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Community and Q&A

Retrofit HVAC installation in Zone 6a

Joe McRae | Posted in Mechanicals on

Our house is a bungalow (1200 sqft) with a 600 sq ft split addition near Toronto, Canada ~ 6A. It is reasonably tight (spray foam throughout).

Our HVAC Retrofit will include replacing an old oil boiler with a NG appliance and installing a ventilation and AC system. When making decisions about this retrofit, IAQ is our most important criteria, followed by operating cost. My wife has a few health issues that make IAQ very important to us.

The two systems I am considering are:
1. Keep existing rads, install a NG boiler for heat and DHW, add an HRV with ducting, and a split AC 2 head unit.

2. Remove rads, install central fan, ducting, furnace and AC with an aircycler g2 to provide balanced ventilation.

In both systems I’m concerned about dehumidification in the summer (we rarely use our AC, but would like to keep RH below 50% for IAQ reasons). I’m think of seeing how the AC works, then adding a portable dehumidifier if needed.

1. Is one option preferable when IAQ is considered?
2. Does my plan for dehumidification seem reasonable for us since we don’t plan on using our AC very often? Or should I look at a whole home system?
3. If IAQ should be similar with both systems – which one would you prefer?

Thanks so much for your help!

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to assess whether your dehumidification plan will work, because it's hard to know what you mean by "we rarely use our AC." How rarely?

    If you use your AC when humidity begins to creep up -- in other words, when you're uncomfortable -- your plan will work.

    If you usually avoid turning on the AC, even when the indoor temperature is 80 degrees F, then you may have trouble controlling your indoor relative humidity (RH) with your plan.

    It's certainly true that a central air conditioner, in conjunction with a portable dehumidifier, will control the indoor RH in Toronto. But for this approach to work, you have to use the equipment.

    Either of your two options -- the boiler with the HRV, or the furnace with the Air Cycler -- will work, as long as the equipment is properly sized (oversizing is a risk) and the distribution system is well designed.

  2. Joe McRae | | #2

    Hi Martin,

    Thanks so much for your quick reply. My wife ends up being uncomfortable when our home is cooled significantly below 80 F -- she loves hot weather, and home conditions. We would both be happy with a thermostat set at around 78 F.

    Just to confirm - if we used AC when temperatures reached above 78 F, a portable dehumidifier should allow us keep humidity below 50%. I worry most about months like September and October, where temps rarely reach above 78 F (so we would not use AC) and average RH levels are still high -- cycling between 58% and 94% on an average day).

    Thanks again for all of your great articles - I will work with my HVAC contractors to make sure our systems are properly sized.


  3. GBA Editor
    Martin Holladay | | #3

    In your case, since your wife has specific humidity requirements and a preference for high indoor temperatures, you will probably be more comfortable if you install a whole-house dehumidification system from Therma-Stor Products.

    Whole-house dehumidification is a lot simpler if your house has a forced-air system than if your house has a hydronic system.

  4. Reid Baldwin | | #4


    You might want to look into the Minotair ( It would provide ventilation, humidity control, and some degree of air conditioning. You would have to run a heat load calculation to determine if it would eliminate the need for a separate air conditioner. The efficiency numbers as an air conditioner are not impressive, but efficiency is much less critical for a family that rarely uses air conditioning.

    BTW, when you say aircycler, I assume you mean Central Fan Integrated Supply (CFIS) which is a supply-type ventilation system rather than a balanced ventilation system.

  5. D Dorsett | | #5

    There's no way your indoor RH is cycling between 58% and 94% unless you run a shower for hours on end without exhaust ventilation. At 94% RH @ 25C (a bit shy of 78F) there would be massive plumes of mist coming out of your exhaust fans in October as the exhaust air cooled to the outdoor temps.

    50% RH @ 78F (your preferred conditions?) is a dew point of 58F, which means that most of the year the interior humidity could be managed by adjusting the ventilation rates. From early September through late June the average outdoor dew points are below 58F in Toronto. See the dew point graph at the bottom of this page:

    During the July-August higher humidity period, an air tight 1200' bungalow can be dehumidified to sub- 58F dew points with a single room type portable dehumidifier if there is ANY amount of air exchange within that space. If the 600' addition is doored off from the rest it may take another. Only if you insisted on high, or very-high ventilation rates would a whole house dehumidifer be called for. Otherwise, it would be like chasing a fly-sized latent load with a sledgehammer.

