Predicted and Actual Energy Use of a Building

A few interesting things happened today. First I got a call from a client I had given a fee to a few weeks ago for an energy saving feasibility study on an existing building. The phone call went along the lines of, thanks for the fee, we will have a purchase order with you in the next week. This was an end user client, so no arguing over the fee! A nice start to the day I thought.

The second interesting thing was a tweet I saw with a link to this blog and comment. In case you don’t want to read both the comment and the blog (though I suggest you do) I will summarise.
The blog is written by Sophie Pelsmakers who is an architect by trade I believe. Sophie writes an interesting blog about energy performances of buildings. This blog looked at the problem of the performance gap between energy predictions and the measured energy use of buildings.

While the blog was correct in highlighting this difference, the tone of the blog inferred that somehow this was due to problems with design, modelling tools, the build or other factors and that the architect and designers would be embarrassed about this gap.

The comment which Sophie had directed people to, in my opinion hit the nail on the head. Basically the comment talks about how Part L modelling software is not intended to give any predictions about actual energy use of the building. The design team should not pretend it is and furthermore we should not even be making energy use predictions as far too many variables, outside the control of the design team are present in this complex calculation.

I whooped and hollered as I read the comment. Someone was saying things I have long said. As a recent convert to blogging I had already decided I would write a blog on this topic, however this has made me bring that writing forward, so here goes.

I will use the term SBEM within this post to refer to the calculation procedure used in Part L to calculate criterion 1, the CO2 emissions of the building. The reality is SBEM is but one calculation tool, the least accurate, but people understand what is meant by this term.

SBEM splits the energy use of the building into two main categories, regulated and unregulated energy.

Unregulated energy is the energy used by non building services elements within a building, such as computers, printers, kettles, cooking, photocopiers etc etc. This energy is not used in the prediction of the CO2 emissions from the building. However, the heat the equipment emits is taken account of, just to confuse even further.

As many will have already spotted, unregulated energy can often account for a vast amount of energy within a building, though it is not included within Part L. Yep, this will for starters contribute to that gap between design and installed energy use.

Regulated energy is split in to the following categories, heating, cooling, hot water, lighting and auxiliary (fans and pumps).

I am going to look at how SBEM calculates all these energy uses and demonstrate why this should never be used to inform a client of the likely energy use of his building. I will hopefully also demonstrate why the building services engineer, or any member of the design team should not be trying to estimate the likely energy use of a building.

I will start with regulated energy, as this is the energy which is looked at with the SBEM calculation. Each sub category will have an explanation of how SBEM calculates it’s energy use and why this will not reflect the reality. I will list the variables below along with a note about who is responsible for this within the design and construction team.

Heating
Design of the building – architect responsible
Building fabrics (U values) – architect to specify
External temperatures – defined with limited weather files in SBEM
Internal set point temperatures, defined as part of room type in SBEM
Heat gains from people and electrical equipment – all set within SBEM, levels of people and equipment as well as the patterns of operation and occupancy
Heat gains from lights – lux level is set within SBEM even if this is not the design lux level, so the actual heat gain and pattern of operation, i.e. hours switched on is set within SBEM
Air tightness – specified by architect, relies on the builder meeting the criteria
Hours of operation – set within SBEM
Efficiency of the heating system – defined by the building services engineer

Cooling
Design of the building – architect responsible
Building fabrics (U values and thermal mass etc) – architect to specify
External temperatures – defined with limited weather files in SBEM
Internal set point temperatures – defined as part of room type in SBEM
Heat gains from people and electrical equipment – all set within SBEM, levels of people and equipment as well as the patterns of operation and occupancy
Heat gains from lights – lux level is set within SBEM even if this is not the design lux level, so the actual heat gain and pattern of operation, i.e. hours switched on is set within SBEM
Air tightness – specified by architect, relies on the builder meeting the criteria
Hours of operation – set within SBEM, though the actual hours will depend on the client
Efficiency of the cooling system -defined by the building services engineer

