Replies

1. GBA Editor
   Martin Holladay | Dec 15, 2011 01:19pm | #1

   Sean,
   The main reason that HRV systems that use furnace ductwork for distribution cost more to operate than HRV systems with dedicated ventilation ductwork is that the former systems usually depend on the use of the furnace fan to distribute the ventilation air. Furnace fans are notorious energy hogs, especially compared to the fans included with the more efficient HRVs.

   Furnace fan energy use varies widely, however, and can be as low as 250 watts or as much as 800 watts. So it's difficult to generalize.

   A 2002 NAHB Research Center study estimated that an HRV using a furnace fan to distribute ventilation air cost about 65 cents a day to operate (2002 costs). A separate study conducted by LBNL researchers (1998) estimated that an HRV with dedicated ductwork cost between 46 cents and 53 cents a day to operate (1998 costs). However, the LBNL researchers included the annualized capital cost of purchasing and installing the HRV when calculating daily operating costs.

   Obviously, the costs shown in these two research studies are not directly comparable, so I can't really answer your question. It is nevertheless clear that an HRV with dedicated ventilation ductwork will cost less to operate than one that depends on the furnace fan for distribution.

2. 
   user-871583 | Dec 15, 2011 01:32pm | #2

   Thanks Martin - I understand it is difficult. I am comparing to an ECM motor so assume that we will only be looking at furnace fans at the lower end of the energy usage in the range. The problem is that if the savings is small and the payback period is huge, then is it really justified using a dedicated system considering all the additional materials needed and their embodied energy.

   Considering that the ventilation season only is probably 6 months (rest of the time, the furnace is probably operating enough time anyway to provide the ventilation requirements in the Pacific North West)

   Do you have the link to the two studies? I want to try and chart the actual savings.

3. GBA Editor
   Martin Holladay | Dec 15, 2011 01:45pm | #3

   Sean,
   I'm not sure what you mean when you write that "the ventilation season only is probably 6 months." I think you mean that the ventilation season is 12 months, but for 6 months the furnace fan is operating anyway, so that there is no additional energy required to distribute ventilation air.

   Here is the link to the December 1998 study from LBNL: Recommended Ventilation Strategies for Energy-Efficient Production Homes.

   Here is the link to the 2002 NAHBRC study: Field Investigation of Mechanical Ventilation Strategies in Residential Construction. Unfortunately, the study costs $100.

4. 
   user-871583 | Dec 15, 2011 02:01pm | #4

   Thanks Martin - Yes that is what I meant. For 6 months - you would be running the furnace enough anyway. I will try to extrapolate based on the free study. Anyone else have any info on the differences in cost?

5. 
   Foamer | Dec 15, 2011 05:21pm | #5

   A year or so ago I asked one of the engineers at Ultimate Air, makers of the RecoupAerator ERV, about dedicated duct work versus furnace ducts. He stated that furnace ducts will work fine and that it is unnecessary to run the furnace fan to push the air through the larger ducts. He was quite adamant that the fan in their ERV is enough and that the air will find its way through the system.

6. 
   user-871583 | Dec 15, 2011 05:54pm | #6

   Not very comfortable with that. There is typically NO balancing of furnace air and the HRV is moving a much smaller volume of air. You would typically end up with parts of the house getting ventilated and parts not in my view..

7. 
   user-871583 | Dec 16, 2011 05:40pm | #7

   OK – I am trying to calculate the cost difference between the following two systems and could use some help.

   Both systems would have the same HRV and both systems would use a Gas Fired Forced Air Furnace for heating

   System 1 – Combined
   HRV ventilation supply air connected to a central upgraded furnace with an ECM motor (utilizes furnace ducts for distribution)

   System 2 – Independent Ventilation Ducting
   Furnace has standard motor
   HRV ventilation air circulated through dedicated ducting

   My Assumptions To date (please let me know where off track)

   Accurate Local electricity costs $0.085 per kWh
   Total Hours per year 8760
   Assume 6 month ventilation only season (4380 hours)
   A Furnace with an ECM motor is roughly $1000 more than regular. $1000 would cover most dedicated duct installations (material & labour) so these two would wash each other out.
   2800 ft2 house = 22400 ft3
   ASHRAE Ventilation Requirements = .35 ACH (too high I know, but rules)
   Believe this works out to a continuously ventilation rate requirement of 130 CFM (7800 CFH) for home
   Furnace Fan would run full time for the 6 month ventilation period in scenario 1 and not at all in scenario 2 during ventilation only season
   Best Furnace ECM motor consumes 250W or .25kW at low speed
   Average HRV consumes 120 W or .12kW for 130 cmf air flow

   Because cost of dedicated ducting would basically wash with the cost of the upgraded furnace fan, I am ONLY going to look at the cost differences for the ventilation only season. This I believe would be the worst case savings scenario and will save a lot of hassle figuring out during the heating season what percentage of the time the furnace would be on high for heating and ventilation, and then what % it would be on low for ventilation only.

