Does the boiler room have a window? If so, have you tried opening it (maybe just a little)?

No windows.

If you have tight construction you should be using gas appliances with outside air intake. An atmospheric vent gas water heater is a bad idea.

Your furnace / boiler should have an outside air intake and an atmospheric vent water heater shouldn't be a thing in your situation.

That said, install a damper that opens on negative pressure to avoid the back draft situation that you just had. If this is a NEW house, get the builder to fix it.

There is a lot of information missing, like the total BTUs, the size of room, HRV/ERV, louvered door, perimtting, etc. But if this is a new build and the mechanical room does not meet volume of air, and there is no additional outside combustion, ventilation, and dilution air then it has not been built to code. You should go back to the builder immediately. Also, if you have replaced a <400cfm range hood with one >400cfm, that introduces make-up air requirements beyond the gas-fired appliance requirements.

Jake The Wonderdog

The issue is the boiler and hot water heater were already existing and where fine because the house was old and not tight. I did a complete reno but left that room as is but now the house is tight. We havent had issues until we finally had a scenario where we were cooking and using the kitchen vent at the same time the Hot Water Tank was heating.

What negative pressure damper would you recommend?

3onthetree

Room is 11'x6'.

Boiler input = 105k BTU

Hot Water Tank input = 40k BTU

Door isnt louvered.

We have an ERV buts its not running yet.

So this is a remodel, not a new build. Were there openings in the mechanical room, like into the attic or crawlspace, that you covered up during your reno? Gas appliances require combustion air whether the house is old and leaky or not. And did you install a range hood exhausting more than 400cfm or not?

Be clear that there are 3 separate issues here: 1) It sounds like you have no combustion air for the boiler or water heater; 2) the range hood is interfering with the operation of those gas appliances; 3) you have a "tight" house.

All need to be solved:

3.) ERV is for the "tight" construction, requiring fresh air just for occupancy (IRC 303.4/M1505.4.3)

2.) Your range hood, if under 400cfm, is creating a backdraft either because placement near the mech room or whatever, requires makeup air (IRC G2407.4). If it is over 400cfm, it requires makeup air (IRC M1503.6). So either way a damper to operate when range hood exhausts.

1.) Your mech room provides ~528cuft of indoor air and gas appliances require ~7250cuft. If you cannot provide permanent openings for indoor/outdoor air, then a mechanical damper must be interlocked to both boiler and water heater to provide >51cfm of outdoor air (IRC G2407.9)

The building code requires a make up air system must be installed for range hoods that operate about 400 CFM. This will solve the negative pressure problem.

3onthetree

Mechanical room didnt have openings prior.

Yes, we installed the range hood.

Can all dampers be wired to both the hot water tank and the boiler?

If i have a damper that opens when either or both are running then i dont need any make up for the range hood right?

First, if the hood is >400cfm makeup air is required with any fuel burning appliance in the house anywhere, period. I don't see anything in IRC that exempts requirements for makeup air if even one is locked from operating while another is being used. IRC just pulls verbatim from IMC, but I doubt IMC would have anything additional like that.

I have seen the combustion air damper and fail-safe wired to multiple appliances, however the nuts and bolts of how to do it are beyond my pay grade.

I either want one that operates when either one is operating or one that operates when there's a pressure drop. You know if the pressure ones work well?