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  1. #1
    Join Date
    Feb 2003
    Posts
    27
    I bought a home where the HVAC installation is suspect at best and Im 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). Ive got young children and want to make sure Im 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. Id really appreciate any guidance you can give for any the areas discussed in this long post. Ive 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 Ive 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. Ive 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 wed like to keep two recessed lights if possible. After discussing options, the HVAC consultants 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, Im 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 couldnt explain how this was calculated as a computer apparently crunched the numbers. Given the backdraft potential of the room, I dont 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. Im 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 dont always use the exhausts and are hoping that this isnt truly the case due to the noise of all that air movement. It seems from the HVAC guy Ive hired for consulting and calling around that most residential HVAC technicians dont have a whole lot of experience with these big residential kitchen exhaust systems. Im 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 owners 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 havent used this yet due to our concerns about no combustion air for the spa heater and potential chemical / humidity issues. My HVAC consultant didnt get to this particular topic. Weve 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? Its been great reading all the information on this site. If it isnt against site rules, Id 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!

  2. #2

    Talking

    All I have to say is wow. You seem to have more technical info than anyone Iv seen. But anyways..problem solved this should take care of your delema. Let me know if this helps. This paired with your exisitng HRV should provide the positive pressure you need. Or just upgrade you exisitng HRV to a larger unit. It would be a good idea to run it constantly at low speed if you interlock it with your furnace.

    http://www.iaqsource.com/index.php?m...=0&prod_id=122

    P.S. You must be an engineer or architect am I right?


    [Edited by acmanjeff on 08-17-2005 at 06:57 AM]

  3. #3
    Join Date
    May 2000
    Location
    Urbandale IA. USA
    Posts
    4,915
    My question is: who built that house WITHOUT make up air???

    It sounds like you have a plan that will work.

    Those who dance, appear insane to those who do not hear the music.
    Those who believe, appear ignorant to those who do not know God.

  4. #4
    Join Date
    Feb 2003
    Posts
    27
    As I understand it, the HRV is meant to be a balanced system meaning that it introduces fresh air only to the extent that it exhausts stale air. Additionally, the residential HRVs arent designed to be integrated with a kitchen exhaust system as the grease would really goof it up. Therefore, an additional HRV or a higher speed on my current HRV will not solve the makeup air issue. Im trying to ascertain how much fresh air should be introduced with my existing HRV if possible.

    One of my original questions to the local HVAC consultant was why shouldnt I just dump a bunch of fresh air in the lower level when the exhaust fans are running. Well, this would increase the heating and cooling load of the whole house as the air would need to pass through a lot of space to get to the kitchen. The makeup air needs to be introduced near the kitchen. The big question is how to properly mate the makeup air system to two independently operated exhaust hoods.

    Originally posted by cehs
    My question is: who built that house WITHOUT make up air???
    I was also curious about this and called our citys building inspector. I think the exhaust hoods were both installed when the home was built in 1984. The inspector theorizes that at the time there were very few homes that had these commercial type hoods and that makeup air probably wasnt required by code. He suggested that these issues only started coming to light in the mid to late 1990s as homes were being built tighter and tighter.

  5. #5
    Join Date
    Apr 2002
    Posts
    11,808
    First off,you are asking a lot of questions to people who cannot see your home.

    It is a big house, work out the total conditioned space and have some one calculate the load for you. If total conditioned space was 6,000 square feet it sounds to me like your furnaces are oversized.

    A possible solution to combustion air problems in the mechanical room

    http://www.tjernlund.com/combustionairintake.htm

    Could also go with properly sized 90% furnaces, directly vented vertically through the roof. You would be really lucky if they had 8 inch bvents for each of the furnaces, you could use the b-vents as a chase. The crushed 5 inch combustion air duct would still be challenged to supply that water heater. Can you horizontally direct vent new 90% furnaces through a wall. For that matter there are gas water heaters available that could also be direct vented.

    Direct vent meaning sealed combustion, small combustion air and vent pipes directly connecting the appliances to the outdoors?

    It sounds to me, that you are going to have to rennovate to solve your problems, opening up ceiling spaces, not what you want to hear.

    For the kitchen, you can get a two speed make up air fan, or a pair of fans like you thought. They have duct heaters with SCR discharge air controls, this means the electric heaters only use enough power to heat up air to a set temperature. 15 kW could heat up 600 CFM of air by about 79 degrees. So for -15F outside it would be tempered up to 64F. Your posts mentions 600 and 1200 CFM of exhaust, so it is a little confusing.

