Can Under Floor Air Conditioning Save Cost, Time, and Energy?

Underfloor hydronic heating works well, partly because convection conditions are favorable for moving heat into the room air. But I don't think underfloor cooling would work as well; it would create a zone of cool air near the floor, with scarcely cooled warm air above. Maybe there would be such a thing hydronic ceiling cooling? (Just a speculation.)

Your comments are useful, Thank you.

There are chilled ceilings but these have a problem with condensation problems and need special control when the humidity levels are high.

Underfloor fan asisted air terminals push the air gently to high level and the draught is almost un noticeable. These terminals have on board controls that allow users to adjust fan speed and temperature to suit themselves.

If, as office layouts or even bedroom layouts are re arranged the terminals can be easily moved to another location in a minute or two. These sytems rely on the use of raised access floor made of 600mm or 2ft square panels laid on a substructure between 200mm and 400mm high. The cool air passes through the void under the floor panels without the need for ducting and that is why the terminals can be easily relocated.

Sometimes engineers use up to 100% outside air, when the temperature is suitable, and this does away with the need to run expensive refrigeration chillers. (Free Cooling) At night this air can be circulated through the floor voids to cool down the concrete slabs a degree or two and this again helps to maintain a good office environment almost free of charge.

Your experience with hydronic heating is more related to pipes laid in the concrete or cement screed I think and I totally agree that if you passed chilled water through the pipes this would lead to cold feet and hot heads which everyone knows is an un acceptable environment for user comfort.

Thanks for your comments.

The only place I have seen this done is in a computer room of old where there were large servers. The access to the cable was the issue to make fast repairs or changes. It's much faster then fishing them through a recessed ceiling. Plus you don't have the cable hanging down from above.

With the changes to computers in storage volume it is very rarely done.

Construction cost I think makes less likely to be used as the floor needs to support weight. A drop ceiling supports little weight are made of light weight materials.

Dear OZZB

Thanks for your comments. I take on board your comments about access to cables being far easier and faster than climbing into ceilings.

In Europe I know Raised Access floor is now a standard element of most intelligent buildings because most offices now have a huge demand for data cable throughout the office and the floor makes an easy home for it as you explained.

The extra cost of the raised floor is part saved by not needing to lay a cement levelling screed and then the major cost advantage comes from the 10% height saving.

If you think of a 50 storey tower and then think of all the materials needed to build 5 floors such as Glass facade, lift shafts, stairwells,structural columns, ventilation duct risers water risers and electrical risers I have been advised by cost consultants that there is a simply huge saving to be made in construction cost and for the green issues think of all the embodied energy saved.

In China they say using these systems "BUILD 10 GET 1 FREE!!!" not bad

I question the cost savings. The building still has to have lighting and the ceiling has always been the choice spot for that. What little space you save from removing the duct work out of the ceiling and pushing it up. You take in putting it under the floor. Plus optimally the returns work better in the ceiling. Might save money in duct work but eat it up in the floor.

As far as construction cost in savings because of building height what is the elevation of the raise floor off the slab? The ones I have pulled cables through standing on the slab the raised floor was about at my knees.

Generally the raised floors used for offices with just cable ways is around 150mm to 200mm clear void. Underfloor Ac systems work well in voids of around 200mm to 350mm. Dont forget that atleast 100mm has been saved by not laying a levelling screed and the associated weight of that screed reduces the cost of the structure a few cents.

The air velocity is very low and the floor void works with a pressure of around 10 pa compared with a ducted system of 250-750pa

The height saving argument and the on going cost saving needs to be looked at on each project but where you are already planning to have raised floor for easy cable management of say 200mm if the floor is then increased to 300mm to further permit AC to flow that 100mm increase compared often with 500-600mm duct space above light fittings offers a net saving in the region of 400 mm. Most office buildings are around 3800-4000mm floor to floor and so the net saving is around 10% and the cost consultants in Europe quote savings of around 7-13% construction cost as a result.

I have to say I am delighted by the response to this and am learning a lot about the Stateside perceptions. Thank you all so much for taking time out to comment.

100 mm of leveling screed? Sorry contractors here level concrete as it's poured. Usually no reason to back and pour sceed over slab to level it. 100 mm is about the thickness of the slab to begin with. Your not saying that your not pouting any slab at all?

I am delighted to hear this.

