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Just Say No to Furnaces in High Performance Homes

Sealed Combustion Furnace

This is the first article of a two-part guest post by David Butler of Optimal Building Systems in Arizona and a frequent commenter in the RESNET BPI group on LinkedIn. I’ve known David for a couple of years now, and I can attest to his expertise in the field of HVAC. When an HVAC question stumps me, I call on David because he’ll most likely know the answer and will explain it in great detail.  If you’re a home energy pro, make sure you download and read his paper, The Elephant in the Room.


Here’s an interesting fact: As you add insulation to a home and tighten up the building envelope, the heating load drops much faster than the cooling load. This makes sense because the winter delta-T (temperature difference between inside and out) is much greater, and infiltration rates are typically higher.

This creates a dilemma when sizing a furnace. The furnace must be sized to handle the cooling CFM even if that means selecting a model with several times more heating capacity than the load. In hot climates such as Tucson and Phoenix, it’s not unusual to see furnaces oversized by 300%, 400%, or even more.

Unitary equipment manufacturers have shown little interest in adding small capacity furnaces to their model lines. Status quo furnace designs, having changed little over the last couple of decades, are strongly biased toward inefficient homes and cold climates. Often the choices come down to big, bigger, or even bigger. Although this has always been an issue in mild climates, it’s becoming a problem in cold regions as homes are built tighter and more efficient.

The new reality: a gas furnace is usually a poor choice for high performance homes. Simply put, gross oversizing causes comfort issues. Homeowners who invest thousands of dollars in a super efficient shell expect their homes to be more comfortable, not less comfortable.

Consider what happens when, on the coldest day, the furnace only runs 15 minutes per hour. Each time the heat cycles off, the house begins to cool from the outside in. The thermostat is necessarily located away from exterior walls and windows, typically in an interior hallway. By the time the thermostat “sees” enough of a temperature change to activate the furnace, perimeter zones may have dropped by 4 or 5 degrees.

There’s more. Virtually all modern furnaces have electronic controls that enforce a minimum run time. When a furnace is grossly oversized, the furnace will experience significant overshoot during spring and fall, when peak heating loads may only be a few thousand Btu per hour.

Many dealers push expensive multi-stage variable-speed furnaces for high performance homes to resolve these issues. Multi-stage burners mitigate the capacity issue somewhat, but there are less expensive alternatives that do a better job of capacity matching.

In part 2, I’ll make the case for why heat pumps, hydronic forced air systems, and combination systems are ideal for heating high performance homes.


Read Part 2:

Heat Pumps and Hydronics – A Great Team for High Performance Homes



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This Post Has 12 Comments

  1. Is this not a call to action
    Is this not a call to action for heating product manufacturers to invest in R&D; for new gas heating products?

  2. Mike, not so much R&D; as
    Mike, not so much R&D; as simply offering more options within existing model lines. What’s needed are models with 20k to 30k burners matched with 2 and 3 ton blowers for cooling. 
    While this may seem like an ideal application for a variable speed blower, the X-13 motor, already used in many air handlers, is much less expensive and provides equally wide range of operating speeds at identical efficiency. Most furnace control boards already support separate speed taps for heating and cooling modes.  
    But I’m not holding my breath. The good new: heat pumps and gas-fired hydronic heat give up nothing to a furnace. Stay tuned to Part 2.

  3. I agree with the premise and
    I agree with the premise and conclusion completely–especially in new construction where it can really be dialed in. 
    I’m not sure I buy this part of the argument, though, as applied to ultra efficient homes:  
    Consider what happens when, on the coldest day, the furnace only runs 15 minutes per hour. Each time the heat cycles off, the house begins to cool from the outside in. The thermostat is necessarily located away from exterior walls and windows, typically in an interior hallway. By the time the thermostat “sees” enough of a temperature change to activate the furnace, perimeter zones may have dropped by 4 or 5 degrees. 
    In ultra high-performance homes–and even less than ultra home that I’ve lived in–high efficiency, with even a modicum of ventilation and air distribution don’t see that sort of temperature differences throughout the space. Possible, yes, but pretty easily resolved. Now, in inefficient homes, that’s a common occurrence.  
    Again, I don’t think that changes the conclusion at all. And warmer the climate, the more this holds true. 

  4. Hey Mike, thanks for your
    Hey Mike, thanks for your comments. You’re correct that a ventilation system that runs independently of the heating system could mitigate zonal temperature droop during peak loads. However, ventilation systems are not usually designed for optimum circulation throughout the home (nor need they be). 
    I have first hand knowledge of two Energy Star homes in Charlotte where this was a problem. At first I thought it was a duct balancing issue. With help from one of the builders, we figured out what was really happening. That was a eureka moment for me. More recently I experienced this effect in my current home. I’ve since switched to a heat pump. BTW, none of these homes could be described as inefficient.

