figure 1. Winter comfort zone for indoor air.

Maintaining minimum requirements may involve more than you think. However, while there are some areas - such as certain sensors - where you won’t want to skimp, owners do have some flexibility in reaching acceptable performance with lowest life-cycle cost.

Sometimes a requirement for humidification in a building is poorly defined, over-designed, and either over or underutilized. Humidity levels are notoriously difficult to control and maintain in buildings of any type. In particular, older buildings with leaky envelopes can lose so much moisture so quickly that it is almost impossible to maintain acceptable relative humidity levels. Fortunately, the human body has a great tolerance for comfort, and we are used to dry skin and static discharge in winter. So perhaps that is why building designers, owners, and operators are happy to leave humidification to specialized buildings like data centers, pharmacies, hospitals, galleries, and museums - especially in the Northwest, where extremely dry conditions only last a few days a year.

For the purpose of our discussion, we will focus in humidification in health care applications - hospitals, laboratories, clinics, and extended care homes.

Comfort Guidelines

ASHRAE recently published “Standard 170-2008 Ventilation of Health Care Facilities,” which requires many areas within hospitals, like operating rooms, to maintain humidity levels between 30% and 60% rh at temperatures between 20°C and 24°C (70° to 75°F).

The standard quite closely follows the winter thermal comfort guidelines also developed by ASHRAE many years ago. The lower boundary of 30% prevents discomfort from cold and dry conditions that exacerbate many health conditions. The 60% rh upper boundary is meant to reduce the opportunity for mold growth. Stagnant warm and humid air above this control region is a breeding ground for mold growth and mildew, the spores of which when airborne can cause respiratory problems. The psychrometric chart in Figure 1 illustrates the accepted winter comfort zone. The psychrometric analysis is used to calculate the relative energy in moist air, among other valuable physical properties, as we will see in the next example.

While it is nice to have a broad guideline like this to allow some flexibility for control of conditions, why would we ever want to maintain temperature or humidity at the higher rh and temperature states, and what would be the cost to do this? Well, the answer to the first part of the question in winter - we don’t need to unless we are getting too many comfort complaints or have special medical conditions that require it. For energy cost considerations, the acceptable low region should be the target at all times during occupied periods.

Table 1 illustrates the acceptable scenarios for a cold day for a 40,000 cfm system humidifying outdoor air at realistic dry winter conditions. As can be seen, the difference between the low end of 20°C and 30% rh and 24°C/60% rh is a substantial energy cost increase from $290 to $819 or $529/day. The latent energy requirement nearly triples between low and high conditions.

Humidity Requirements

Unfortunately, we can’t just keep the humidity and temperature in the optimum energy state for various reasons - not the least being the inability to measure and control accurately with standard HVAC-grade sensors and controllers. So what is required to humidify a space like an operating room or a delivery room?

Well, there are several systems that can be used, but the simplest method for general humidification - though perhaps not the purest method - is the use of plant steam piped into a humidifier dispersion grid located in the supply duct work of a major air-handling system. The addition of low pressure (10 to 15 psi) steam from a humidification coil enables water to be injected and evaporated into the airstream within the minimum length of duct run. If the coil is large enough and properly designed, and the air velocity is kept low enough (1,000 ft/min or less), then this method will evaporate steam in distance of less than 6 ft (1.8 m). Good dispersion will avoid the saturation of any duct insulation and filter media.

Table 1. Acceptable variation in rh and temperature vs. cost and energy for a 40,000 cfm 100% outdoor air system.

Steam Concerns

Boiler water chemical additives are often cited as a concern, but according to ASHRAE, plant steam can be introduced into health care spaces - as long as it meets the USDH FDA requirements of 173.310 in the Code of Federal Regulations CFR21. This code, of course, cites numerous possible chemical additives and their limitations in the steam, especially if the steam comes in contact with milk or milk products such as in food preparation areas.

Fortunately, these requirements are reasonably easy to meet, but a chemical analysis of your plant steam is recommended. The requirements can be found at Health Canada discusses safety “Guidelines for Incidental Additive Submissions” under its Food and Nutrition division (see and mentions boiler water treatment compounds, but unlike the FDA document, no direct reference to humidification is made.

ASHRAE Standard 170P restricts the use of reservoir and pan-type humidifiers because these systems contain standing water that is open to air and can induce mold and bacteria growth. Other types of closed steam-injection type humidifiers are acceptable. Steam-to-steam humidifiers that use a heat exchanger supplied with plant steam or electric heating elements to boil steam from a clean water source like a water purification system - can also be used, but this can be very expensive. Use of domestic (potable) water for this type of application can lead quickly to maintenance problems if mineral levels in the water are too high and cause buildup on control components like fill valves.


In some cases, humidification may be required to be delivered at the source, like the example given above with a large general supply fan, or at the load like added to air supply at the branch that serves the space - like an individual operating room. The problem with this method is that the owner is required to maintain several small humidification systems and service each one.

For example, at Vancouver General Hospital in British Columbia, Canada, there is an operating room (OR) suite of 20 ORs fed from four main AHUs (serving the ORs and other less critical areas). The AHUs have general humidity control, but in addition, each OR had its own direct steam humidity control valve and dispersion grid. After a few years of failed attempts to control the humidity in the spaces, all of the humidifiers were abandoned after problems were discovered with poor absorption of the moisture into the air that resulted in wet steam condensing on duct surfaces and supply grills.

As a consultant who looks at mainly retrofit projects in health care and other commercial and institutional buildings, I have seen numerous abandoned humidification systems that were either too difficult or too costly to maintain and control.

figure 2. Minimum elements for general humidification and control.

At the very least

So if you plan to maintain acceptable control of temperature and rh in a critical space, at the same time controlling operating and maintenance costs over the long term, then what do you need to do at minimum?

The minimum number of control elements required are actually more than you may think or want. You need a reliable source of steam, a good control valve that is sized well to supply only the minimum requirement at the peak conditions, and at least three and perhaps four rh sensors - a supply air humidity duct sensor to ensure you don’t overspray the air, a high-limit rh sensor mechanically interlocked to the control valve to cut the humidity supply in case of a failure, a return or exhaust air duct sensor, and possibly a space rh sensor to reset the supply rh. Figure 2 illustrates the required elements.

Of course, the whole thing will be controlled from the building control system, and appropriate failsafes and alarms need to be set up to capture potential problems. And naturally, each control element will have its own challenges during its lifetime (which needs to be as long as possible). The supply air rh sensor needs to be reliable or it will saturate one day when too much moisture hits it, and never recover. The high-limit rh sensor should be a manually operated device that only provides a relay dry-contact output to open the control circuit. The return or exhaust air rh sensor is probably the most critical since it is one used for rh the control in the end. Use a high-quality sensor for this - even if it costs $1,000, it will pay for itself easily in a large system.

The supply air rh sensor is often used to control the steam valve, but it is ultimately reset by the return/exhaust air element. Unless the OR has independent humidity control, the rh sensor in that space will just be an indicator of conditions. Keep in mind that if the space is cooler than average return air, the rh may be quite a lot higher, which is yet another reason to keep rh levels at minimum acceptable in the supply.

The use of these systems is obviously very seasonal, so it is a good idea to isolate the whole system back at the source in early spring so that no heat is wasted. Just letting the control program shut the valve on an outdoor temperature limit is not enough.

So in conclusion, some humidity control in these critical environments is important, but a lot of flexibility is allowed to help the building owners and operators cope with issues like steam quality, controllability, operating cost, and energy usage. Keep it to the minimum, keep it as simple as possible, and use high-quality control elements in order to enjoy an acceptable environment in your critical spaces. ES