It sounds like you are getting backdrafting which can be very dangerous. Usually 600 CFMs shouldn't pose a problem unless your house is very airtight. In certain states they require makeup air with anything more than 300 CFMs. One solution is to always open a window when you use the vent but the better solution is to have a makeup air system installed which can be costly.

Why is back-drafting dangerous?

To be clear, we do not have a fire going in the fireplace (it is summer). Its surprising that the house is "airtight" as its 60 years old, but it must be... or the chimney is very drafty.

Any suggestions for how to DIY an air make up system? it cant be that complicated

It's dangerous because it can bring carbon monoxide and smoke into the house, if you have fire going. It can even reverse the draft in the chimney in a worst case scenario, causing the fire to draft into the house, instead of up the chimney.

If you have a 60 year old house that isn't tight and a kitchen vent causes the problem, you may have serious issues. That fireplace might need to be looked at by a pro.

In the meantime, take a look and see if you can use the product line linked below.

Here is a link that might be useful: Broan Automatic Make Up Air

You can also backdraft your furnace as well as your chimney - that's when it gets really bad.

MUA systems can become quite elaborate, but it may be best to start with simple DUI approaches to see if they work well enough. You want the house pressure to not fall much or the fireplace will at best not draw properly. Fireplaces only draw as it is because there is normally a slight differential pressure between the hearth and the chimney exit.

Say that you have a true 600 cfm going out, and some house leakage that we will assume is unknown. Then you need up to 600 cfm of make up air; more if there are also bathroom fans in use, or a dryer, water heater, furnace, etc. You could adapt the air conditioning intake ducting to pull in outside air when the pressure became negative.

The issue is how much air will be pulled in through that path versus the house pressure. If it isn't enough (the pressure is still too low), then some additional flow is needed. This could be done by blowing outside air into the air conditioning ducts. This may work in very temperate areas where heating is not needed.

A less lossy path might be considered where outside air is pulled in through a damper. Again, cold air may be a problem.

A blower may be needed. Unlike commercial hood and MUA setups where the ventilation is usually run at a single speed and hence the MUA can be run at a single speed, the residential hood blower is usually variable. Balancing the system for various conditions, including open windows or not, bathroom exhaust fans on or off, etc., can be a challenge -- one I am still working on for my own system.

Another stage of complexity is to heat the incoming air. This means another furnace circuit, or an electric heater and some large conductors.

And all of this requires more ducting, a suitable intake assembly on the roof or exterior wall, controls, etc. Sometimes the physical configuration of the house makes such a system impractical (giving the benefit of the doubt that any MUA system is practical).

You may wish to plan on open windows in a remote room that has a very good heating system to temper the air. A kitchen window can work, but tends to disturb the rising effluent of the cooking process, causing some to miss the hood.

kas

If you have gas utilities, then you could possibly backdraft the furnace and water heater. If you don't have a CO detector, get one (or two) today.