducted outside. Our HVAC person said this would be the best approach and simplest/least expensive way to address the issue. Our house is very leaky. We probably wouldn't have had any issues without the MUA system.