I bought a home where the HVAC installation is suspect at best and I’m trying to correct it for health and safety reasons. I have hired a local HVAC person on a consulting basis and have his recommendations but would like to solicit some additional opinions on the best approach to solve my problem(s). I’ve got young children and want to make sure I’m receiving good advice. My post is long in order to provide the details that many of you seem to ask for and to give a snapshot of the issues my eventual HVAC contractor will have to resolve. I’d really appreciate any guidance you can give for any the areas discussed in this long post. I’ve read perhaps thousands of posts on the HVAC-Talk forum trying to learn more so that I can choose the best HVAC contractor to solve the problems I’ve inherited.
The home was built in 1984, is located in Minnesota (Minneapolis/St Paul area) and is approximately 6,000 finished square feet. The furnace room is on the lower level is about 68 square feet (652 cubic feet) and contains an updraft 120K BTU power vent furnace, a downdraft 100K BTU power vent furnace and a 100 gallon 80K BTU water heater that is a standard atmospheric vent. The kitchen has a 70K BTU cooktop and a 36K BTU grill. The laundry room has a 22K BTU dryer and a separate spa mechanical room has a 55K BTU Raypak spa heater. There are also two fireplaces – each having an approximate 864 square inch opening.
OK – now for the frightening: The only combustion air into the furnace room and entire house is a 5” flex duct that was crushed when an electrician pulled a wire over the top of it at some point in time. There is a Vent-A-Hood updraft 600 cfm hood for the grill and a Thermador downdraft 600 cfm hood for the cooktop with no make-up air. There is no source for combustion air in the spa mechanical room.
Item #1 – The furnace room. The local HVAC person initially suggested that I need the equivalent of a 12” rigid pipe to the outside for combustion air. There is no way to accomplish this. I’ve removed the ceiling in an adjacent room which reveals I have 3 joist spaces which I can use to get to the outside. While the joists are 9” H x 14” W, electrical wires limit the usable height to 6” in one space and 5” in the other two and we’d like to keep two recessed lights if possible. After discussing options, the HVAC consultant’s suggestion was to run 7” insulated flex duct to replace the crushed 5”, run 2 pair of 3” PVC and create two 100 square inch high/low openings to communicate with the adjoining area (approx. 21,000 cubic feet). The logic is that this will provide the necessary combustion air and allow me to switch both furnaces and the water heater to sealed combustion devices when I replace them. Is this a good strategy? What would you recommend? Once I implement the recommendations, I hope to not have to take down the ceiling in the adjacent finished room again. Secondly, I’m not sure the two 100 square inch openings are sized correctly. If I read the code correctly, I need one square inch per 1K BTU but not less than 100 square inches each which would mean with my 300K BTUs in the furnace room that I need each grill to be 300 square inches. While two 8” x 20” grills were installed, the contractor couldn’t explain how this was calculated as a computer apparently crunched the numbers. Given the backdraft potential of the room, I don’t want to leave anything to chance.
Item #2 – The kitchen exhaust fans. The only place I can introduce make-up air is into the kitchen ceiling via the attic. There is no way to dump any of the makeup air in the furnace room. As we are in Minnesota, the air will need to be tempered which will be done with a planned 15kW electric duct heater. The problem is how to best setup the make-up air system. The 600 cfm Vent-A-Hood (used occasionally) is comprised of two 300 cfm fans. The 600 cfm Thermador stovetop exhaust (used regularly) is on a variable speed switch from 0 – 600 cfm. Minnesota code specifies 11” rigid for a passive 145 cfm for 600 cfm of exhaust and 1200 cfm of exhaust requires a minimum of 660 cfm powered makeup air. From my research on this site, it seems that many of the techs here suggest between 70 – 80% makeup air. My initial thought is to have two independent fans to reduce the system complexity set to deliver 366 cfm each (CFM AXC200A @ .25 sp) which is about 67% of the exhaust. I’m assuming we would then be required to always run the exhaust fans at full speed so as to not create positive pressure in the kitchen from the makeup air. A complication is that my HVAC consultant stated that the hoods and make-up air need to be interlocked with the gas stovetop and grill. We don’t always use the exhausts and are hoping that this isn’t truly the case due to the noise of all that air movement. It seems from the HVAC guy I’ve hired for consulting and calling around that most residential HVAC technicians don’t have a whole lot of experience with these big residential kitchen exhaust systems. I’m hoping someone here can point us in the right direction for a makeup air strategy. Also, is the biggest possible air grill the best way to minimize noise and air velocity from the makeup air system?
Item #3 – The spa mechanical room. There is currently no combustion air source from the outside. The current exhaust vent has a powered fan assist helping the air out the side of the building on a 9’ equivalent length run. The spa heater needs a 5” insulated flex for combustion air. Is there any merit in oversizing to a 7” or 8” duct for the purpose of additional combustion air for the furnace room (draft to furnace room via crawl space between main and lower level ceilings) or to introduce additional fresh air into the house?
Item #4 – Indoor Air Quality. There is a Lifebreath 195DCS HRV installed in the spa mechanical room with a single intake duct from the lower level and the fresh supply air ported into the cold air return on the furnace (no other method would work) for the upper level. The HRV is not interlocked with the furnace at this time. Each furnace has its own AirCycler currently set to run the fan at least 10 minutes out of every 30 minutes which is providing very even temperatures and air distribution throughout the home. My HVAC consultant suggested running the HRV continuously at low speed to provide about 50 cfm. A tech on this site suggested following the ASHRAE 62.2-2003 Standard. of (7.5)(number of bedrooms+1) + (0.01)(conditioned floor area). Our house has approximely 6,000 ft2 floor area with 6 bedrooms (though we generally use 4 at a time). The ventilation requirement would be (7.5)(6) + (0.01)(6,000) = 45 + 60 = 105 cfm. What ventilation rate should I use? Should the HRV be interlocked with the furnace or run continuously? The HRV has 5 speed settings, however, the owner’s manual does not state the speed for each setting. If I know the desired setting, can the HVAC technician measure the system performance and select the correct speed?
Item #5 – 150 gallon hot tub in Master Bedroom. We haven’t used this yet due to our concerns about no combustion air for the spa heater and potential chemical / humidity issues. My HVAC consultant didn’t get to this particular topic. We’ve never owned a spa or hot tub previously. Will I get a lot of excess moisture / window condensation if I use the hot tub in the winter? Will the chemicals used in the hot tub corrode parts in the HVAC system? Is an air exchanger, ventilation fan or dehumidifier needed exclusively for this room? To complicate things, the only way to add equipment to this room is via the garage or attic. Should I simply not use the hot tub or what are your recommendations?
Thank you for checking out this long post. I really do appreciate your feedback and suggestions. Is there anything else I'm missing or forgetting? It’s been great reading all the information on this site. If it isn’t against site rules, I’d also welcome recommendations of either HVAC consultants and/or installers in the Twin Cities area that you know of who might be able to do a good job for me. Thanks again!