Updated: Dec 8, 2021
A significant shortcoming in the face of the coronavirus pandemic and the volume of COVID-19 patients in need of care is hospital bed availability. More specifically, we must also deal with the suitability of these beds for treating a respiratory virus. How do we find a place for those patients, and how do we do so without putting the staff treating patient and rest of the healthcare facility in undue jeopardy? One answer is to create a negative pressure ward through manipulation of existing system controls and balancing.
Step 1: Location Selection
The first hurdle in successfully converting hospital space for Covid-19 patient care is selecting a location which will readily lends itself to the transformation. Aside from the obvious question of how large of a suite would be needed, it’s important to review the existing HVAC systems and the existing spatial relationship with the rest of the hospital’s general patient care and public spaces.
Because the driving factor in creating a negative pressure ward is removing contaminants and contaminated air from the space, the ability to fully exhaust all airflow is paramount. Even HEPA-filtration of recirculated air will not provide the 100% protection from coronavirus contamination. Most commonly, a dedicated outdoor air system with space terminal units will not be a good candidate for conversion. While is could be used, care must be taken, in that the re-circulating space terminal units will become contaminated themselves, making maintenance much more complicated. This should only be seen as a very short term solution when need is extremely urgent. A complete variable air volume system is likely to be one worth exploring, as these are typically designed to be able to operate at full-flow and full-economizer simultaneously.
Not only is it important to remove all contaminants from the space but it’s equally important that the removal be controlled: contaminated airflow must be ejected from the building to protect neighboring suites and spaces. Choose a location which is already somewhat isolated from the rest of the hospital. Intensive care units are often built this way to begin with. They often have one way in and one way out, versus those with several secondary exit(s), which makes re-balancing a less complicated exercise and monitoring much simpler.
Step 2: Air System Controls Manipulation
If the space chosen has full direct digital controls, it’s possible to do most work from a facilities management system control terminal. First and foremost, the AHU should be placed in full economizer mode and the mixing box locked out. Depending on the individual circumstances, consider controlling to a constant volume flow to ensure the newly created isolation ward will remain negative to the rest of the hospital. Consider the alarms installed in the system and consider deactivating any which might shut down the all-important exhaust fan. If you’re outfitting a space that happens to have return air control as well, revising flow setpoints from the front end is the quickest and simplest way to attack the problem.
Step 3: Air System Local Re-balancing
There is still work to be done in the space, however. It’s likely the return system (which is now effectively exhaust) must be re-balanced to create that needed negative pressure relationship. Air change rate is now driven by exhaust flow. Consider reducing supply air as much as possible. If the system is to be in an isolation set-up only in the short term, consider the time of year and the expected load. It may be that the needed supply flow may be lowered even further.
Review the space for pressurization concerns such as open ceilings, transfer grilles, and door undercuts. During planning, consider determination of an “acceptable” reading for the negative airflow relationship. This will hopefully expedite testing by eliminating uncertainty.
Step 4: Testing and Verification
Lastly, the conversion should be confirmed through testing and verification. It’s unlikely that the rooms already have differential pressure measurement provided, so a hand-held pressure-test device is a good method. Upstream, hand-held smoke generating devices will prove flow direction where pressurization isn’t possible or practical.
There are many other considerations outside the scope of this discussion. If the AHU is to be set up for 100% outside air flow in the long-term, modifications must be made to ensure its year-round performance. There must be a plan in place to bring the space back to normal intended use. The staff at the facility must be trained to operate under this revised intent. The list could go on.
As the pandemic evolves and hospital resource peaks are hit and capacities exceeded, expediently and resourcefully using what we have to meet changing needs will make a world of difference.
Terence Boland, PE Principal & Mechanical