Switching from natural gas to electric resistance heating is NOT what decarbonizing is about!

Even in a state with a relatively low-carbon electric grid, electric heat is more carbon--intensive than on-site natural gas combustion. True, it will get better as the grid improves, but neither electric resistance not natural gas is a step in the right direction.

Check out the topics on heat pumps and heat recovery chillers, which are 2.5 times more efficient than electric heat.

Typical hospital design would put an entire floor - maybe several floors - of patient rooms on a single AHU. That makes sense in many ways, of course, which is why we've done that for decades.

Consider a patient tower with long building facades to the east and west. In the morning, we may need full cooling on the east face, so our opportunity for Supply Air Temperature Reset (SATR) is limited. Meanwhile, we're reheating on the west side of the building, because those rooms haven't seen the sun yet.

In the afternoon, the opposite happens, and we're controlling supply air temperature to keep the west side cool, while reheating on the east.

If we're serious about reducing reheat in an all-air system, we have to leverage SATR. Zoning AHUs by building face, maybe serving the east side of multiple floors with one unit and west side of those floors with another significantly improves the opportunity to raise the supply air temperature, reducing both cooling load and reheat. We can do the same with north/south facades. We just need to readjust our thinking to prioritize minimizing reheat.

Do you know any surgeons that want to run the OR colder?

Thought so.

With 20 air changes per hour in ORs, colder is easy. In humid conditions, though, colder temperatures mean higher humidity. The classic solution is subcooling - taking the supply air temperature lower to wring out humidity. We used to do this with DX coils and condensing units, but now mostly do that with a dedicated chiller, usually air-cooled, often with some glycol to allow low-temp operation and protect it from freezing. (For those of you in Southern CA, "freezing" is a theoretical condition where the air temperature dips below 32F. Crazy, I know.)

The reality of OR air systems is that outside air is the primary source of moisture. In the classic system, we mix moist air with dry return air, then subcool that air to dehumidify, then have to reheat to keep from overcooling the OR. Wasteful, both on cooling and reheat energy.

The first improvement is to dehumidify the outside air separately - before mixing with return air. Two benefits - lower cooling load of course, but also a lower reheat load. When we mix 20% cold dry air with 80% return air, we get a mixed air temperature that needs less reheat - and we can cool it a bit (sensible only) if we need a lower temp.

But even better is to do the dehumidification with an active desiccant. Again - this is only the outside air stream. In most cases, that means just 20-25% of the air. Desiccants allow us to dehumidify without subcooling, so we don't need a separate chiller system. Desiccants do need heat to drive off moisture, but that can be done with energy recovered from the exhaust stream. Add some additional cooling at the AHU, and we have cool, dry air going to the ORs - at a much lower energy cost than the old ways.

I've watched a 20,000 cfm air system run at 55 degF 24/7 - even though we had SATR implemented in the control sequence. Why? Because "someone" buried the thermostat in an IT closet. With the thermostat set on 62, the zone was never going to be satisfied. So, that call for additional cooling kept the entire AHU operating at its lowest setting.

The solution? A little attention from the humans tending the system. First, of course, setting the IT closet to something reasonable - and limiting what the well-meaning IT tech could set it to. 78 is not an unreasonable temperature for an IT closet. Getting control over setpoints is crucial to SATR strategy.

In most systems, there are lots of zones where we don't really care if the temperature goes a few degrees above setpoint. Being selective about which zones are allowed to take control of the supply air temperature can make a difference.

Then there are the zones that just always need full cooling. This is where design engineers have a role - raise the airflow setpoints on these spaces so that they can provide adequate cooling at higher temperatures - maybe 60F, or even higher. There's some judgement involved here, but designing for a higher flow of warmer air in high- or constant-load zones can make the difference in cooling and reheat energy.

I often hear that there is a conflict between SATR and supply fan Static Pressure Reset (SPR). SATR tends to raise the SA temp, causing an increase in airflow, while the goal of SPR is to reduce pressure (and fan power) in response to the throttling of VAV terminals.

The reality in our healthcare buildings is that we are very often delivering more air than we need for cooling, based on code-required air change rates. When that is the case, the opportunities for reducing airflow are limited. If we're stuck with delivering excess air, then we have to raise the temperature to prevent overcooling. That can be done at the reheat coil, of course, but it's way more efficient to just do less cooling in the first place - by raising the supply air temperature at the AHU.

SPR functions independently to reduce fan energy as VAV terminals back off from full flow. There's just no reason not to do both - especially in healthcare.