Make up air unit with a 600CFM range hood

[djmr2]

Anyone know if I need a make up air unit with a 600CFM range hood? House is being built with two furnaces and necessary fresh air intake for that but not sure how to figure out if I need more air.....

[frozenrice]

With 600cfm you’ll at least need to interlock it with one furnace. It’s also recommended but not necessary/code to add a preheater to the intake air of the furnace that is interlocked. If and whenever you run the hood fan when it’s freezing (-30) cold outside you’re going to be drawing in cold air for the furnace’s burners to combust with. The cold air and heat from the burners will cause excess condensation which over time will cause the burners to fail. Under normal circumstances this isn’t an issue but with a hood fan it makes everything work more often causing things to fail quicker.

[djmr2]

Thanks!

Can an interlock be wired after the fact or should i get them to do it while the house is being built?

[frozenrice]

That’s a good question. I’m not sure what kind of actual wiring is required but I imagine it can be done afterwards since the key to the interlock i believe is a current sensing relay which can be wired near the electrical panel. Don’t quote me on that part As I’m not an electrician. It’s probably best to do it during the build anyways. Since you’re talking furnace controls etc it’s better to let someone else mess with it. If something goes wrong they’re on the hook for it not to mention the furnace is not something you’d want to mess up on.

[djmr2]

Makes sense. I already told them its a 600 CFM fan and it's on the appliances order spec sheet. They said they weren't concerned as it's apparently pretty common nowadays to have higher CFM fans. The home does come with a HRV/ERV system as well so it's possible there is enough intake from that.

[88CRX]

We had to sign a waiver for our 600cfm hood fan stating that we would run the furnace fan and/or open a window if we run on max speed (ie: 600cfm) for any extended period of time.

600cfm can and will put your house into slight negative pressure.

[frozenrice]

The make up air system has more to do with the furnace combustion part of the intake for the heat exchanger. HRV/ERV etc has more to do with the actual “breathable” air circulation. The two cross over at the heat exchanger in the furnace. Similar concept as air passing through your car’s heater core. The two should never mix hence why you don’t want the furnace’s burners/heat exchanger to crack/rust/fail.

[realazy]

We had to sign a waiver for our 600cfm hood fan stating that we would run the furnace fan and/or open a window if we run on max speed (ie: 600cfm) for any extended period of time.

600cfm can and will put your house into slight negative pressure.

We did as well in our current build. The make up air system was quoted at $3,500 so we declined it and signed a waiver.

[goldfish168]

Morrison charged us $207 to interlock the furnace with the hood fan.

It's a requirement of the building code to do this when the hood fan is between 300-600 CFM. Greater than 600 CFM, requires make up air. You shouldn't have to worry about it though, the builder needs to figure out what needs to be done to pass HVAC inspections based the selections you make for your range hood. If not, they'll be given this deficiency to correct when the city does the inspection.

[macman64]

Cedarglen interlocked free of charge. The run the cable to the fan as a standard thing.

[The_Rural_Juror]

My hood fan is over 600 CFM. How do I tell that a make up air unit has been installed?

[frozenrice]

^^Sort if depends on when the house was built. At one time it was code so you’d have a preheater unit on the intake air for the furnace. That’s one way to tell. Since the switch over to direct vent furnaces they’ve been more relaxed on the make up air system requirements. Now the main way (or another way for that matter) to tell is if your furnace comes on when you turn on the hood fan (the interlock). Don’t ask me what physical features to look for as I have no idea what an interlock relay looks like.

[BokCh0y]

Great info here.

Hood fan died, going to upgrade to one of those asian ones so i can stink up my neighborhood. Any ideas on who or where I can go to to have the range hood interlocked with the furnace? I assume I won't need a make up air unit as I don't intend on going higher than 680 (specs on the one I am looking at states max cfm to be 680) but we'll never be running both fans at the same time or at least not at max at the same time. I'm fine with a 600 cfm unit too.

I know there's no interlocking right now between the furnace and the range hood as the furnace doesn't start up when the range hood is on.

TIA.