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Unit ventilators

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This unit ventilator has been retrofitted into a classroom.

By Roy Collver

Hydronic heating systems are often dismissed for sometimes being incompatible with cooling and ventilation requirements. However, there is a hydronic heating cooling and ventilation solution that has been used successfully for years in commercial applications – especially in classrooms.

I believe they should be much more popular, especially for offices, health services and vet clinics – many applications come to mind. These devices – unit ventilators – can be simple heating and ventilation units operating from a wall thermostat with a manual outdoor air control dial, or they can be expanded with cooling coils, ECM fan motors and sophisticated automatic indoor air quality (IAQ) controls and sensors.

From what I have seen online, and in the field, common capacities range from 500 CFM to a little over 2,000 CFM, depending on the manufacturer. I see no reason why they can’t be made in smaller residential sizes and configurations and can make a strong case for manufacturers to develop unit ventilators to service apartments, condominiums and hotel rooms.

Heating and cooling loads are shrinking very quickly as building envelopes are increasingly sealed and insulated. These energy saving initiatives are resulting in the need for increased but properly controlled ventilation methods. The hydronics industry needs to develop practical HVAC equipment to service spaces with these reduced loads.

Air filters and dampers

The basic unit ventilator design consists of a fan cabinet with air filters and a set of modulating dampers. Air from outside is mixed with return air and heated or cooled as it passes through the active hydronic coil.

The diagram I prepared (Fig. 1) illustrates a vertical upflow design where the supply air can be ducted to diffusers, the same as with a standard air handler or furnace. The primary difference is the ability to introduce and temper outdoor air from just a small amount up to full room air bypass and free-cooling.

The many variations available from different manufacturers include ducted or direct-to-the-space horizontal and downflow designs. Type “unit ventilators” into your search engine and you will get an idea of what is available and who is making them.

This is the same retrofit installation, showing the hydronic supply and return piping.

Control technology makes a big difference in the utility of these machines, with CO2 sensors and related software/hardware commonly used in classrooms to position the dampers where needed. Smaller units need not be overly complicated, and manufacturing in volume can bring some sophisticated IAQ technology to the table at reasonable costs.

As long as standards for quality and efficiency remain high, this type of device could become popular, with some innovation from builders, installers and the engineering community.

Noise complaint

One complaint that comes up repeatedly in small living spaces is the issue of noise. It is a universal complaint that many hotels and motels choose to heat and cool the guest rooms with ridiculously noisy thru-wall heating/cooling units. You pay money to rent this little room usually for two reasons – to get a good night’s sleep, followed by a hot shower in the morning. But right next to the bed you have a noisy, banging, wheezing and groaning piece of value-engineering that is guaranteed to keep you awake all night long.

When the hotel pesters you to fill out a customer survey detailing how much you enjoyed your stay, please call-out the crappy heat/cool unit (but don’t hold your breath expecting a response). At least you can choose to not go back to that hotel, but someone purchasing a condo or signing a long-term lease is going to be stuck with the noisemaker for the duration.

Unit ventilators have traditionally been designed for school classrooms as well as hospitals because they are quiet by design. If manufacturers take up the challenge to design smaller units for smaller spaces, they must make quiet operation a priority, which could be a challenge in some high-density urban environments since each unit has a wall penetration to the outside. Acoustic liners and other methods of sound attenuation will be required.

In this classroom installation, supply air ductwork has been installed above the ceiling grid.

It makes little sense with today’s reduced heating loads to provide individual combustion appliances for heating smaller living spaces in multi-family buildings or row housing. One solution will be to provide central boilers and/or heat pumps to serve multiple units with distributed heating, chilled and domestic hot water, and take advantage of the ability of hydronics to move energy through small non-invasive piping systems. Unit ventilators could be an ideal way to provide quality HVAC for these types of buildings.

I am not a big fan of huge central distribution systems, so providing individual systems floor-by-floor, or by block of row houses, can simplify things mechanically and avoid the need for large piping and custom accessories (pumps, valves, etc.).

Relief air

When using unit ventilators, exhaust air needs to be addressed. Without some kind of relief air, unit ventilators will pressurize today’s tightly sealed buildings. Some units come with a form of pressure-activated damper (think barometric) that exhausts air directly back to the outside from each unit as positive pressure builds.

Another method being used is to have central modulating dampers activated by pressure sensors, relieving pressure from a number of units in a common pressure zone. Disadvantages of that approach include the requirement for transfer ducts from each unit into a hallway for instance (fire separation becomes an issue), and the challenge of heat recovery from the exhaust air.

A number of potential solutions suggest themselves including integration with bathroom and kitchen exhaust systems. The ideal is to have a neutral pressure balance in the living space, and there isn’t one simple method that will serve every application. The type of structure and its use will determine how to deal with this issue, with fire suppression in multi-family construction being the priority.

There have been many methods tried over the years to heat and cool smaller spaces – high velocity hydronic heat/cool units and mini split heat pumps come to mind. Using small unit ventilators would be another variation in the search for an ideal approach and would keep hydronics in the game by providing required ventilation.

The ascendancy of air-to-water and geo-exchange heat pumps will favour low temperature hydronic solutions, and heating coils will need to be sized accordingly. Domestic hot water can be provided via indirect storage water heaters or on-demand units depending on load profiles. A two-pipe distribution system will work well in applications where heating or cooling is a seasonal switch-over, but in many buildings a four-pipe system will be needed so that individual units can choose between heating and cooling at will.

Brian Kingsley of Rocky Point Engineering, Nanaimo, B.C., commissions a unit ventilator.

Multi-family buildings currently require the services of a professional engineer, and that won’t change, but the advantages to the builder will be the reduction of intrusive ductwork and simplified installation.

Every time I attend an industry tradeshow, I seek out the manufacturers who are displaying smaller hydronic air handlers to ask them about their products and where they are being used. A common question is “what is the smallest unit you make” – usually followed by “could you make a smaller one,” “could you add a cooling coil,” and “could you put in an outdoor air intake damper.” The answers to the last three questions are usually “yes, yes, and yes.”

Perhaps you are a manufacturer reading this and you already produce such a machine. If so, please drop me a line so we can talk about it and discuss where it is being used. If you don’t produce such a machine, but could do so – now’s your chance, the commercial guys are selling all kinds of larger units – there’s a niche for you too.

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