Heating, Cooling, Hot Water: What are my options?
The nice thing about a Passive House is that you can't build a passive house unless its modelled. Passive House is a performance standard not a prescriptive one. A prescriptive standard, like the National Building Code of Canada, tells you how to build a structure to maintain safety, longevity and the health of the inhabitants but it doesn't tell you anything about the performance of the building. It tells you about the performance of the whole package, upfront, before the building is erected. As long as you can meet the performance measurements i.e. blower door test, insulation performance, your building will, for the most part, behave the same as the model.
In the previous post I presented the required heating loads for the home. They are pretty small. The total load according to wrightsoft is about 2.4 kW. My designer has used the PHPP software to map out several options for heat/cooling sources. The modelling has provided me with a way to compare site energy usage for different heat/cooling sources. See the table below:
The base passive house case uses electricity for everything, but no cooling. Basically electric resistance heaters for heat, a electric resistance water heater and appliances as energy star with some allowances for energy usage. Total usage would be somewhere around 12365 kWh per year. about 3100 kWh is for heating, about 4000 kWh is for hot water and the rest is for appliances. Keep in mind actual appliance usage will probably be less given that he hasn't derated the power usage based on our appliance choices.
Adding a DMS (ductless mini split) decreases energy usage by about 2155 kWh/year. Keep in mind that the heating demand was only 3100 kWh but the DMS modelled here also includes cooling. Total energy usage would be 10210 kWh/year.
Using a HPWH wouldn't actual do anything. Since a tank HPWH starves the building space of heat, the DMS will work more in the winter to provide heat scavenged from the space. However, in the summer, the HPWH would cool the space effectively since it extracts heat from he space and sinks it into a hot water tank. No energy savings but it is an efficient setup.
How about a wood stove in a passive house. You just need to add a air intake and make sure that the stove has closed combustion....But..woodstoves typically get really hot and finding one with less than 4 kW output is pretty impossible. That being said, using a wood stove is about as effective as adding a DMS to the house in terms of total energy savings and total yearly demand. In this case the demand is about 10350 kWh.
How about if you have a system that can use multiple sources? What if you had a way to heat water, to provide space heat in the winter, and a heat pump water heater to heat the water in the summer? Now you could have an interestingly efficient system which would allow you to be less dependent on the grid. Using a wood stove for ambient heat, to create hot water, along with low temperature hydronic radiators the energy usage drops by almost 4355 kWh/year to a total of 8010 kWh. Now if you add a heat pump to the mix (inside the thermal envelope) the energy usage decreases to 7289 kWh/year! At $0.0972/kWh, the total yearly electrical bill including heat, lights hot water will be about $708. The heat pump would be the add on type (http://water.nyle.com/r-series/) and it sits in the mechanical room and can be ducted to the south rooms of the house. It would draw in warm air from the dining room and expel cool air to the living room since it has the most glazing.
Our hybrid system will be the first of its kind used on one of Passive Design Solutions passive houses, so it is a bit experimental but we trust the software. The components will be
1. WallTherm gasification wood stove (http://www.walltherm.ca). It will provide air heat, hot water radiation through low temperature rads (30% more efficient than standard rads), and all of our hot water during the heating seasons. Its a beautiful stove and with it's secondary burn chamber its quite a site to watch on a cold evening.
2. The heat pump (http://water.nyle.com/r-series/) will provide us with our hot water in the summer. It will feed heat through a solar coil exchanger built into the boiler. The boiler naturally stratifies the water so the hot water stays at the top of the tank.
3. A couple of electric elements will be added to the boiler tank for those times in the shoulder season when you don't want to cool or heat the space.
So there it is. Basically our hot water and heat will be taken care of in the heating seasons using about 0.5 to 0.75 cords of wood. In addition, I won't have to run the stove daily if I don't want to. I can store water at 80 C! So i can burn on a cold day and store enough heat for hot water to get through a couple of sunny days! Or if it's cold outside, the water in the tank can be used for heating the space. If we are on vacation, the electric elements in the tank will keep the tank warm. as long as the water in the tank is above 55 C, the electric elements will never turn on. In the summer, the heat pump will cool the space inside the house while providing hot water by dumping space heat into the hot water boiler. Our space cooling requirement appear to almost match our hot water heating requirements so the space should be nice and cool in the summer....Hopefully!
It looks like a beautiful solution and we are going to implement it....I may still add the necessary wiring for a ductless mini split....Just in case!