Flashes of insight aren’t that common. And when they do occur it’s easy to miss them, dismissing our intuition over what may be well-maintained orthodoxies, even if incorrect. But we can’t doubt our hunches when they unrelentingly demand our sustained attention. Such was the case while poring over about 3,000 lines of javascript code in order to create the next, better version of OBJECTIVE’s Interactive Sankey diagram – it became obvious to me:
“What if TEDI (Thermal Energy Demand Intensity) really isn’t an envelope-centred metric after all, but really is just what it says it is – the amount your heating system needs to contribute since gains alone typically can’t heat the whole building?”
Basically the amount of energy your heating systems needs to output, over a year to make up for the shortfall from the sum of internal gains and solar gains, in order to match your total losses. It’s easier to see as a picture, that’s why TEUI3 always had a Sankey-diagram in its user-interface roadmap – because a picture is worth 1,000 words, or in my case 3,000 lines of code.
We could equally call TEDI simply ‘Heating Demand‘, representing the energy your heating system needs to output to match not just your envelope losses, but your Total building losses (less total gains), so that the right side of the Sankey diagram pictured below equals the left, then Energy Balance achieved, Supplied = Lost. Buildings lose energy in a bunch of ways, through envelope components like Floors and Walls and Roofs and Windows and Doors and Air Leakage – sure – we all know that stuff. But it also loses energy through Ventilation Systems, and Gains-that-cannot-be-utilized (ie. opening a window on a balmy October afternoon), and also by System Inefficiencies – such that the small streams of heat that exit the exhaust pipes of DHW/SHW systems and Gas Furnaces – yep even efficient ones like 98% AFUE condensing gas furnaces, and even unrecovered thermal energy lost down our drains – not transmitting out of our building envelope. When you look at TEDI, it clearly isn’t an envelope metric AT ALL! And when you look at it on an energy balance diagram like in PHPP or our Sankey tool – TEDI needs to be on the ‘Energy Supplied’ side, the left-side of the graphs, not on the right, on the losses side. But what if we collect up all the losses through the envelope only, and parse out ventilation (because arguably, that ain’t part of the envelope), what then would you call that group of losses? (ps – admittedly, I have always wrongly thought of these as = TEDI, until now).
I’m gonna go out on a limb and say, since we seem to love 4-letter acronyms that end in ‘I’ – I nominate TELI = Total Envelope Loss Intensity, where TEL is the total, and TELI is TEL/Conditioned Area. I’m not really making up anything new here. This can be thought of as a legitimate set of losses that reflect the total Envelope Losses without conflating ventilation losses with our clear envelope-rating metric. As our principal advisor Dr. Ted Kesik has always said, “…Ventilation should never have been part of TEDI as a pure envelope metric, since occupancies change, and are on schedules, where the envelope is not. To be a measure of building resiliency, we must have a metric that addresses the envelope on its own terms, and improve on that as we cannot rely on ventilation systems in a power blackout…” (I hope Ted doesn’t mind my paraphrasing here – but I’ll ask later ;). Correction: “…Mechanical ventilation is an entirely different kettle of fish. It varies with occupancy and hence demand for acceptable indoor air quality. It has nothing to do with fabric or air leakage (assuming our buildings do not come even close to leaking enough to ventilate the space)…” Close enough? Dr. Ted prefers to call TELI “TEDI à la TEDDY“, but I’m gonna stick to TELI rather than pick a needless fight with the Passive House community 🤪.
There’s something for everyone here, Passive House practitioners already have a set way of determining and using the TEDI metric in PHPP’s energy Balance Diagram, and the rest of us that never wanted ventilation in a pure envelope rating metric now have something else we can use – TELI! We have a balanced ledger. I’m just gonna TELI it like it is!
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