If the slab can be poured and power floated immediately that seems perfect to me. One reason for screed was to submerge electrical conduits so that power could feed to outlets across the the slab in large floor plates to come up by desks, not flexible though and size restricted so future expansion limited. This then gave rise to the use of the computer floor throughout the office space for tenant freedom. The other reason was that often floor plates are made with pre cast planks which are not very even when laid down due to different camber and surface roughness and the screed covers over all this while infilling the joints bewteen planks. Anyway this is a small item in the overall cost

I guess USA cant see the point in these floors but as the whole of the rest of the world does I find it quite amazing.

Have a nice day!

The one thing that a computer room with cold air pumped through a raised floor and into the equipment cabinets can improve efficiency is that the equipment heated air can be either plenum directly into the drop ceiling air return or just naturally flow along the tile of the ceiling until the warm air return captures this air. The air space that people occupy in servicing this equipment does not get this heat load directly.

Dear Redfred

I totally agree. The raised floor in the computer room provides an ideal means of distribution for the cool air. The problem is that often the computer room has a ramp up onto the floor from the office slab and the cables distribute at high level through the office and then down poles to the desk, that a another contributor told us are difficult to access and maintain.

By using raised floor throughout the space you avoid ramps, improve safety for escape etc improve flexibility in the use of the space and maintain all cables easily at low level and bring cool air to the people in exactly the same way as you do for the computers. The big difference being that the colossal amount of air flowing in computer rooms is often introduced through large floor grilles without control or directly into the equipment itself whereas for users the Fan assisted terminal allows the user to adjust the flow close to them to suit their personal desires.

Thank you for your input.

The only places I've seen cold air pumped through flooring was at facilities with architecturally planned computer rooms. So no ramp was required for the removable floor level was the same elevation as the fixed floor levels.

Dear Redfred

This is fascinating stuff. What do you suppose was the additional cost to prepare such a slab and what was the effect on the headroom and services on the floor below.

In multi tenant space where all tenants want server rooms on their floors and not necessarily in exactly the same spot as the guy below how would that work.

Thanks again for your input- Its all most helpful.

All of the locations that I saw a computer floor with cooling and cable routing were in purpose built buildings with only one tenant. The architectural economic and spatial impact of doing things any other way were never a concern of the architect since this was a requirement of the building design operations.

Have you ever been in a "mobile" home, IE trailer park? Registers are on the floor in most that I have seen and they have no attics to speak of. This leads me to believe there is a cost savings involved.

There is a lot of talk about "chilled beams" which I guess are in the ceiling.

I hope the beams don't drip condensation.

Many modern Mobil homes and modular buildings in the Southern half of the U.S.A. use flex duct in the crawl space beneath the floor. and a Package Unit outside to both Heat and cool air enter the building through registers usually located along the walls, but sometimes in the middle if larger rooms. One can cool the building much faster by opening vents in the ceiling.roof of the larger rooms at least until the naturally heated can evacuate.

This is a most interesting comment.

The speed of warm up using warm air compared to radiant heating has been noticed in Europe as well.

Your comment about opening a ceiling vent is correct but in multi floor buildings of course users can't do this. Having fan assisted terminals however,the cool air can be thrown to high level and bring about a speedy pull down of the temperature in the same way as the heating does.

The systems you refer to make use of ducting to the floor registers and the registers are fitted into a wooden sheet floor I guess and are therefore rather difficult to relocate if the furniture layout demand a location over the top of a register. Raised access flooring in offices allows easy access to cables as well as being like a chess board for moving air terminals and electrical outlets

Thanks for your comment. The speed is what I take from your message.

Thank you for your thoughts. I understand that when cold air is introduced from high level users tend to complain of draught because the neck is one of the most sensitive areas of the body and cold air hits this often unclothed area of the body for both men and women.

Old displacement techniques of underfloor ventilation worked on the principle that cool air was delivered across the floor at low level and body heat and equipment energy warmed this and with bouyancy it would rise to high level where it could take out additional heat from lighting and then be exhausted or re cooled and delivered back at floor level. The problem again was draught so temperatures of supply had to be limited, demanding greater volumes of air to be delivered to achieve cooling. The limit came at about 50-60watts per sq metre. Under floor air conditioning is different in that it makes use of terminals that throw the air vertically and induce room air into the plume and this does away with the draught problem whilst increasing the cooling capacity to normal office demand of around 100-160 watts per sq metre. I understand that is why there is less complaint. This then leads to greater productivity of staff and thus raises another area of cost advantage in the reduction of absenteeism. Thanks again for your input

under the floor .... maybe depending on the conditions of the location. had a customer who decided to convert all their stores to "under the floor air conditioning" seemed like a good idea at first, then the problems with water from condensate and ground water started, particulate counts were very high since alot of nasty contaminates seemed to find it's way down to the awaiting ductwork. like most folks have all ready said, clean computer/server rooms, usually not on the 1st grade of the building and are built up level with the existing floor, are the best to benefit from under the floor cooling.