    Fireplaces are probably wood burning right? Having a cake and eating it too. Gas inserts are available that are direct vent.

    The 80% make up requirement is a commercial kitchen idea, and locally the kitchen is kept negative to contain odours. The whole building is not negative. Some one would have to see your house and determine if maybe some make up air could be dumped in an open area, some into the kitchen.

    The HRV in the spa mechanical room sounds like it was put in to control humidity and condensation in the spa area in the winter. You may want to temper the air from this HRV, or not, depending on how stiff you like her nipples to be.

    You sound like a smart guy, go over all your concerns with the present consultant, and if he cannot answer them all to your satisfaction, then PAY for a second opinion, whether it is a good engineer or a good contractor. You have quite a few issues here for people online to answer without having the luxury of being there or being compensated for their time.


    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    Tjerlund is in White Bear lake, probably next door to you

    I reread your post, and can see that the HRV was just installed in the spa mechanical room. My mistake, I skimmed through and suspected it was for a large hot tub room.

    The problems you describe started in the late 70's in Canada. A house of that size in Canada would probably require a mechanical ventialtion rate of about 240 CFM.



    [Edited by Carnak on 08-18-2005 at 12:51 PM]
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
    Join Date
    Feb 2003
    Posts
    27
    Thank you for your reply. Yes you are right, I am asking a lot of questions for someone who cant see my home. As most of you on this forum already know, there are good installers and bad installers. Im trying to do my due diligence to make sure I find someone well qualified to do things right.

    I had another contractor stop by today who was recommended by Shelter Supply of Burnsville, MN a wholesaler and IAQ specialist that I had already found mentioned in another forum thread. The contractor hopes to get back to me within a couple of days with preliminary findings.

    One neat product Shelter carries is the Make-up Air 700 (http://www.sheltersupply.com/search_...+-+Make-Up+Air - their site is slow sometimes) made by Electro Industries (http://www.electromn.com/gen/makeup_air.htm). It appears to be a nice all in one solution however it doesnt look like we can get it (or likely two of them) into the attic. It seems likely that well have to use individual components to fit everything into the attic through a closet access panel.

    Ill bet youre right about oversized furnaces. Ive got to go Ill post more information later. Thanks again!

  8. #8
    Join Date
    Feb 2003
    Posts
    27
    After talking with my wife, Ill bet youre right about the furnaces being oversized. We recall them running in spurts even on the cold days this past winter. Ive used HVAC-Calc before for my dads house and found his furnace was also oversized. It was a lot of work as I broke everything down by room but the results turned out great. The furnaces we have both have a single PVC pipe running from them I may have mistakenly called them direct vent instead of power vent or whatever the proper term is.

    There will be no luck going up through the mechanical room ceiling. Ill just have access to the 3 joist spaces leaving the room horizontally. Ill do the HVAC-Calc program and post what I find and ask the contractors Ive hired as neither of them have yet suggested a load calculation. I already brought up the idea of a direct-vented water heater they said they would be quite loud with the fans running. Based on the 5 bathrooms with a family of 5, the recommendations were primarily for commercial type heaters. One recommendation was the AO Smith Cyclone http://www.hotwater.com/frame.html?t...al/comhome.htm with a muffler which would have fully sealed combustion. The other recommendation was a pair of AO Smith Power-Vent residential water heaters. I havent a clue as to what would be best when our existing water heater goes out.

    My post mentioned 600 and 1200 cfm because we have two 600 cfm exhaust hoods for a total of 1200 cfm. In all likelihood, we would only use one hood at a time if the temp were below 0 F so 15kW to 20kW of tempering should do the trick. Again, Im working on the assumption that we are not truly exhausting 1200 total cfms due to the inherent static pressure of this system as each exhaust is rated at 540 cfm @ .2 SP (1080 cfm combined output @ .2). Therefore, the makeup air requirement should be at least 660 cfm to meet Minnesota code if both units run simultaneously and not greater than the amount of air being exhausted. By the way, do you know of a 2-speed fan that might work in this application? Hopefully, Ill find out more about where the makeup air can be dumped. It looks as though it will likely that all of it will have to go into the kitchen because of the inaccessibility of getting to space on other levels or to the mechanical room itself.

    Yes, the fireplaces are huge brick works of art and are wood burning. They are spectacular looking with gorgeous custom-made glass doors that the previous owners must have spent a fortune on. We havent even tried them yet as we were afraid to use them until weve figured out the combustion and makeup air issues. In fact, we have 8 CO detectors scattered through the house with two of them being low level detectors given my fear of CO. The paradox is that none of them showed any sign of CO this winter which I would have surely thought would have happened. The walls are 6 inches thick stick frame with good insulation but air must be infiltrating somewhere which is probably why things were OK. Im thinking we may need a blower door test once the makeup air is added to test the entire house.

