Kitchen exhaust make-up

[olc]

Kitchen exhaust hood suppliers have been providing direct fired gas makeup air units for the makeup air in kitchens. We need to get some of the outside air to the dining room for ventilation. I do not like to use dirct fired natural gas units for ventilation because 1. of the added humidity & 2. I just never liked the idea of direct fired units. Of couse the owner does not want the added expense of the indirect fired unit. Am I the only one who does not use direct fired for ventilation?

[Carnak]

You are correct, do not use the direct fired for ventilating the dining area.

How is the dining area heated and cooled now?

[firstmech]

Typical design around here is the make up air system provides 85% make up compared to exhaust total. The remainder make up is through the hvac system ecnomizer. This can be as simple as ninimum position settings or through the use of building static pressure controllers.
Depending upon the amount of additional make up air needed through your dinning room equipment without proper tempering this air the customer will probally complain of drafts.
The Lennox L series has an option that allows for supply air tempering by adding a supply air sensor downstream of the unit and selecting the option.
This set up can be retroffited to most gas fired hvac units without too much truoble.
We see alot of restaurants using the L series way of total make up, but we I not sure this is a great option.
Bottom line is with additional make up air or not the air being exhausted has got to enter the building one way or the other.

Good Luck
GB

[ac/dc]

I agree with carnak, cant outside air be ducted to the
system(s) currently serving the dinning rm.? Or is it
served by package unit(s) on the roof?

[olc]

The ventilation to the dining room is the driving factor. It can be 33% or more of the total air to the dining room. So we can use the economizer. It is just hard to control on very cold days. I will look at the Lennox thing.

[Carnak]

Not sure where you are but 33% outside air can be hard on a packaged unit in a cold climate, even with stainless steel HX.

Maybe you want an add on HRV.

[valdelocc]

its a common design,to keep the kitchen area at lower pressure than the dining section.
Around here I've seen the kitchens being serve by a 100% outside air, direct fire unit like a captive-air and a few exhaust fans. For the dining area, one or more package units w/ economizer.
olc. are we talking about a new intall or you a retrofit?

[olc]

It is a cold climate. Actually I have two similar jobs which are both sort of new construction using part of an existing building. Wood frame construction with sloped roofs and no space. The HVAC for the dining will be split systems (it would be nice every once in a while to have a flat roof with 3 feet of ceiling space).
I think most developers and owners just expect the make-up air to come in the back door. It's tough when you have a 3000CFM exhaust hood in a 2000 SF building that needs 600CFM or more for dining room ventilation.

[olc]

valdelocc - that is what I am talking about. Replace the captive-air direct fired unit with a larger indirect unit. Heat the air to 60ish. Roughly 2/3 of the air would go to the hood makeup connections. The other 1/3 would go to the inlet of the dining room HVAC (split system) unit.
If we do not do this - The design temp. here is 0 deg. It would be common to have to operate when it is 10 deg. out. If we just us the HVAC for the remainder of the make-up and it is 33% of the total, that gives use 50 deg. entering a single stage (or maybe two stage) split system furnace. Maybe OK but iffy.
What do you think?

[valdelocc]

are you going to use mini-splits or a evp. coil seating on a gas fired furnace? an air handler w/ elct. heat? or heatpumps?

[Carnak]

The direct fired make up for the kitchen will give lowest fuel consumption.

You want the kitchen to be slightly negative so the make up air is obviously less than the exhaust air on the hood.

The fresh air to the dining area will make it slightly positive so air will go from the dining area to the kitchen.

Going the route of a Big Indirect unit to supply the kitchen and the dining area would probably work but it will drive up operating costs.

And with packaged units more than 25% outside air could void a warranty on a gas pack. Can look at stainless steel HX or something that recovers heat or a small make up air unit.

Small clamshell MUA units do not seem to have much lifespan on 100% outside air.

[valdelocc]

carnak is right,33%OA at 0-10 OT would be bad for a heat exchanger and most likely will void the warranty, you could install an electric re-heating coil in the make up air duct, but the operating cost will be higher.
I' think that maybe you should consider,heat pumps w/ electric heat back ups for the dining area and leave the kitchen the way it is.

[Carnak]

Originally posted by Carnak

Small clamshell MUA units do not seem to have much lifespan on 100% outside air.



Should read "Small STAINLESS clamshell MUA units do not seem to have much lifespan on 100% outside air.", would not even consider one that was not stainless.

[fat eddy]

Could he go with a small direct fired MUA, for the dining area, that runs a two position 80\20 or 50\50 outside mixed with return ? I don't beleive the extra vapor will be a big deal if the kitchen is properly ventilated already, and the energy costs would be less ? he would also need to have cooling in this unit.

[olc]

OK, here's an actual situation.
Hood Exhaust - 3600CFM. Dining room ventilation - 1000CFM. Dining room cooling load 7 tons. So the hood direct make-up will be 2600CFM (captive-air will go up 80%). Design temp is 0 deg. BTW.
A. 2600CFM direct fired unit for the hood make up. 175MBH indirect fired (probably direct vent) make-up air unit with cooling coil in discharge duct. Probably a split coil with two condensing units. 2800CFM.
or
B. 3600CFM 300MBH indirect fired makeup air unit. 2600CFM to the hood and 1000CFM to the inlet of the HVAC units. The HVAC units in this case may be two 3-1/2 ton split systems with furnaces.

[Carnak]

They will probably go 80% make up air to the hood supply plenum. You can still use a ceiling diffuser to add more make up air. Don't let diffusers blow on the hood.

80% of 3600 is 2880 so that leaves an imbalance of 720 CFM and that sounds a little too much to be transfering from the dining area.

1000 CFM, will be quite a load,it depends where you are but there won't be much left out of that 7 tons for space cooling.

You need to look at washroom exhaust rates and see if they are putting a hood over the dishwasher too.

[Carnak]

3240 CFM direct fired unit, about 250 MBH. If Captive Aire says it is too much for hood supply plenum, dump some air to a ceiling diffuser, do not blow air on hood.

15 ton packaged unit with 300 MBH heat STAINLESS STEEL HX, should be two stage heat and cool.

You could very well be 300 CFM per ton if that outside air is humid, so maybe 4500 CFM total. Bring in your 1000 through this unit. Heating up that 1000 CFM will use up about 75.6 MBH output so you would have 164 MBH left for space heat.

The kitchen will be short 360 CFM, a couple public wash room exhausts are maybe another 400 CFM.

1000-360-400= 240 CFM positive pressure. Maybe there are other exhausts like staff washroom or dishwasher hood.

Allow a means for air to transfer from the dining area to the kitchen, and make sure this transfer air does not make a bee-line for the hood.

[Edited by Carnak on 03-30-2005 at 03:51 PM]

[olc]

I have no problem with transfering 1000 CFM from the dining room to the kitchen.
Captive Air designed it for 80% make-up to the hood, but if I tell them 72%, they will do that.
We are not cooling any air that is going to the hood make-up connections.