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Mechanical Systems

PH-style Mechanical Systems

  • 16-Apr-10 16:19
    Reply
    Message # 326671
    Stuart Fix
    Passive House buildings are designed around the principle that, with a high performance envelope, the building's heating needs can be supplied solely through a direct ducted ventilation system. So for the average sized PH, your heating load is limited to what you can deliver in about 100-200CFM, while not heating the air above 50deg C (above which dust burns).

    This is how PH's can be cost-effective; eventually the cost of extra insulation is countered by the savings in a vastly simplified HVAC system. There's a tipping point here, if you don't go far enough, you'll still need a boiler/furnace and your super-insulated house will be very expensive. From my knowledge, this is why the design heating usage must be under 15kwh/m2yr; if you use more heat than this, you can't deliver it solely over the ventilation system. At the Whistler conference, I learned that the peak heat load rule of thumb for a PH is less than 10W/m2.

    I've recently started doing residential HVAC design, and have done 3 jobs now that were shooting for PH performance. I've used the HRAI heatloss/heatgain methodology, and all three homes have reached about 10-12kW peak heating load. To reach PH performance levels, this peak load needs to be closer to 1-4kW. This shocked me! These homes all have about R30 slabs, R40 walls, and R60 ceilings; how can they still require 2-3 times more peak heat delivery than a Passive House!?

    I looked deeper in the PHPP, and realized that the PH peak heat load design method factors in both solar gains and ventilation heat recovery. HRAI ignores both of these. Further, a PH needs to get about 40%(+) of its heating energy from the sun (look at Guido Wimmers' Passive Design Toolkit on the city of Vancouver website). Clearly one cannot use the HRAI mechanical design methods, which ignore solar gains, to design a PH mechanical system.

    So HRAI is out, PHPP in. But, the PHPP only gives you the whole house peak heat load; it doesn't break the home down into room-by-room heat loss/heat gain. So how does one actually design a Passive House mechanical system? PHPP will size the in-line heating element that you'll need, but it doesn't tell you how to break down the heat delivery requirements of each room. So I'm a bit stumped at this point.

    I'm looking forward to taking the upcoming PHIUS Passive House Consultant training at Ryerson, to learn how the pro's do it!
  • 17-Apr-10 10:31
    Reply
    Reply # 327001 on 326671
    David Elfstrom (administrator)
    Stuart Fix wrote:
    So HRAI is out, PHPP in. But, the PHPP only gives you the whole house peak heat load; it doesn't break the home down into room-by-room heat loss/heat gain. So how does one actually design a Passive House mechanical system? PHPP will size the in-line heating element that you'll need, but it doesn't tell you how to break down the heat delivery requirements of each room. So I'm a bit stumped at this point.

    I think you could still use the HRAI method to break down the room-by-room heat loss and then use the same proportions for the PHPP calculated loads. That would be extra work.

    Perhaps a simpler technique would be to proportion based on exposed surface area per room, accounting differently for any rooms without glazing or with mostly glazing. When all the outside surfaces have such a high R value it means those surfaces will also have a higher surface temperature. Air circulation would even out the temperatures across the home. Except.... we're aiming for 100% outdoor air ventilation. Which means you're limited by one-inch increments of duct size. So most of it will all come down to the damper setting on the end of the run!
  • 17-Apr-10 20:41
    Reply
    Reply # 327175 on 327001
    Stuart Fix
    David Elfstrom wrote:
    I think you could still use the HRAI method to break down the room-by-room heat loss and then use the same proportions for the PHPP calculated loads. That would be extra work.