Hot Water
Hot water usage – defined by room type in SBEM
Efficiency of hot water generation – defined by the building services engineer
Lighting
Design of the building – architect responsible
Amount of glass – architect responsible
Light transmission of glass – architect responsible
Hours of operation – defined within SBEM for room and building type
Lighting level – defined within SBEM for room and building type
Switching – defined by the building services engineer, though SBEM makes assumptions as to how efficient switching regimes are
Dimming – defined by building services engineer
Efficiency of light fitting – defined by building services engineer

Auxiliary Energy
How much air is distributed through the building – defined by SBEM for room type, building type and HVAC system
How much water is distributed – defined by SBEM for room type, building type and HVAC system
Efficiency of fans – defined by building services engineer,
Efficiency of pumps – defined by SBEM

What is clear from the list of things above is that the building services engineer is responsible for the efficiency of the heating, cooling, hot water and lighting systems and that’s about it. He is not responsible for the biggest factor in energy use, the energy demand.

The remainder of things which contribute to the regulated energy use are defined either by the architect or by SBEM.

A couple of things to note. Many things are a function of the occupancy of that room, it is likely that the client will know how many people will occupy a room, however SBEM will not let us input our own occupancy levels. Instead SBEM uses a default figure. This then goes on to determine the amount of hot water used, the amount of fresh air supplied and the amount of heat gain from those people.

In short, SBEM uses normalised, standard figures for the following, all of which can’t be altered.
Occupancy numbers
Hours of occupancy
Fresh air supply volumes
Temperature set points within each room
Lighting levels
Hours of operation of the lighting
Amount of hot water used
Hours the heating plant operates
Hours the cooling plant operates
Hours the ventilation plant operates
Amount of heat gain from people
Heat gain from unregulated energy
External temperatures

It is likely I have missed things off, but you get the idea.

Unregulated energy as described above is the remaining energy used (mostly electrical) from things such as computers, photocopiers, cooking etc. This can contribute a large proportion of the building total energy use, but is not taken account of as part of the CO2 emission.

Unregulated energy in reality is the single biggest potential for errors between predicted and actual energy demand. SBEM makes an assumption on the likely amount of unregulated energy along with the hours of operation.

The reality of any buildings energy use is how the building gets used. SBEM allows no scope to make this bespoke to our particular client. How the building is managed and operated makes such a huge difference in energy demand. Along with this, we all know different people like different temperatures, lighting levels and amounts of fresh air. This level of uncertainty caould never be factored in to a energy use calculation.

Take a school as an example, SBEM will have a set of hours from which the school is open, these can’t be changed. So, if we know our school will operate late several nights a week and be open weekends for community use etc, SBEM can’t take account of this.

Different teachers will have different preferences for the temperature within each class. Some teachers may switch the lighting off at breaks, others may not. SOme teachers may leave windows open, others may not.

Also, some buildings are well managed, all lights are switched off on a night, windows closed computers shut down etc, but this is not always the case. All these factors make a huge impact. The management of a building is something which could never be taken account of from an energy use point of view.

So, are other modelling tools available, well of course. Should a client really want an accurate assessment of the predicted energy use of the building, they would need to provide a massive amount of information. This information would then need to be input into a model, and a huge modelling exercise undertaken by the services engineer and then, it would still only be an estimate. The reality is a lot of the information the client would need to provide would not be available, such as;
Energy use of computers, printers, photocopiers, monitors etc
Exact hours of operation of the building
Exact occupancy patterns of each room
The management regime of the building, i.e. will all equipment, lights etc be shut down at night? Will all windows be shut and at what time etc

Note, none of the above could possibly be decided on by the services engineer, nor the architect, nor the builder, yet they make a vast difference to the amount of energy a building uses.

Too many variables are involved in predicting the energy use of buildings. The building industry isn’t guilty of building buildings which fail to meet the predictions of the energy use of the building, it is the building users and the management of the building who do not match the predicted patterns of use and management.

What the building industry is guilty of is telling clients we can predict the energy use of a building, when the reality is we just can’t. What we are good at is predicting the energy use should all the variables be as per the assumptions within SBEM, however, this is never likely to be the case.

So when a client asks you to tell him what will the likely energy consumption of the building be, we should all just say no!

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