  5. Troy, the type of combo
    Troy, the type of combo system that uses the water heater for both space and water heating can work well in homes with low heating loads. It makes no sense to do that with an electric water heater, however, because using electricity to make heat through electric resistance, even though it’s 100% efficient, isn’t as efficient as the alternatives (gas or heat pump water heater). 
    I don’t have any direct experience with them, but I’ve heard they can be difficult because you’re asking two different trades (plumbing & HVAC) to work together.

  6. The blog mentions a part 2
    The blog mentions a part 2 where a distinction will be made, apparently, between gas fired furnaces and other types. Having installed many hydronic forced-air furnaces (Airia’s Lifebreath Clean-air furnace) that were powered by a combination of a 4×10 solar thermal panel and storage tank backed up by high-efficiency on-demand water heaters. This system was also provided domestic potable hot water. This is absolutely the best solution for areas with relatively minimal heating demands but domestic hot water demands equal to other climate zones. I’m in a zone 5 with 6000 HDD. The LifeBreath had a high efficiency ECM motor and a built in HRV for highly efficient in-door air exchanges. It works very well and should be the standard for zones 2, 3, and 4 that get good solar exposure. Hydronic in-slab radiant has been the standard in our market, but it inherently lacks the ability to exchange and distribute fresh air, something a good hydronic furnace with and HRV can do very well.

  7. Troy, gas-fired boilers and
    Troy, gas-fired boilers and tankless water heaters have adequate capacity to handle both dhw and space heat, although a buffer tank may be required if the load is more than about 25% of heater’s max output. 
    I recently designed a system for an ICF home in St Cloud (MN) that will use electricity to heat the water. The utility offers a 5-cent off-peak rate for storage heat. That’s the equivalent of $1.25/gallon for propane. The design load for that home was only 32k btu at -17F (2800 sf)! I touch on combo systems in Part 2.

  8. Kim, the Lifebreath air
    Kim, the Lifebreath air handlers are great! However, I’ve never found solar thermal space heat to be cost effective, except perhaps for do-it-yourselfers. Perhaps the subject of another article?

  9. David, 

    As a builder of high-performance homes I tried a variety of these systems. I look forward to your part II discussion. For larger homes I liked the Lifebreath air handler combined with a high efficiency gas water heater. For smaller homes without cooling loads an HRV can actually be used as the air handler and a fancoil purchased separately for install on the supply side. Stirling has a nice fancoil unit for sale now. Colder climates can incorporate a limited amount of radiant heat to supplement the HRV/Fancoil unit. This solution is being touted for Passive houses with their small loads and need for high efficiency HRVs.  
    I’m still holding out hope for the CO2 refrigerant heat pump water heaters to reach our shores sometime this decade. With COPs up to 5.0 they could be installed as a replacement for gas units to deliver the hot water for heat and domestic hot water.

  10. “Consider what happens
    “Consider what happens when, on the coldest day, the furnace only runs 15 minutes per hour.” 
    I found this interesting – I just checked the thermostat in my “high performance home” – actually a 1927 stucco in MN, with retrofitted wall & attic insulation, and some attempts at air-sealing, and it said it had run for 2h52m today, as of 10:20pm. That’s even less than 15 min per hour! 
    I’ve just got a plain gas-fired boiler. I don’t think it’s grossly oversized, though I don’t know if the installer did a heat-loss calc or not. I’m considering an outdoor reset to modulate the temp, and it seems like this might even it out a bit. 
    But this makes me wonder; is my 1927 retrofitted home “high performance” now? 🙂

  11. Hi Eric, less than 3 hours
    Hi Eric, less than 3 hours run time on a cold day indicates an oversized boiler — but not necessarily a high performance home 🙂 
    Is your entire system forced air, or does the boiler support radiant as well? If you have radiant, then you can’t judge boiler sizing based on thermostat run-time. It’s more complicated. Also, the run time over a 24 hour period is pretty much irrelevant, since your home’s heat load isn’t peaking all day. 
    Perhaps I should explain the procedure… 
    Cycle timing should be done under design conditions. The outside design temperature (for heating) is defined as the 99th percentile temperature for a location, based on historical climate data. Design temperatures are published by ASHRAE (-9F for Minneapolis). 
    Using a watch, time two or three complete on-off cycles when the outside temperature is at or just below the design temperature. Then average the cycles. 
    For example, if the average on-time is 20 minutes and the average off-time is 10 minutes, then the duty cycle is 66%. This means the system is roughly 50% oversized (1 divided by .66). 50% is about the limit I recommend for a conventional furnace. A furnace that is 3 or 4 times larger than necessary is where you get into comfort issues (e.g., a duty cycle less than 33% at design conditions). 
    The good thing about hydronics is that oversizing becomes (almost) a non-issue. Water temperature can be modulated as the load changes, eliminating temperature droop and wide swings. This isn’t possible with an oversized conventional furnace, which is the point of the article.

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