    Perhaps a simpler technique would be to proportion based on exposed surface area per room, accounting differently for any rooms without glazing or with mostly glazing. When all the outside surfaces have such a high R value it means those surfaces will also have a higher surface temperature. Air circulation would even out the temperatures across the home. Except.... we're aiming for 100% outdoor air ventilation. Which means you're limited by one-inch increments of duct size. So most of it will all come down to the damper setting on the end of the run!
    Yeah, I think you could use the HRAI conductive/infiltration numbers (though we've discussed the fact that their tables don't have deep enough R-value data). I agree that breaking down the ventilation losses on an average basis is pretty valid, the real tricky one is solar. I can see a few ways to factor in solar, but they're all pretty damn complex, and you might as well use a modeling program instead. I guess I don't feel like reinventing the wheel here, we'll see what the PHIUS folks have to say on the matter.
  • 29-May-10 23:57
    Reply
    Reply # 351364 on 326671
    Lyndon Than

    Hello, and best wishes to all,

    My wife and I are designing and building a Passive House in Toronto, hopefully to start construction within a couple of months.  Preliminary plans have had their first review with the city, and we are hopefully completing permit drawings within 3 weeks or so.  We have a blog, very simple, though, for now - takes too much time...

    www.passivehousetoronto.blogspot.com

    So many questions: 

    How far away from the house should the earth tube be to avoid drawing heat away from the foundation?  I guess this depends on the soils...ours seems to be a clay-sand mix.

    Is there a PHLuft translation out there?  I've struggled through the German using online translators, but it isn't pretty.

    How do Passive Houses in Europe deal with the Kitchen range hood issue?  I've given up on the clothes dryer, and have been designing the house with space instead for hang drying.

    No mention in PHPP of the plumbing vent stack - doing all we can to have no more than one vent stack on the house, despite numerous fixtures.  The plumber does not think the city would accept a whole-house AAV.

    Thanks so much for your inputs, anyone out there.

     

     
  • 02-Jul-10 15:24
    Reply
    Reply # 375461 on 351364
    Jon Schmid
    Lyndon Than wrote:

    Hello, and best wishes to all,

    My wife and I are designing and building a Passive House in Toronto, hopefully to start construction within a couple of months.  Preliminary plans have had their first review with the city, and we are hopefully completing permit drawings within 3 weeks or so.  We have a blog, very simple, though, for now - takes too much time...

    www.passivehousetoronto.blogspot.com

    So many questions: 

    How far away from the house should the earth tube be to avoid drawing heat away from the foundation?  I guess this depends on the soils...ours seems to be a clay-sand mix.

    Is there a PHLuft translation out there?  I've struggled through the German using online translators, but it isn't pretty.

    How do Passive Houses in Europe deal with the Kitchen range hood issue?  I've given up on the clothes dryer, and have been designing the house with space instead for hang drying.

    No mention in PHPP of the plumbing vent stack - doing all we can to have no more than one vent stack on the house, despite numerous fixtures.  The plumber does not think the city would accept a whole-house AAV.

    Thanks so much for your inputs, anyone out there.

     

     

    Hello Lyndon - do you have an HVAC consultant involved in your plans?  I see the date on this post is from several months ago, so I assume you've come up with some ideas.  

    I am developing 2 residential PH's scheduled for 2011, both intend to eliminate the kitchen range hood and dryer vent by incorporating them into the HVAC system.  This will eliminate unwanted moisture and odors and recapture latent heat generated by the range and dryer (although this will add more upfront $$ to the projects, it will decrease heat loss and control humidity - a definite balancing act.)  My HVAC consultant is excited about the project, and we have begun working with local inspectors already to satisfy their requirements.
  • 25-Aug-10 10:49
    Reply
    Reply # 408288 on 326671
    Stuart Fix
    Hi Lyndon, Jim,

    From what I know, many Passive House projects have been getting around the dryer vent issue by using a condensing dryer. These are available from various manufacturers.

    For kitchen range hoods, they are using recirculation fans with various filtration options, to deal with smoke & cooking fumes.

    A proper PH ventilation system will extract air from the bathroom, removing the need for bathroom exhaust fans, and likewise in the Kitchen. BUT, the ventilation air intake in the kitchen must be away from the stove; otherwise you'll plug up your HRV with kitchen grease.

    - Stu
 
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