Week 14

This was the week of the last concrete pour and the last concrete patty of about 8 left by the pumper and concrete delivery trucks for later disposal. 

The water supply rough plumbing on the first floor was also completed.  The first floor is now ready for the wall board installation next week to cover everything except the windows and doors. 

We even received the stairs to the basement which came in two sections that were quickly installed and had walls framed around them. 

Outside, the trim, siding, shingles,  gable vent,  and attic entry door were completed on the west side.  This was achieved with the help of Balky, a Genie Boom rental lift that had to be pampered just right to do any lifting.








The Cost of Heating

In week 4 we stated the basic energy need equation, En = EL - EG.  The energy needed, EN, to heat the house to a desired temperature T is the total energy lost, EL, to the outside minus the total energy gained, EG, by internal activity and solar heat gain. We defined the desired temperature at 68 degF.

In week 13 we estimated the solar gain contribution of the windows which on an annual basis is 2370 kwh, not counting June, July, and August contributions as they are too much to meet the heating need and need to be blocked.  One other gain previously discussed is the occupant generated heat, around 2.4 kWh/day-person which for one person is 876 kWh/yr.

Several weeks back we determined that the house is estimated to use around 140 BTU/degF. Using this number and combining it with the Vermont average monthly weather data we arrive at an annual need of 6500 kWh after a 2370 kWh passive solar contribution. Other heat losses are from the heat recovery ventilator, uncontrolled air leakage, ingress and egress activities, and water usage losses. 

Heat Recovery Ventilators are needed for air tight houses during heating days to avoid humidity and CO2 build up.  These units will draw in fresh outside air and heat it from the heat of the stale inside air which is expelled to the outside.  This occurs in a two chambered non-mixing polypropylene heat exchanger.  There are various ventilation standards with the Minnesota standard requiring .05 CFM/sqft + 15 CFM/person which would be 55 cubic feet/min for this house with one occupant. 
The chosen ventilator is a Venmar Constructo 1.0  which is rated at 45-96 CFM with an efficiency of 76%  at a 70 degF difference between the inside and outside air and 50 CFM.  According to the data sheet, which is not very specific, at 32 degF external and 50 CFM, 44 W of electricity are consumed.  If this ventilator were to run all the time it would annually consume almost 400 kWh just to move the air.  Judging from the two motor design layout, about half the motor heat will be exhausted to the outside, the other half will go to heat the incoming air .  It is unclear how much energy is consumed by the defrost cycling which occurs below 23 degF.

The ventilator would most likely not be used during the warmer months as the windows would then be used for ventilation and temperature control instead.  For the 264 days of usage (Sept.-May), the average outside air temperature is 38 deg. Assuming this air to be at 50% humidity, heating it to 68 degF would take 15 KBTU/day as can be determined from a psychometric chart and assuming that 2/3 of that heat can be recovered from the exhaust air.  For the heating season this would compute to an annual consumption of  1160 kWh.  Using it at less than 50% would half this to say 580 kWh/yr. Adjusting the motor heat consumption for the heating days and the 50% duty cycle of operation, would subtract 74 kWh/yr for a final 500 kWh/yr heat need.  (Disclosure: I am not a thermodynamicist just an electrical engineer who had to take thermodynamics almost a half century ago)

All water entering the house enters estimated at 50 degF.  It will most likely exit at the house temperature of 68 degF.  Thus for an estimated daily usage of around 100 l (25 gal) this represents a heat loss of around 600 kWh annually.

Uncontrolled air leakage losses are a function of the tightness of the house construction, the temperature difference of the inside and outside air, and the external wind pressure.  I will assume this to be zero currently and will wait until it can be measured with a blower door test to be performed by Efficiency Vermont.

Ingress and egress losses are a function of the life style and habits of the house occupants.  Every time a door is opened during the heating season a certain amount of heat is lost in the air escaping and a certain amount of heat is needed to heat the person's belongings and clothing from the outside temperature to the inside temperature.  This is assumed to be zero for now and will be measured later.

We thus have optimistically, the following heat needs in kWh:
En = conductive losses + HRV losses + water losses + uncontrolled heat losses + ingress/egress losses - solar gain - occupant heat
En = 8855 + 500 + 600 + 0 + 0 - 2372 - 876 = 6700 kWh
Thus at the 0.15cents /kWh expected later this year, the heat need would cost 1000 $/yr.  The only other electricity costs not covered by this would be external lights (1) and the HRV motor heat loss of 74 kWh/yr.

Next time we'll see whether this is good and how this cost is covered - or not.

Please send me comments that add to the discussion and correct any errors of comprehension.