    From a total year-round comfort point of view, keeping the rads and right-sizing a modulating condensing boiler would be far preferable to ducted air. Ducted systems move high air volumes, and keep particles in suspension, and un-balanced ducted systems drive outdoor air infiltration. If you used air conditioning at a high duty cycle there might be a rationale to use ducts for distribution, but smaller ventilation-only ducts and a heat recovery ventilation system (under dehumidistat control for most of the year, duty-cycling during July/August) that has a redistribution (no outdoor air exchange) mode, probably makes more sense.

    You can probably air condition the whole house with a single 3/4 ton or 1-ton mini-split head mounted at the top of the stairs (assuming it's a 1.5 or 2 story bungalow), which is certainly the case in my ~2400' bungalow in a climate with a comparable 1% outside design temp, and similar latent loads. (With my shading factors I can get by with a half ton window-shaker, but without the PM shade I'd be looking at about 1-ton.)

  6. GBA Editor
    Martin Holladay | | #6

    For more information on the Minotair, see Another North American Magic Box.

    For information on the CERV, an appliance similar to the Minotair -- one manufactured in the U.S. -- see A Balanced Ventilation System With a Built-In Heat Pump.

  7. Expert Member
    Dana Dorsett | | #7

    Since you probably have a heating history on the place, it's worth running a heating load calculation based on fuel use measured against heating degree-days to size the heating end of it correctly. The basic methodology on how that is done can be found here:

    Since most natural gas service in Canada is billed in cubic meters, the important conversion factor to BTUs source-fuel needed is is ~35,500-36,500 BTU/m^3 (depending on the exact mix of natural gas, which varies.) If you simply used 36,000 BTU/m^3 it would be close enough for this type of calculation.

    If I had to guess, most reasonably tight 2x4 framed reasonably tight 1800' houses would have a heat load of between 24,000- 32,000 BTU/hr at a temperature difference of 70F. The 99% outside design temp in Toronto is +1F, so even if you kept it 76F indoors all winter (a temperature difference of 75F), it would keep up with less than 35,000 BTU/hr of boiler output, and even at the 10 year temperature extremes you'd be more than covered with 50,000 BTU/hr of boiler output (and it could be as low as or lower than 35,000 BTU/hr to cover the 10 year low temp.) With history as a guide, odds are pretty good that the pre-existing boiler's output is more than 75,000 BTU/hr, or even 100,000 BTU/hr, with enough radiation to deliver that at 180F, in which case the radiators would be big enough to deliver the heat to the house on the coldest day of the decade at highly efficient condensing temperatures.

    When sizing a modulating condensing boiler to work with your radiators it's important to measure up the radiation per zone, and make sure that you can run it at condensing temperatures without short-cycling the boiler. The napkin math on that can be found here:

  8. Joe McRae | | #8

    Thanks Martin, Dana and Reid - I'm blown away by the great advice you have given me so quickly today!

    Reid - I could be wrong, but I think the aircycler g2 is closer to "balanced" than supply only because the controller of the air cycler also can turn on a bath exhaust fan. Also thanks for the Minto Air suggestion - I've been in touch my distributor and I think this could be a great system for us. (I have some math ahead of me to confirm).

    I'm now leaning towards a mod-con boiler (again some math ahead) and a Minto. I see a dedicated HRV at around $3500 installed, a split AC system at around $3000, and from a quick conversation with my Minto rep, I think the cost for a Minto will be about the same.

    Dana - I've used the article you have referenced in my search for a boiler contractor, so far most don't seem to be concerned with the issues you've mentioned - so I have kept moving on. Hopefully soon I find the right contractor.

    I'm truly grateful for how all of you readily share your expertise!

  9. Expert Member
    Dana Dorsett | | #9

    I just re-read the original post- the original boiler was an oil burner (sold in liters, I presume?). The source fuel energy content would be about 36,500 BTU/liter. It's definitely worth running the calculation.

    If the radiators are high volume high-mass types you can get away with a lot of oversizing, but with a mod-con boiler you'd still like to size it to modulate almost continuously. More than half of the people installing mod-con boilers don't have much of a clue as to how to tweak the maximum efficiency out of them, but it's not rocket science.

    A pretty-good choice that has fairly low minimum-modulated output would be the smallest Laars Mascot FT series, the MFTHW80 (the same boiler under the sheet metal as the HTP UFT-080W), and should have good support in Toronto, given that the Canadian HQ of Laars is located in Mississauga.

    The HTP and Westinghouse branded versions of this exact boiler are pretty cheap (under USD $2K), and I would expect the MFTHW80 to be similarly priced. The manufacturer is a first-tier Korean boiler manufacturer (Kiturami), built with primarily Korean-sourced components, but unlike their home-country competitor (Kyung Dong Navien), they don't seem interested in developing their own North American sales & support network (yet.)

