VRF vs. Chilled water system

[ssn61]

I have designed a chilled water system for a HVAC renovation of a school that has existing chilled water system. Classrooms used to have unit ventilators and I recommended to replace them with new ones. One of the mechanical contractors that are also engineering compnay has made a comment as why not go with a VRF (Variable Refrigenrant Volume) system. The HVAC and electrical design is done and out for bid and kind of late to redesign. Any thoughts as why we need to go VRF system?

 

[Drazen]

that has becoming typical case in industry nowadays, sometimes i think whatever i choose contractor will propose opposite, so it is very important to get client's comprehensive design brief as early as possible.

on good point against vrf in your school is that school management will be bound to one vrf manufacturer, the one they choose to install, while in chilled water system each and every component is replaceable with many alternative products all up to zone controllers.

that can be remedied by good project engineering negotiations for long-term maintenance contract, but public entities are rarely doing that job independently.

 

[EnergyProfessional]

You should weigh the pros and cons adn also perform an energy simulation to see if VRF actually saves you anything. i large applications like your project the chiller can be better and you also can recover heat and use for perimeter heat if the core needs cooling etc.

I think VRF is good in smaller applications. Depending on your climate you need a separate heating system.

VRF can work with your (Honeywell or other) room thermostats. At least Mitsubishi ones. However, the way VRF are advertised they pretend you have to use their controls... but their controls don't allow DOAS etc.

As Drazen pointed out, you marry your client to a manufacturer. In addition you have condensate lines throughout the building, refrigerant, and have filters to clean in each zone or hallway.

It really depends, there is no black and white when it comes to any HVAC technology. If your client already has room for AHU etc., a chiller system is more likely. i currently work on a small project where there is no AHU space and i would have to create one (and reduce other spaces), for that VRF may be the lesser evil.

It all depends, but the contractor likely is the least knowledgeable and most biased person to be your sole source of information.

show me one single HVAC system that is the best for every application....

 

[NeoMeche]

My two cents on this system comparison.

You should weigh the pros and cons adn also perform an energy simulation to see if VRF actually saves you anything. i large applications like your project the chiller can be better and you also can recover heat and use for perimeter heat if the core needs cooling etc.

VRF systems offer heat recovery options as well. Unfortunately, my office only uses Daikin so I'm unfamiliar with what the other manufacturers offer in this area.

As Drazen pointed out, you marry your client to a manufacturer. In addition you have condensate lines throughout the building, refrigerant, and have filters to clean in each zone or hallway.

You'll need condensate drain piping to each evaporator, AHU, fan coil in both chilled water systems and VRF.

It really depends, there is no black and white when it comes to any HVAC technology. If your client already has room for AHU etc., a chiller system is more likely. i currently work on a small project where there is no AHU space and i would have to create one (and reduce other spaces), for that VRF may be the lesser evil.

Both options allow you a number of options for evaporators. I have not seen a large difference in cabinet size for evaporators between the two systems. They'll both just essentially be a small fan, coil, filter, control box, etc.

One of the mechanical contractors that are also engineering compnay has made a comment as why not go with a VRF (Variable Refrigenrant Volume) system.

This is surprising to me. In my experience, I still find contractor knowledge of VRF systems to be very limited.

The primary advantages to a VRF systems are as follows.:
-Less rigging, a lot of condensing units weight less than 1000 lbs
-No boiler plant is required, like Herr said, this is dependent
-Refrigerant piping is smaller than chilled water piping
-Less landmarks concerns versus a chiller
-Less structural concerns versus chiller system.

The disadvantages are:
-Installation & Maintenance: Possible contractors may
not be familiar with system.
-Refrigerant piping run lengths are limited
-Potential for refrigerant leaks.
-Large quantities of refrigerant within the building.

That's the short list of advantages. I could tell you a bit more if I knew if it was a water or air cooled chiller.

 

[EnergyProfessional]

but the AHU is typically located far away from the occupant space. for the VRF you have condensate in each space (or nearby spaces if you use ducted units). Those also have a filter that needs maintenance.

sure, the AHU had filter and condensate as well, but this is confined in a mechanical room and only one, not as many as you have zones.

Not a deal breaker, but something to consider if maintenance is supposed to go into classrooms or hallways during schoolday to service those.

 

[dbill74]

Another thing to consider is your outside air source. If you don't pre-condition the outside air with a DOAS, you might want to make sure the zone AHU's are properly sized to handle the outside air load.

My office is currently working on a hotel design using VRF and in a recent coordination meeting I was forced to relocate domestic water piping out of corridors to above the guest rooms because the OA duct, VRF control boxes and piping was filling the above ceiling space. So the "space savings" VRF manufacturers promote is ... questionable.

By the sounds of your original post, the design is done and to switch to VRF would mean a complete redesign of the HVAC system adding time and redesign costs to the project (you don't want to do a second design for free do you?). To answer the contractor's comment "why not go with VRF?" Why go with CHW at your renovation project? Surely there was some reason to go with CHW versus VRF or any other system.