  9. #9
    Join Date
    Apr 2002
    Posts
    11,808
    Wait until you have all your make up air resolved before you run the loads.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  10. #10
    Join Date
    Feb 2003
    Posts
    27
    Originally posted by Carnak
    A house of that size in Canada would probably require a mechanical ventialtion rate of about 240 CFM.
    Carnak, how did you arrive at your HRV fresh air requirement or what makes the most sense? The 105 cfm I calculated was based on the ASHRAE 62.2-2003 Standard. The old ASHRAE standard 62-2001 calculates out to 336 cfm which seems quite excessive. The CSA F326 and Canadian Building Code Method listed in the Venmar installation guide works out to 170 cfm. There was no condensation on windows this past winter without the HRV running, it will be serving the whole house and its not dedicated to the spa. Our reason for having it is to simply ensure that there is a reasonable amount of fresh air entering the house throughout the year. Maybe it comes down to this is it better to err on the side of over-ventilating or under-ventilating?

  11. #11
    Join Date
    Feb 2003
    Posts
    27
    Originally posted by Carnak
    It is a big house, work out the total conditioned space and have some one calculate the load for you.
    Thank goodness for HVAC-Calc! Surprise the furnace are actually sized properly! The unadjusted heat load for the lower level is 95,619 BTUH and the furnace is sized at 100K BTUH. The unadjusted heat load for the main and upper level is 99,508 BTUH and the furnace is sized at 120K BTUH.

    I called Don to ask about the various types of makeup air. The makeup air for the kitchen exhaust is not included in this calculation since it is intermittent. Of course, we will temper and possibly air condition the makeup air for the kitchen. For the HRV, assuming the introduction of 100 cfm with an 80% efficient unit, you would add additional 20 cfm of forced intake to the home. I also asked about combustion air intakes and their effect on infiltration. Don stated that they are good for the home and slow infiltration but there is no need to enter these into the HVAC-Calc calculation.

    Since the electric utility offers an energy conservation which includes a blower door test, I will schedule this for after the kitchen makeup air system is completed. Ill then be able to get the actual infiltration rates and be able to do a worst case depressurization test.

    Im also still questioning the calculation for the combustion air requirements for the furnace room. 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 I would need each grill to be 300 square inches. The room will also be getting combustion air from a 7 flex duct so there must be some combination formula I cant find.

  12. #12
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by zorax2
    Originally posted by Carnak
    A house of that size in Canada would probably require a mechanical ventialtion rate of about 240 CFM.
    Carnak, how did you arrive at your HRV fresh air requirement or what makes the most sense? The 105 cfm I calculated was based on the ASHRAE 62.2-2003 Standard. The old ASHRAE standard 62-2001 calculates out to 336 cfm which seems quite excessive. The CSA F326 and Canadian Building Code Method listed in the Venmar installation guide works out to 170 cfm. There was no condensation on windows this past winter without the HRV running, it will be serving the whole house and its not dedicated to the spa. Our reason for having it is to simply ensure that there is a reasonable amount of fresh air entering the house throughout the year. Maybe it comes down to this is it better to err on the side of over-ventilating or under-ventilating?
    The canadian method is worked out on so many CFM per habitable room typically 10, a master bedroom gets 20, and an unfinished basement gets 20. A lot of times it works out to about 0.3 air changes per hour.

    You mentioned no condensation problems last winter and the HRV was not running. This suggests to me that you have a high infiltration rate, probably due to all the large exhaust applainces you have and two wood fire places.

    Were you running a humidifier?

    Good thing the water heater has not spilled.

    Something built in an area about 3 hours north of Duluth, at around the same time period was most likely a little better insulated and more air tight would have a total heat loss in the neighbourhood of 140,000 Btu/hr for total conditioned space of 6000 sq ft.

    Double pane windows low e, R20 walls, R40 ceilings, sealed vapour barriers

    The lowerlevel furnace is a 100,000 Btu model, meaning 100,000 Bth per hr input with perhaps an output in the low 90,000 range? If so it is sized to be just shy of what you have calculated. You would have seen it running for long stretches of time if that was the heat load, unless of course it was a very mild winter in the Twin Cities.




    [Edited by Carnak on 08-19-2005 at 04:57 PM]
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  13. #13
    Join Date
    Mar 2005
    Posts
    997
    with the hrv sounds like you should have plenty of outside air.

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