   Questions:

   1) Is it generally accepted that the lower range for a good ECM furnace motor will be 250 watts consumption at low speed?
   2) When the HRV ventilation supply is connected to the furnace plenum (system 1), can I assume that the furnace fan can ‘suck’ the air through the HRV core and therefore only the exhaust fan on the HRV would need to run to balance the flow??
   3) If the above is true, then you would have the additional power penalty of ½ the HRV on system 1. If not true, then you would be running both fans on the HRV anyway and would have an additional 100% HRV power penalty.

   Based on these above assumptions, I have charted the savings in a Google Document https://docs.google.com/spreadsheet/ccc?key=0AgBCE6BxI8QedDlPdi1hZnFsREdiYlRvbUlrNS1vSHc#gid=0. Please review and provide comment/critique at will.

8. GBA Editor
   Martin Holladay | Dec 17, 2011 05:15am | #8

   Sean,
   There are a lot of assumptions in your calculations, and it's going to be hard to tease them all apart.

   I'll start with your assumption that your 2,800 square foot house needs 130 cfm of ventilation.

   Using the ASHRAE 62.2 formula, that would only be true if you have 13 occupants. I imagine that your family is smaller than that.

   ASHRAE 62.2 calls for 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. If there are 4 people in your family, that means that your 2,800 square foot house needs 58 cfm of ventilation.

   More information on calculating ventilation rates can be found here: Designing a Good Ventilation System.

9. 
   user-871583 | Dec 17, 2011 12:56pm | #9

   No problem - just increases savings. I do not use ASHRAE but thought it was a 35% OR Occupant Load whichever was LARGER. Anyway, I have substituted the British Columbia requirement which is calculated by bedrooms and comes out to 75 cfm for 4 bedrooms into the spreadsheet. Closest HRV puts out 87 cfm at only 52 watts. Savings now $83.40 from $70.74

   Anything else that needs tweaking?

10. 
    user-626934 | Dec 17, 2011 10:27pm | #10

    "There is typically NO balancing of furnace air and the HRV is moving a much smaller volume of air. You would typically end up with parts of the house getting ventilated and parts not in my view.."

    Why don't you balance the furnace air ducts, then?

11. 
    user-871583 | Dec 17, 2011 10:42pm | #11

    Hi John - point is that no one typically does balance a furnace duct system beyond some basic dampers. To make it an effective ventilation system would require spending money on control dampers. If it is already cheaper to run independent ducting, where is the incentive to spend even more making furnace ducting do what it was not designed to do. I am just trying to confirm everyone agrees it is cheaper to run independent. This is just an exercise to solve an argument I am having with an instructor. I personally would never put in a furnace anyway as I believe hot water systems are much more efficient carriers of heat.

12. 
    user-871583 | Dec 19, 2011 11:53am | #12

    Martin or Others, are there other gaping holes in the spreadsheet logic?

    The required Ventilation I list is from the BC Code. The HRV watts used is from an EneReady HRV unit I have the spec sheet for. Your references have indicated an ECM on a furnace is in the $1000 range. My experiences have shown this to be the cost of a typical HRV ducting system.

    What else needs to be vetted?

13. GBA Editor
    Martin Holladay | Dec 19, 2011 12:39pm | #13

    Sean,
    Unfortunately, I don't have time to double-check all of your numbers; I have already suggested that your original ventilation rate was too high, and you noted that change.

    There are advantages and disadvantages to either ventilation approach. Clearly, you have a fairly good handle on the anticipated costs -- so choose the ventilation method you prefer. I think you are mistaken in your belief that the choice can be boiled down to a precise mathematical calculation. There are many possible variables and surprises; installation costs are impossible to nail down, for example.

    Either approach will probably work fine. If you prefer one, that's the way to proceed.

14. 
    BrucePPH | Dec 21, 2011 05:30pm | #14

    A $1000 for a furnace upgrade to ECM is a rip-off. I OEM'd a 1/2hp ECM moter for $385 and replaced my PSC in about an hour. Not hard to do. The price is less if you don't have to buy a mounting kit. Throw in a 6" duct to outside with a mechanical damper and an aircycler and you have a workable cost effective solution.

15. 
    user-871583 | Dec 21, 2011 06:35pm | #15

    Thanks Bruce - what would you say a fair adder is $750? I was only going by the article that Martin provided the link to.