Construction Design-Concreteless Slab on Grade-Mechanical Systems.

The concreteless slab on grade home had a lot of unique building materials and techniques, at least for my area of Northern Minnesota.  The mechanicals of the home were no different.  Everything from the water heater to the electrical system was designed to conserve, and in the case of the electrical service panel, track energy use.

Lets start with the electrical systems.  As I mentioned in an earlier post, the entire ceiling was strapped with 2 x 4 lumber so that nearly all the electrical wiring in the ceiling could remain inside the building envelope.  I liked this method so much, I’m trying to incorporate it into a couple other designs I’m currently working on.

All the electrical wiring and boxes were able to remain inside the building envelope.

The electrical service panel is a newer technology we installed in this home.  The panel is manufactured by Leviton and features WIFI enabled circuit breakers.  We installed standard breakers, which includes mostly code required arc fault breakers for the majority of the circuits, but I wanted the ability to track energy usage of the heating system.  There are a pair of two pole breakers that have the ability to monitor and record energy use.  The system will be set up so that both the homeowner and myself will be able to track the electrical use of both the breakers.  Gotta love new technology.  We only installed the two because of budget concerns.  At nearly $100 per breaker for the two pole versions, the cost would have skyrocketed if all the breakers were WIFI enabled.  We can always go back and change out breakers at a later date if the homeowner needs additional energy usage tracking.

One nice feature about this panel is all of the electrical connections are terminated on a bus that is fixed to the panel.  No wire is attached to the breaker itself.  A couple other features, a WIFI enable breaker can be turned off remotely from a phone, tablet or computer.  The breaker has to be manually reset.  Handy if your away from home and you notice the well pump using more energy than normal, could be the result of a plumbing leak.  Another nice feature is the arc fault breakers can receive a firm wear update should a common nuisance tripping event occur.  This update would be provided remotely by Leviton.

One additional electrical feature built into this home is a provision for a generator.  The electrical panel has a special breaker control that will allow the connection of a generator so that the entire panel can be energized without back feeding the utility supplied power.  The homeowner will have to be selective on which circuits are energized.  He will be able to utilize his water heater, but most likely won’t be able to supply power to anything else during that time.  The generator will most likely be a wheeled 3-5 kW unit.

This home received a Marathon water heater, which has a plastic tank and shell with 2 1/2 inches of spray foam between the inner and outer shells.  This insulation greatly reduces standby heat loss.

The heater is also on a load control program which means it is controlled by the local power provider.  The heater only is taking a charge in the middle of the night.  We wanted to provide enough hot water to last all day, so the water heat temperature was increase to 140 degrees and a mixing valve that will only allow 120 degree water to reach the faucets was installed.  Because of the control by the electricity provider, the customer receives a reduced rate for the water heater.  This rate is roughly half what the normal rate is, saving the typical family $10-$20 per month.

The water heater is controlled by a special radio receiver and meter.

The heating system is a cold weather air source heat pump.  Most of the cold weather versions are mini-splits, meaning there is the typical outdoor condensing unit with a wall or ceiling mounted fan unit inside.  Not every room gets a heating head.  These are a great systems, but I worry about maintaining comfort in rooms without a wall mounted fan unit, especially if a door is closed.  Because of this, I chose a central ducted cold weather air source heat pump manufactured by Mitsubishi.

This is a PVA multi-position air handler heat pump with a two staged electric plenum heater supplying additional heat that may be needed during very cold weather.  The heat pump is rated at 24,000 BTU and the electric plenum heater is a 8 KW unit.  The SEER rating of the unit is 20.5, very efficient unit.

The outdoor piece of the air source heat pump.  The unit is elevated.  Notice the time of the year that it is being installed.  lots of snow and ice on the ground.  This becomes an issue later in the winter.

Because of how tightly constructed the home is, I knew balanced mechanical ventilation was very important to control the humidity levels during the winter months and also to supply the owners with fresh air.  Minnesota also requires by code that all new homes have  balanced mechanical ventilation.  This was accomplished by this heat recovery ventilator (HRV).  I know a lot of areas of the country install energy recovery ventilators (ERV), but HRV’s are usually installed in my climate.  One of the big differences is an HRV will go into a defrost mode periodically to eliminate the frost that builds up on the inner heat exchanger core during cold weather, most of the winter.  This water that accumulates after the defrost cycle is then discharged out the plastic tubes seen underneath the unit.

I was working at the home one day this winter during a very cold spell with windy conditions.  I snapped this picture to show how much moisture can be present in a home during construction.  This is the vent for the HRV unit before the unit was installed.  The warmed, moist air is moving through the vent and has condensed on the siding.

The homeowner requested a second source of heat to be used as a back-up.  This property is very rural and can experience prolonged power outages.  The homeowner originally wanted a wood burning appliance but the builder and myself were worried about combustion air and proper drafting in this tightly constructed home.  We were able to talk him into a thermostatically controlled propane gas fireplace.

Though this isn’t a high efficiency unit, it’s ability to operate without power was the reason it was chosen.  The fireplace’s heat output on high is twice what the home requires, but can be varied to a lower setting.  This fireplace can quickly raise the temperature of the home, whereas the air source heat pump works much better at maintaining a temperature.

OK, now for the one problem we have encountered this winter.  We have an icing issue under the outdoor condensing unit of the air source heat pump.

We are looking to make some modifications to the heat pump stand to elevate the unit a little more and final grading should also help.  The unit became operational in early winter, after the ground had frozen and we had a couple feet of snow on the ground, none of which helped the icing issue.  Something else to monitor in the coming years.

The energy tracking of the heating equipment has yet to be set up.  I will be writing a future post as soon as I have this data.  Stay tuned.

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