    The min-fire output at condensing temps is in the 7500 BTU/hr range, which is well below your expected design heat load, and probably less than half the mid-winter daily average heat load. And the high-fire output is about 75,000 BTU/hr which means this boiler would work reasonably well in about 19 out of 20 homes in North America.

    When you have narrowed down your boiler options based on what you think make sense, calling the manufacturer or distributor for contractor recommendations can be a good way to separate the clueless hacks who can't read the manual from those who can install & support it with some confidence. The distributor is in a position to know which contractors are installing a dozen or more per quarter, and which are tying up the technical support lines with issues already spelled out in the manual, or making warranty claims on boilers damaged by incompetent installation/system-design.

  10. Joe McRae | | #10

    Hi Dana,

    I'll jump back on with my calculations soon (but it seems they have closely matched your approximates anyway). I have one question about the LAARS MFTHW80 - The literature I see on the LARS brochure states the 80 only has a turndown ratio of 5:1 - would this mean it's min-fire would be 15000 BTU/hr, not 7500? When I check the equivalent HTP model it states a 10:1 turndown ratio - so it gets to 8000 BTU/hr.

    I'm guessing, if LAARS is brochure is correct, this wouldn't be a great boiler for me.

  11. Joe McRae | | #11

    My calculations come up with the following:

    A design day in Peterborough is -4F. An average day in Peterborough is ~15F. I would like my themostat at 70F (I also measure the actual sq ft of my house today - 1500 sq of living space and a 900 sq ft unfinished basement)

    My boiler states a rating of 560 sq ft water and 1 gph max firing rate. Based on 36500 BTU/L and the conversion shown at the link below for sq ft of water, I calculated the boiler efficiency at 57%. This seems to be in the right ballpark for a 60 yo oil boiler.

    My 60DD heat load comes out to be 22000 BTU/h and with a 1.4 factor about 30000 BTU/h.

    My average seasonal heat load is 15400 BTU/h.

    I have only 40 ft of fin tube radiators in the house. Plus one large cast iron radiator (45"x45"x4") in the basement. When I account for the cast iron (~4 ft of fin) I have about 44 ft of rads in the house.

    Condition 1 - If I purchase a boiler that can fire at 8000 BTU/hr (96% AFUE), I would have ~160 BTU/h /ft so I should condense?

    Condition 2 - 120°F AWT, my one zone can emit: 8700 BTU/hr. Since this is less than the 7600 BTU/h boiler output.

    So a boiler with min output of 8000 BTU/h and max of 80000 BTU/h would be great for my heat.

    Does this look right?

    Thanks again so much! Joe

  12. Expert Member
    Dana Dorsett | | #12

    I apologize- though it's substantially the same boiler as some of the other Kiturami imports, the MFTHW80 bottoms out at 16K-in, which isn't low enough for your loads!

    TheHPT UFT-080W or Westinghouse WBRUNG-80W really do drop to 8KBTU/hr-in, which would be more appropriate.

    I have to run (now), so I'll peek at the radiation numbers later...

  13. D Dorsett | | #13

    The 40' of baseboard can emit ~200 BTU/hr per foot, 8000 BTU/hr at an average water temp of 120F (125F out, 115F return), which for most mod-con boilers will result in the low mid-90s efficiency.

    The radiator is probably ~90 sq. ft. EDR, which at 120F delivers on the order of 50 BTU/ft^2, or 450 BTU/hr. There is some amount of thermal mass there, but it's bit of an issue if it's a separate zone of it's own. If it's a separately zoned basement you could just monitor it, see if it short cycles very much. Odds are that if you dial in the outdoor reset curve for the 40' fin tube zone so that the boiler runs nearly continuously, most of the time calls for heat from the basement zone would overlap those of the other zone.

  14. Joe McRae | | #14

    Hi Dana,

    Thank you again for all of your help! I now feel armed with the information I need to chat with contractors. I've contacted my local wholesaler to ask about HTP contractors and it turns out that they are getting out of HTP business. They have some stock left that they are selling at 40% off. Including a Versa (PHE130-119). It is a huge tank, but I'm wondering if this would let me create smaller zones in my house and not worry about short cycling? Also, the price is great. Thanks again, Joe.

  15. D Dorsett | | #15

    The Versa combi can be a great solution to micro-zoned systems, since the thermal mass of the water makes in inherently immune to short cycling.

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