Saturday 22 June 2013

Heat Inputs

The estimate arrived at in my previous post expresses how much heat must be added to the living space every hour by a heating system. However the heating system alone is not the sole source of heat in a dwelling with occupants, electrical appliances and, particularly in my case, solar gain all acting as sources of heat. Ken Matesz (1) states that "each person will be adding at least 230 BTU/hr into the living space and that electrical appliances will contribute approximately 1600 BTU/hr. From the outset my objective when designing the house was to maximise solar gain in the winter months and rely upon the high thermal mass proposed for the house construction to store, distribute and radiate the stored heat. I saw this solar gain as a key source of heating as in my original research I found that it is possible to receive up to a couple of thousand BTU's per square foot of double paned glass windows. Assuming, for estimation purposes, a solar gain of half this, say, 1000BTU's per square foot from the total glazed area for my design of approximately 80sq.m (i.e. approx 850 sq.ft) of the SSE facing glass wall yields a solar gain of 850,000 BTU's per day.

However only a portion of the heat entering will be absorbed by the thermal mass of the house. Assuming that the unfinished concrete surfaces that I am specifying for the floors, walls and ceiling will only be 50% efficient the solar gain will be 425,000 BTU's per day i.e., say, 18,000BTU's/hr.

Thus the total passive heat gain for the house with 2 occupants will be approximately 20,000 BTU's/hr.

Right at the bottom of the range of the heat required!!

Having spent last summer frustratingly observing the site throughout the heat of the year, due to the inordinate delay in obtaining the Building Permit, it was apparent that cooling was more important than heating.

To check the effectiveness of the overhang I had specified to keep direct sunlight off the glass wall during the hottest part of the days I constructed a model which simulated the height of the SSE facing wall of glass and the corresponding overhang. I then ran an experiment in the mid July heat by orientating the model as the house will be orientated and photographing the shadow cast by the overhang from early morning until evening to prove that the glass wall remained totally in shadow throughout the day. This was successfully achieved.

No expense spared model to monitor shadow of overhang on very hot day in mid-July 2012

I know my engineers have software available to simulate such a task but it was very rewarding and confidence building that I could equally simulate such conditions. It was clear from careful observation throughout the day that there would be a lot of heat reflected from whatever surface is outside the overhang shadow area. We personally experienced this effect in our rented apartment throughout the summer!! Our solution in the apartment was to close the large double shutters but that made our room very gloomy and claustrophobic throughout the day. Not what we are seeking in our new house. The solution, rather than shutters or drapes, is to have a heat reflecting coating applied to the inner surface of the outer pane of the double glazing which can greatly reduce the heat entering the house. The problem is, of course, that this restricts the solar heating capacity of the window wall in the winter months. An interesting dilemma!

At the moment I am keeping all options open.

As I mentioned in my previous post (Heating Requirements) I had originally planned for needing both a wood fired range cooker and a wood fired masonry stove/heater to provide cooking and heating for the main living space in the coldest months of the year and a small wood burning stove for the main bedroom with ensuite bathroom for a quick burst of heat for the chilliest of days/nights.

A typical range cooker (eg. Esse 990) produces 3-5 kw/hr. Using a conversion ratio of 1kw/hr=3,412 BTU/hr  this gives a range of 10,000-17,000 BTU/hr. Given that the cooker will also be being used in conjunction with the solar water heater during the winter to provide hot water I am assuming the lower end of this range for estimating heat input to the main living space. A similar level of heat should also be available from the small wood burning stove planned for the bedroom but this will be for a few hours each day and only as and when required to meet any sudden very cold spells where a quick source of intense heat is required for only a short time so will add relatively little to the overall thermal mass stored heat.

I will address Masonry Heaters in subsequent  posts.

(1) "Masonry Heaters: Design, Building and Living with a Piece of the Sun" - Ken Matesz