Construction Design-Code Minimum House-Mechanicals

At the time of this writing, construction on the code minimum house is coming to an end.  This has been a fun and rewarding project for me because not only was I able to design much of the home, I was also directly involved in the construction.  A quick review on why I named this project the “code minimum house”.  This was a budget build without much in the budget to increase the home’s performance.  No walls with thick insulation, basic off the shelf windows and no fancy mechanical systems.  Where we were only able to increase this home’s performance in air sealing.  The final blower door test results and a recap of the techniques used for the air sealing package will be in an upcoming blog post.

Before we get to the mechanicals of this house, I want to discuss something I had completed before construction began, a third party heat loss calculation.  A local HVAC contractor in my area purchased a heat loss program which he uses for his own work.  Because he’s not the HVAC contractor on this project, he charged a small fee to calculate the heat loss.  (I recommend having a heat loss calculation done on all new construction.)  The design parameters were R-10 sub-slab insulation, R-21 Walls, R-60 ceiling, R-4 windows, and an air infiltration rate of 1 ACH50.  He also received a digital copy of the floor plan which he imported into the program.  The design criteria for my very cold climate is -23°F, the 99% design temperature.  We have right around 10,000 heating degree days and roughly 500 cooling degree days.  Cooling isn’t much of a factor in my climate though most new homes being built in my area include AC.  Total heat loss for the project was 43,271 BTU with the attached garage accounting for 13,305 BTU.  So roughly a 30,000 BTU heat loss for the main home.

The main heating system for the home is a natural gas combination boiler meaning the boiler heats not only the living space, but also provides the domestic hot water.  There are a couple good and bad points when dealing with a combination boiler.  Bad point, the boiler is grossly oversized for the heating needs of the home.  The reason being is it also supplies instant hot water, which requires a lot of BTU’s.  We ended up with a 100,000 BTU unit.  Now the good point, the boiler is modulating.  If your not familiar with this term, the best analogy I have is it’s like driving a car.  You vary how hard you push the gas pedal depending on the speed you need, most of us rarely push it to the floor.  A single stage heating system is either to the floor or off.  This system modulates from 100,000 BTU down to 12,000 BTU.  Perfect for this home.  My brother happens to be the HVAC, plumbing and electrical contractor on this project.  I let him choose the boiler manufacturer.  The reason being is he has more experience with the manufacturers than I am, he will be the one called if something isn’t working correctly.

This installation of the heating system actually started at the beginning of the project.  Heating tubes were placed before the concrete was poured.

Hot water in-floor heat tubing before the concrete pour.

In our area, we use roughly one lineal foot of tubing for every one square foot of living space.  This home has 1,500 lineal feet of tubing with another 600 in the attached garage.

The hot water in-floor heat is the main heating system.  We also added an air source heat pump mini-split system (ASHP).  The issue with in-floor heating systems is during the shoulder months of April and May in the spring and September and October in the fall is that the in-floor heat is often to much.  The heating need is there during the night when temperatures are cooler, but during the day, the floor can be too warm causing a comfort issue that is often corrected by open windows to cool the home.  Not very efficient.  The reason this happens is the warmed concrete slab is a large heat sink, it is slow to rise in temperature but also slow to cool.   I will be recommending to this homeowner that they do not start the in-floor system until mid-November and turn it off in mid-April.  The cold weather air source heat pump being installed in this home is more than enough to heat during the spring and fall plus the ASHP will provide any needed air conditioning.  We also installed a thermostatically controlled natural gas fireplace.  Though the system is large enough to heat the home, it is intended for casual use and as an emergency heat source.

Because this home is being built in Minnesota, the state codes require balanced mechanical ventilation.  The codes do not specify the need for a heat recovery ventilator (HRV) or energy recovery ventilator (ERV), just that the ventilation is balanced and mechanical.  I always specify an HRV because of my experience with past installations, I know they work.  In the future I may try an ERV, many manufacturers are producing models that work in a cold climate, but my lack of experience and the fact that I know once the home is turned over to the homeowner, I loose control of the humidity levels, which could be an issue for a home in my climate with an ERV system.  Minnesota has a specific code for sizing a mechanical ventilation system.  Total ventilation (cfm) = (.02 x square feet of conditioned space) + (15 x (number of bedrooms + 1))  The code minimum house is just under 1500 square feet and has 3 bedrooms so the calculation tells us we need 90 CFM of balanced ventilation.  If the ventilation equipment operates continuously, the ventilation rate can be cut in half.  We will be shooting for around 50 CFM.  The code minimum home got a Broan HRV T90 which is capable of exchanging 35 to 120 CFM.

We installed the HRV as a fully ducted system meaning all the ducts used to bring in fresh air from outside and exhaust stale from inside are dedicated to the HRV.

I made some changes in the way moisture is handled in the bathrooms of this home.  Instead of installing the usual bath fan, I decided to use the air exchanger instead.  Because of the tightness of the home, I wanted to limit the exhaust only equipment.  This home has an electric dryer which will exhaust around 100-200 CFM and venting range hood, again in the 100-200 CFM.  (The range is electric.)  If we had installed two bath fans, there would have been an additional 50 CFM each of exhaust only ventilation.  I decided to use the HRV with a timer switch as the bath fan.  The timer will bump the HRV into a high speed, maximum 120 CFM of exhaust, but an equal airflow will be coming back into the building.  I will be instructing the homeowner to start the timer during every bath or shower.  I’ve had some discussions with other HVAC contractor’s who won’t use an HRV as a sole bath fan.  The reason being is if the unit isn’t maintained regularly, the reduction in air flow can cause high moisture loading in the bathroom.  No mater what, ventilation equipment needs maintenance, just like it’s important to change the oil in your car, HVAC filters need to be cleaned or changed.

The switch to the right is the bathroom timer for the HRV.

One last system that was installed in this home, the Humidex exhaust fan for the garage.  I’ve written a blog post about this fan, read it here.

I’ve hit the major mechanical systems in the code minimum home.  I’ve gotten to know the owner a little since we began this project.  I will be visiting him from time to time to be sure the home is satisfying his expectations.  Time will tell.

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