Mechanical Systems
In an earlier post, we discussed our HVAC strategy overall. For those interested in the nuts and bolts of system selection, air tightness and makeup air, this update is for you!
We’ve decided on a 3 Ton Carrier 38MURA High Heat model with SEER 18 and HSPF 10.5. First - some terminology:
– SEER is the Seasonal Energy Efficiency Ratio, or total cooling output divided by the energy consumption during that time.
– HSPF is the Heating Seasonal Performance Factor, or total heating output by energy consumption.
– Heat pumps are sized based on the amount of energy the output. 1 Ton is equivalent to 12,000 BTU per hour.
– The High Heat model means our system’s output won’t drop off as much as the equivalent standard heat pump during cold months, e.g. our unit is specced to still be capable of providing 34,500 BTU of heat (almost 3 Tons) at 5 degrees F.
A rule of thumb that engineers use for heat pump sizing is ~1 Ton per 500 sqft home area. We will be about 3,000 sqft and so that has us at a 5 or 6-Ton system. Because of our highly insulated walls and roof, as well as air tightness of the home, we are able to reduce this to 3 Tons. Note - our energy model actually specifies a max heating load of just over 2 Tons. We are oversizing slightly to account for unmodelled effects and extreme temperature events.
Other factors to consider for heat pump selection are physical size (affects the pad it’s placed on), local servicing (life span is ~15 years but how easy it is to get replacement parts if needed during that time) and noise (typically between 55-60 dB, some quieter than others).
Our heat recovery ventilator (HRV) will operate separately from the heat pump. The energy model recommends 75 CFM (cubic feet per minute) based on square footage and air tightness (target 1.5 ACH). With very little natural “breathing” occurring through gaps and cracks in the building envelope, it’s important to get the ventilation rate right.
HRVs typically operate as completely balanced systems where the volume of air leaving is equal to the volume entering. With an air-tight home the challenge becomes when you run fans for various purposes - range hoods for cooking, clothing driers or bathroom / mudroom ventilation. Replacement air has to be drawn from somewhere, and with an air-tight home, negative pressure can build. Typical room fans are not a problem, but our range hood fan is ceiling mounted and likely running around 400 CFM (cubic feet per minute) power on the highest setting.
We need to provide makeup air somewhere in the house. Options are: (1) manually open a window when turning on the kitchen fan; (2) install a simple lever duct system for makeup air (essentially an automatic opening and closing window) (3) install a mechanical make-up air unit with in-coil heating tied to the range hood operation
Options 2) and 3) come at a cost, additional design complexity, and are not required by building code unless there are specific appliance or air quality concerns, i.e. from combustion equipment such as gas stoves or fireplaces. We’re opting for the window option (1), based on input from our project team, and lived experience from others in airtight homes.
And with that, we have our mechanical system selected! The proof will be in the pudding - indoor comfort over 12 months of operation, and whether our solar array covers the energy needs for heating/cooling/ventilation (#1 use of energy in the house). To be continued!