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Please help with HVAC system decision – West of Boston, MA

smgb | Posted in Mechanicals on


I’ve learned a lot from this resource – thank you for that.  I’d like to ask the experts here what you recommend as options for a heating and cooling solution for my home.  Here are the pertinent details.  Let me know if I’m missing anything.

1.  Garrison colonial built around 1965.  Second story was added / remodeled in the 1980s (by previous owner).  Original house is drafty feels cold in certain spots due to minimal insulation and older windows.
2.  Currently adding a new garage and rear 1-story addition with full basement (see pictures).  2×6 framing.
3.  Space:
  a.  Rough estimate is 2900 sq. ft. of above-ground total living space. 
  b.  Basement is about another 1300 s.f. that I’d like to finish in the near future. 
  c.  Garage is 370 s.f. of floor space, but has a cathedral ceiling that is 20 feet high.  I placed PEX in the new slab for future radiant heating.
  d.  Unconditioned attic with gable and ridge vents.
4.  Windows:
  a.  New addition in rear:  Marvin Elevate and Ultimate sliders and awning windows.  The 16′ wide x 8′ high oxxo sliding doors are: ~0.28 U-factor; ~0.33 SHGF; ~ 0.57 VT
  b.  Existing windows in old part of house are double pane Andersen windows ~20 years old.  Will replace within a year.
5.  Insulation:
  a.  New addition in rear:  R23 mineral wool walls, R49 mineral wool unvented flat roof
  b.  New garage:  ~R23 mineral wool walls and R38 pink insulation roof
  c.  Old house:  2×4 framing with I-don’t-know-what-R-value in the walls.  It feels cold!  I want to upgrade the insulation when the old windows and siding are replaced (looking for options on that as well, but will save for another day).
6.  Existing HVAC:  oil-fired tankless boiler for both heat and DHW.  Willing to retain this for supplemental heat.
  a.  New addition in rear:  baseboard hot water
  b.  Old house:  baseboard (2nd floor) and old convector radiators (1st floor)
  c.  I’ve been using window A/C units in the old house and will retire them this year as the windows get replaced.

7.  I have a 10 kW/hr solar PV system.

1)  I’d like an reliable and efficient HVAC system, and am open to options.
2)  I’m thinking about an ERV/HRV as well to supply the house with fresh air. 
2a)  The new basement has a passive radon exhaust system – I’m considering an inline electric fan to make it an active system.
3)  I’d love to have radiant heating for the new addition on the rear (for how good it feels to my feet!), but am willing to compromise on that. 
4)  I’d also like the basement and garage to have dehumidification, if not full heating/cooling.

Please let me know what other details I can provide.  Thanks in advance!

(10 miles west of Boston MA)

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  1. Expert Member
    NICK KEENAN | | #1

    You mention twice that the house is drafty. Air sealing is separate from insulation. In fact, without good air sealing insulation can be ineffectual. Since you have an unfinished basement and attic, it's entirely possible that both are leaky and could be tightened up without much effort. When air leaks in it tends to spread around the house causing drafts in places other than where the leak is.

    But: you should assume nothing. You can test and measure how leaky your house is through a blower door test, and you can test again after making improvements to see if they do anything. During the blower door test you can also locate locations that are leaking. This short video shows a blower door test being conducted and leaks being found:

    If you buy a new HVAC system and then improve the insulation and sealing, your system will be too large for the house. You won't be as comfortable, and you'll overpay.

    1. smgb | | #2

      DC Contrarian:

      Great point. We actually had the house air sealed and a blower door test done about 5 years ago. I don't have the results on hand, but can say that it made a big improvement. We're one of the few homes on the block without ice dams! I guess I should say it feels cold in certain areas of the house, rather than say "drafty". My mistake.

      I was going to talk about air sealing / insulating my house in a separate thread to keep this one focused on HVAC, but the thought is to use rigid foam on the exterior before new siding is installed (currently have cedar shingles, over horizontal boards/sheathing, on main house).

      Thank you.

  2. smgb | | #3

    I got a quote from a local design-and-build company. They've spec'd a 5-ton unit for the 1st floor (existing house and expansion; unit placed in basement), and a 3-ton unit for the second floor (placed in the unconditioned attic). They are the Bosch 2.0 variable speed units.

    This does not include heating / cooling for the basement or garage. It feels oversized based on some of the other posts I've read on GBA. What do you all think?

  3. Expert Member
    Peter Engle | | #4

    Most likely, this system is dramatically oversized. You can't know for sure until you have a room-by-room Manual J analysis performed by someone competent to do the work. This is generally not the HVAC contractor trying to sell you the equipment. The analysis must include the upgraded insulation that you intend for the existing house.

    The benefits of right-sized equipment extend far beyond better comfort and energy efficiency. The ductwork can be smaller while still allowing for lower velocities (less noise and fan power). In some cases, ductwork can be eliminated entirely, using minisplits instead. This saves space and money. Smaller equipment is generally cheaper, though you give some of that up with more sophisiticated modulating equipment.

    It all starts with a competent Manual J. You just can't make intelligent choices without that.

    1. smgb | | #5

      Thanks Peter. I am trying to find a company that will do a proper manual J and even that is challenging. I will continue to look.

  4. Tinman77 | | #6

    Looking briefly at the floor plans I definitely don't see justification of a 5 & 3-ton system. Granted they are variable speed and would just modulate down to the correct load ( hopefully if installed and commissioned properly).

    I would recommend installing a hot water coil in the air handler if you're going to put ducts in every room, as this give it a lot less water content to heat, thus saving oil. You could also go with a heat pump outdoor units ( dual fuel is the industry term and gets quite be good rebate at the moment)
    Also with variable speed equipment that can be zoned easily without waste.

    I'm a 25 year heating professional and would be willing to help you if possible.

    1. smgb | | #7

      Hi Tinman77:

      I’m willing to go with electric heat pumps and completely do away with oil, or keep it as backup. I’m also very interested in air-to-water heat pumps as well.

      Where are you located? I’m west of Boston.

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