John Melichar has upgraded the furnace in his two-level San Francisco home, one of several improvements that should have made the house more comfortable as well as more energy-efficient. The new furnace has the capacity recommended by his heating contractor, but so far the house seems less comfortable, not more comfortable.
In a post at GBA’s Q&A forum, Melichar explains his concerns:
“Our contractor told us to buy a 60K Btu/h furnace; we opted for 96% AFUE with two-stage variable blower — the Goodman GMVC960603BN.
“We get high winds from the vents,” Melichar continues. “We thought that the variable blower and two stages meant that the unit would operate on a low flame and low fan (cubic feet per minute) level but that doesn’t seem to be the case. The furnace starts out slow then ramps up to gale storm.”
These problems prompt Melichar to ask whether the furnace has been installed incorrectly. Maybe the contractor has recommended a furnace that’s too big for the house. Can some adjustments be made to improve its performance?
Further complicating Melichar’s situation is a language barrier with the contractors who installed the furnace: He’s able to speak with only one of them.
That’s the backdrop for this Q&A Spotlight.
Did the installer do his homework?
GBA senior editor Martin Holladay’s first question is whether the heating contractor has taken all the necessary steps to ensure the furnace was specified and installed properly.
That process, he writes, should begin with Manual J and Manual D calculations to make sure the furnace is matched to the heating loads in the house, and that the ductwork is designed correctly. Following installation, the system should be commissioned to make sure it operates as intended.
“There is a good chance that your contractor never performed steps one, two, or three, unfortunately,” Holladay says. “You should start by asking your contractor about these three steps. You may need to hire a home performance contractor to check your system if (as I suspect) your contractor doesn’t understand the three steps I listed.”
Melichar suspects that Holladay has a point.
“I wouldn’t be surprised if your suspicions about our contractor not completing those steps are right,” he says. “My conversation with the furnace installer hired by our contractor was limited to, ‘How big should it be? 60K BTU? 66K BTU?’ He said ’60.’ We were responsible for shopping for it. The contractor made a suggestion of a single-stage, single-speed blower system even though we stated we wanted the opposite, so I’m not sure how familiar they are with improved technologies.”
Here are some tweaks you can make
Jon R suggests that the specifications for the furnace Melichar has installed don’t show much of a difference between high and low stage. But, he adds, there is an adjustment for air flow that would reduce the “high winds” the system seems to be producing now.
“Also make sure that the thermostat supports two stages,” he adds. “Adjust zone dampers so that they never close to less than two-thirds.”
“That’s interesting about the dampers not being closed more than two-thirds,” Melichar replies. “It makes sense to me and seems a route to consider given that we are expecting the furnace to sometimes heat the whole 1,500 square feet, most often the 1050-square-foot upstairs and sometimes only the 500-square-foot downstairs.
“As you probably suspect, when the furnace is only heating the lower 500-square-foot zone, it is loud and fast!” he continues. “Reducing that cfm by leaving the upstairs damper upstairs makes sense.”
Reid Baldwin adds that a zone controller, a device installed between the thermostats and the furnace, may be to blame.
“Even if you have two-stage thermostats and a two-stage furnace, the zone controller may only be capable of single-stage logic,” Baldwin says. “When you use a single-stage controller with a two-stage furnace, the furnace is usually programmed to run on low stage for about 12 minutes and then to switch to high stage until the heat call ends. The furnace has no idea how many zones it is feeding. If you have a two-stage zone controller, it would have more sophisticated logic, like using low stage when only one zone is calling for heat and high stage when both are.”
Your new furnace is just plain too big
To Dana Dorsett, the problem is more fundamental: The furnace is way too big for Melichar’s house.
A typical house of that size framed with 2x4s, and one with clear glass double-pane windows or storm windows installed over single-pane windows, should show a heating load of 22,000 to 28,000 Btu/hour with an outdoor temperature of 0°F and the thermostat set at 68°F, Dorsett says. In a house with 2×6 framed walls insulated to R-20, the heating load would likely drop to between 19,000 and 24,000 Btu/hour, he adds.
“So, is a 60,000 Btu/hr two-stage the optimal furnace for your house?” he asks. “I seriously doubt it.”
“Odds are pretty good that even at it’s lowest firing rate the output is probably more than 3x oversized for your actual 99% heat load,” Dorsett continues. “A reasonably tight and insulated 2×4 house at 30°F would have a heat load of maybe 15,000 Btu/hr, 20,000 Btu/hr if it’s all single-panes with no storms — and even an uninsulated brick house would typically come in well under 40,000 But/hr.”
If Melichar has access to records for fuel use, he can check the heating load at the actual outside design temperature (as established by ACCA’s Manual J Residential Load Calculation) based on heating degree-day data.
In an exchange with Alan B, Dorsett’s defends the process by which he estimates Melichar’s heating loads, adding the new Goodman furnace could be as much as 10 times too big for the 500-square-foot zone, and as much as six times too big for the 1,000-square-foot part of the house, even when it’s burning on low.
Even in cases where heat-load calculations have been performed, a variety of factors can throw them off, Dorsett says, including assumptions about R-values, air infiltration, and the effects of plug loads.
“Seems there is not sufficient enforcement of California Title 24, which [if I remember correctly] requires Manual J heating and cooling load calculations even on replacement equipment, and may even require duct leakage testing and remediation (though that might only be for new construction),” Dorsett says.
Another heating option
If Melichar were to start over, Dorsett thinks a better approach, given the heating loads, would be to produce hot water with a condensing tank-style water heater and distribute the heat via a one-ton hydronic coil air handler, such as the Firstco 4CW.
The Firstco is capable of delivering 13,700 Btu/hour with 120°F water and a pumping rate of 2 gallons per minute, and 19,200 Btu/hour with 140°F water.
“That would deliver long comfortable low-cfm flow cycles,” Dorsett says. “The max flow on the Firstco 4CW is 400 cfm, which is lower than the lowest heating cfm setting on the Goodman GMVC960603BN (600 cfm, set to minus 10%.)
“If air conditioning is a ‘must have,'” he adds, “doing the heating and cooling with minisplits might make more sense. But cooling loads are low enough on that side of the bay that most houses that size do just fine with a half-ton window-shaker for cooling.”
Our expert’s opinion
GBA technical director Peter Yost added these thoughts:
I am going to build on Dana Dorsett’s comments on choosing another heating option. I think the series of images below from the free download software Climate Consultant is worth 1,000 words on what I feel is the completely misguided type of space conditioning system that was elected for this project. (For more information on Climate Consultant, see “How to Use Climate Consultant 4.”)
To follow along, simply download Climate Consultant 6, enter the California zip code for John Melichar (94132) and use the drop-down list of charts. And note that the Climate Consultant starting page summary states, “Climate Consultant seeks to translate outdoor conditions into indoor comfort…”
California Climate Zone 3 has about six months of low-grade space heating and then six months with just about zero active space conditioning need. As Dana explains and Climate Consultant 6 reinforces, it’s hard for forced-air systems to be as effective as an integrated package of other design and mechanical strategies.
I think the oversizing and misapplication of the type of system means John should go after the HVAC contractor for such gross errors and then move, if he can, to a hydronic system (or hydro-air system) that allows him easily and effectively to combine his space heating and domestic hot water loads.