Heating Water in a Passive House

People in Canada typically use about 75 litres of water per day.  In a 3 person household, the total hot water consumption would amount to about 225 litres per day.  We can safely assume that the water entering the hot water tank is somewhere between ground temperature (7 deg Celsius) and room temperature.  Lets say 10 deg C.  Using the heat capacity of water, one can calculate the amount of energy required to raise the temperature of the water from 10 deg C to 55 deg C.  That amount of energy is about 11.8 kWh.  Typical electric tanks consume about 0.5 kWh in standby losses and you can safely assume you'll loose another 0.5 kWh in pipe loses.  So the to total energy used for hot water in a 3 person household is about 12.8 kWh.  Assuming average usage patterns, the total amount used per year will be 12.8 kWh/day x 365 days/year = 4672 kWh/year.

This is about the same energy requirements as the total heating load for a 2000 sf Passive House.  Further, minimizing the required energy for heating is pretty much impossible given that it's already been decreased by 80-90% of the heat required by a code built home.  So it seems sensible to concentrate energy conservation measures on the production of hot water.  The pie chart below illustrates the savings in heating and the fact that hot water accounts for one of the largest pieces of the pie in a Passive House.  Its a little misrepresented since the space heating requirements for our climate are more on the order of the hot water requirements.

In my previous post I talked about heating a Passive House with wood.  There are many alternatives but wood seems like a good option given the small heating requirements.  My estimate showed that the heating demand would be fulfilled using 1/3 of a cord of wood.    Keep in mind that the Walltherm stove that I considered in the previous post heats water while you are also heating your space.  So its a little more complicated than I made it out to be in the previous post.

So here it goes.  The Walltherm stove outputs 12.5 kW to water.  Most hot water tanks are set to 55 C or 60 C.  For a system like this, you are dumping large amounts of energy into a thermal storage tank using a circulation loop.  The tank is huge...almost 600 L! or 150 gallons!  I know my daily requirements will be about 12.8 kWh but I can add more energy if I want to get the tank up to 85 C.  If the tank is at 60 C as a result of the burn from the day before, ignoring standby losses, the temperature of the water in the 600 L tank will increase from 60 C to about 78 C.  This is pretty hot!  However, with a thermostatic mixing valve, the temperature of the stored hot water can easily be mixed down to 55 C for domestic use.  If my hot water requirements for the day will be 12.8 kWh, the amount of time that the stove has to burn once in gasification mode (ignoring standby losses and the ramp up time to get the stove the the temperature required for secondary burn) would be 12.8 kWh/12.5 kW = about 1 hour.  The stove takes about 15 minutes to heat up and during that time you would be using some of the fuel purely for heating the home.  The total heat added to the house during the burn would be 2.5 kW x 1.25 hours = 3.13 kWh.  In the dead middle of winter, on the coldest heating degree day the heat loss will be about 9 kWh, which means I will need to add about  9 kWh to the house in order to keep the temperature constant.  If its sunny outside I will gain up to 1/3 of my heat from solar energy, 1/3 from internal gains so 3 kWh from the wood stove will be enough to heat the house for a day and I also get my hot water.  If it's not sunny, I'll just burn the stove until the water temperature reaches its maximum temperature of 85 C.  That will sink 17 kWh into water instead.  the stove will burn for another 25 minutes and generate another 1.5 kWh of heat amounting to about 4.5 kWh.  If the internal gains of the house are about 3 kWh, I'll just need to make up another 1.5 kWh.  I could just let the electric heat kick in or burn a little longer.

I would probably only burn the stove for half of the year.  So lets say 180 days.   My energy requirements for hot water would be 12.8 kWh/day x 180 days = 2304 kWh.  Each cord of birch 0.contains 6400 kWh of recoverable energy.  At 93% efficiency the stove would require,

2304 kWh / 6400 kWh  per cord / 0.93 = 0.39 cords

So 2/5's of a cord.  Based on my previous post, I would need about 1/3 of a cord for heating and a little more than a cord for half of my yearly hot water.  So for heating season I would need about 2/3 to 3/4 of a cord of wood....I'd be looking at burning 4 pieces of birch 4 inches x 4 inches x 12 inches  per day to make up my heating and hot water costs.  Then I need to figure out how to heat water in the early shoulder seasons....and summer....I'll explore this in the next post....


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