VRF Systems in Mechanical Rooms and ASHRAE 15-2013

[tyler_lux]

Hi Everyone,

I am working on a large 40-story condo tower that is looking at using water-source VRF systems on each floor. The heat pump/condensing unit for each VRF system will be located in a small mechanical room on every floor. ASHRAE 15 requires a dedicated MUA/ventilation system for mechanical rooms. To dedicate a single system for all 40 floors would be very expensive to do. We have looked at possibly using a wall vent between the mechanical space and the hallway to essentially increase the total area the refrigerant could disperse to if the entire 30-35lbs of charge were to leak. We haven't had luck getting confirmation from a code official that this would be ok.

Does anyone else have any suggestions? I have heard of other high-rise buildings using water source VRF where they put the heatpump/condensing units in a small closet in the corridor that is not actually occupiable. The closet has large double doors that open up to the equipment, but you cant actually fit in the space with the doors closed. This may be a way to get around the ASHRAE 15 requirement of a 26 RCL for an occupiable space, but does it get around the mechanical room ventilation requirement?

Thanks for the help!!!

Tyler

 

[willard3]

Why are you trying to avoid doing what ASHRAE recommends?

ASHRAE is a pretty good design guide.

 

[dbill74]

ASHRAE 15 seems pretty clear in its requirement for mechanical ventilation.

To dedicate a single system for all 40 floors would be very expensive to do.

As in one fan pulling air from all 40 floors? Yeah, I could see that being a challenge. What about one fan and inlet louver for each floor?
Per ASHRAE 15 8.11.2.1, you need a detector that will sound an alarm and actuate mechanical ventilation. It's not like the fan would have to be on continuously, only when a leak is detected and concentrations levels are sufficiently high.

The requirement is there for a reason, you don't want a maintenance person to open a door or enter a space with too much leaked refrigerant, you will suffocate him.

 

[tyler_lux]

We are 100% on board with ASHRAE's requirements and are not trying to cut corners to make this an unsafe environment - we just want to be sure we aren't missing any options that have been tried and tested to both follow ASHRAE 15 while at the same time being cost effective.

The VRF system is an alternative to a WSHP system. With the heat pumps you obviously don't run into this issue. The owner is very interested in using VRF, but we are concerned the dedicated ventilation system for 40 floors of small mechanical rooms could possible price out that option.

 

[GMcD]

It sounds a little like you are trying to pound a square peg in a round hole. VRF systems and residential occupancies are risky at best, and I would suggest that a well designed and operated conventional four-pipe fan-coil system would work just as well, on a budget, and have a lot less risk with high pressure refrigerant leaking into a place where people sleep. Many countries in Europe are already getting away from VRF systems due to refrigerant phase-outs and their logical approach of using high performance building envelopes rather than mechanical techno interventions for indoor comfort.

 

[willard3]

By cost effective, do you mean the cheapest?

Cheapest is an open fire for heating and a block of ice and a fan for cooling.

 

[tyler_lux]

Well I am glad to see that the advice so far is don't do it, you're trying to kill people, and move back to the stone age....

This project has been well thought out and engineered up to this point. The VRF system with heat recovery has been modeled to be 30-40% more efficient that a four pipe system. Each floor has a total refrigerant charge in each VRF system of less than 30lbs, well below the RCL limits set by ASHRAE for occupied spaces. If you for one minute think refrigeration systems will ever be phased out in the next 100 years you are off your rocker. At this point the US has zero plans to phase out R410-a.

@Willard3 - Awesome contribution to the conversation, thank you for your input.

I came here because we have designed this VRF system option per the owners request. One of the last remaining issues that we have not dealt with previously was many small mechanical rooms and if there was any other way than a dedicated ventilation system to those rooms. My original thought of using a transfer grill to the hallways/core, which have a dedicated ventilation system, was what I had hoped the discussion would have been focused around. If anyone with any actual helpful advice has something to say, please do.

Thanks

 

[GMcD]

I'm not sure why an Owner/developer can dictate to a professional engineer what system to use, but if you are OK with the potential risks and possible benefits, then fine. I have found that VRF systems tend to be a system where the installer/supplier can then work it into a nice service contract, since the VRF systems and controls are very proprietory, and there are limited installers who can install and commission them properly, so the tender pricing can be variable. While 4-pipe fan-coils and two-pipe package heat pumps are, in your terms "stone age", at least there are a larger number of installers and a very wide variety of suppliers who can provide those systems and components, and as well you can use anyones' controls systems with them. There are some very energy efficient fan-coils with variable speed ECM motors, and if the heating/cooling plant is well designed, I have a hard time believing the amount of energy savings of the VRF system compared to a well designed heat pump or 4-pipe fan-coil system. I agree that the VRF system will be more energy efficient than a conventional water based system, but energy modeling is a great game to play if one knows how to work the inputs and outputs.

 

[dbill74]

I'm not sure why an Owner/developer can dictate to a professional engineer what system to use,

There's more than one way to skin a cat.

There are a variety of reasons why an owner/developer would want a particular type of system. Most often it has to with maintenance, specifically what their maintenance staff are familiar with. Good example of this is school districts. They often have a maintenance department that is responsible for multiple campuses or buildings, a single type of system throughout saves on training and spare parts costs.

Bad experience will also cause an owner to prefer one type of system over another.

My original thought of using a transfer grill to the hallways/core, which have a dedicated ventilation system

My concern for this option would be the possibility that the door vent would eventually get removed, either with a new door or blocking on the louver itself. "It looks bad" or "sound of the heat pump is keeping people up at night". People can be funny about things like this.