 

[Drazen]

i designed number of vrf's in hotels, and that was almost always on client's demand, who was approached by very aggressive vrf sales manager even before client approached me.

first issue with vrf's in any but smallest hotels is the limitation related to sleeping room volume/refrigerant charge ratio, which imposes use of number of systems instead of single central plant. that's far away from any space saving.

there are many other limitations as well - use of vrf for domestic water heating, for handling outdoor air... i asked almost every vrf manufacturer i am in contact with about when they plan to introduce indoor unit which is able to treat 100% outdoor air properly, and only daikin mention they work something on that, but even they are very reluctant to make such a unit.

currently daiken offers ventilation units which treat outdoor air only as three speed fan-coil type unit. if you want to install dx evaporator in some ahu, you can do it only with dedicated split unit, together with special interface applied.

for domestic water heating mitsubishi electric offers local heating units that are too large for one rooom, and two small for some central application, so they are very inconvenient in terms of adequate space.

vrf's are good for smaller application, but modulated chiller are coming into scene, and vrf guys will have to work hard to retain their advantage.

 

[EnergyProfessional]

Drazen: I think in a hotel VRF isn't so bad since typically it has a unsightly packaged window unit with electric heat and with guests wanting it cold and hot at the same time VRF can be efficient.
I actually have a harder time imagining to use a chiller and AHU in hotels since space for ducts is more an issue and a recirculating system like VAV would be a problem with people doing all kind of things in the hotel room and that smell going into everyone's room. unless you meant a fan coil unit in each room connected to a chiller, but then you bring that maintenance item back into the room.

but yes ASHRAE 15 will be a problem. Unless they use ducted units in the hallway, which would be better maintenance-wise anyway. I've seen the single split wall-mounted Mitsubishi units all over the Mediterranean, VRF just brings it one step further.

the sales person always will tell you his product is best

 

[RossABQ]

When the Contractor proposes an alternate, it is rarely because it will cost the client less, and usually means more money in the Contractor's pocket, one way or another.

 

[cry22]

1. Ross got it right.
2. I say to the contractors - Hey mister, you're paid to build, not for opinions - I am the opinion, so stick to building. Once a design is done, there is no going back, You should not entertain contractors' ideas, they will play smart ass on you all day. Stick to your job and I will stick to mine is the deal (even if he is right). The quote if from Orson Wells in Citizen Kane - "Hey mister, you get paid for hauling or for opinions"
3. What is your building size?
4. which climate are you in?
5. Heat pumps in general are rated for 47F for heating then the back-up electric heat kicks in - so VRF will use more electricity
6. Heat pumps cannot cool down 100% outside air for the DOAS unit - they cannot dehumidify sufficiently.
7. VRF is good for small residential, supplemental cooling in restaurants, etc.. not really for schools which require substantial amounts of outside air.

 

[EnergyProfessional]

"5. Heat pumps in general are rated for 47F for heating then the back-up electric heat kicks in - so VRF will use more electricity"
not entirely true, they lose performance below the rated temp, true. but they work well below that. Some VRF manufacturer also have hyper-heat etc. versions working at much lower temps (Obviously COP will suffer).
If you are in the north, you want some gas fired backup.

 

[HVAC68]

Questions.

1. How's the fresh air being taken care ?
2. What capacity are we talking about ?
3. Do you also have other areas like accommodation, dining, etc., attached to the school ? If yes, a chilled water solution would be better.
4. Several indoor units would require interference and entry of maintenance staff/technician in classrooms as well (maybe)

HVAC68

 

[Waramanga]

Vrf cons: Poor control on part load. Limited controls options. Loose gas, loose whole system. Gas charge in small room issues. Cant handle high oa loads in cold climates. Shorter lifespan.
Vrf pros: cheaper. Good efficiency. Heat recovery option.

 

[urgross]

Is the mechanical suggesting this rep'ing for a 2-pipe or 3-pipe system? Some people will do things not well thought out in order to get a commission. I've stopped doing business with more than one firm for this reason.

Seems to be late in the game for value engineering, especially if there is a deadline for finishing construction. Liquidated damages should be a concern for changing directions.

 

[317069]

You could check the contractor suggestion and find out what is the different bewteen your design and his suggestion, he might be right, you are still in bid stages and changing it now is better than later.
you said it is a renovation project and there is an existing chilled water system, are you going to change the chiller, piping ..etc or just rooms ventilators.
you could study the contractor suggestion inside your company, because as you said contractor is an engineering firm too, then he will find his way to your client and then you have to answar your client about why did not you go with this option.

 

[CarolinaPE]

Could be they are the only contractor in the area with the expertise installing a VRF (hoping to narrow their list of competitors). Sounds like a question proposed from a pre-bid meeting. Since he is not paying your design fee I would consider that comments from the peanut gallery.