Dear T-stat

I hope you didn't do the design.

Sounds like he needed a consulting engineer to address some issues.

The systems I refer to don't have ducts, they certainly don't have condensation and the floor voids are far cleaner than ceiling voids and ducts because they can be easily accessed if they do get some debris in and can be easily steam cleaned or vacuumed out depending on the degree.Interestingly the Swedish Clinic for Occupational health have made studies along with Healthy Buildings Inc in the States and the spaces are some of the healthiest ever measured.

Perception and poor design are clearly hampering progress on your side of the pond.

I am very interested by all the rsponses so far as they are all less than positive in my view. Fascinating.

HEY, Don't insult any group of engineers here. Your perception of a poor design should never be based on anecdotal information. To do that shows your complete lack of engineering integrity. While I previously had my doubts about your intentions earlier I am now certain of your integrity.

Why don't you enlighten us then?

cold air falls & so does particulate matter...

ducts take up space, floors have to support weight...

what possible advantages to what you advocate overcomes these very basic constraints?

running Cat5 cable is not difficult in floors ceilings or walls

I'm missing the "huge savings in space"

This reminds me of something I was told long ago about absolute zero ... it is when heat = zero. Heat, as far as we know, does not have an upper limit, but it wants to get back to zero. So the more efficient design, will be one that allows it (heat) to do it's own work.

Hot, or hotter, air expands and rises ... let it. As enerygod says, recover above the 'hot stuff' ... heat wants to go there anyway. Push it from below with the 'not as hot' stuff. The question seems to be, from how far below.

Be aware of material cost savings that lead to false economy with regards to the expected savings over the life span of the structure. You might want to spend a bit more to save much more in NRG over time.

In multi-story buildings, the ceiling above is the floor above. I do not see the material cost savings, but perhaps for the top floor. I can see invading the space below for the benefit of the space above being an issue though.

I think it is obvious the return air should be from a central location above, that there must be some minimum circulation, and that personal zones are needed. The circulation would be the end all be all of any design I think. For a single floor structure, I'd think inverted umbrella with the handle being the return, with well insulated sloped ceiling to the highpoint at the return, the finial, the heat pump/compressor, and the ribs, the conduit to about chair rail height.

In floor cooling is OK only as long as you stay above the dew point. If not you have condensation where it makes a mess. as the dew point here today is between 23 and 18 not much cooling would take place.

This is the rage in certain circles as it is viewed as a money saver but - -

Same for all hydronic whether it be ceiling, walls or floor.

Fan coils are too easy to install and use to go for this.

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Screed level - I had never heard of this until working outside the Americas. Seems to be a thing the Brits left behind. Very few concrete masons in Europe and the 3rd world are concrete masons as far as finishing surfaces - they typically leave 50 to 100mm to be placed on top of the rough concrete. That is finished off with weak concrete made of cement, sand and small aggregate.

Since air supplied to under floor system came from A/C system supply, it is inherently above dew point because dew point occurred on the coil surface.

Actually, by using low temp CHW that I described, it is possible to operate ROOM with 25-26 DB temp and 35% RH which FEELS like 22.5C and 50% RH.

This approach has the added value of allowing lower air flow (greater DeltaT) and reduces thermal-cooling load because room temp is closer to ambient temp, reducing thermal transfer and lowering total A/C load by 12-15%.

I am talking about hydronic - water as was mentioned in my post.

What type of floor are you talking about - a raised supply plenum, or ducts under the floor?

I would think under floor ducts would tend to collect dirt more than over head ducts. I have seen mice poop in underfloor ducts. That means there were mice in the ducts. Not my idea of good air quality.

If there is a floor below the floor, then the below floor ducts might be accessible for maintenance if required, otherwise it might be more difficult to access underfloor ducts.

My experience in design is with above floor ductwork. That's the way it's done here in the US, but underfloor is supposed to save some energy. I think it will have a new set of problems that I don't have experience with.

Hello, Friend its new for me, I also don't know about this. I want to know about this all replies are welcome.

Thanks for this useful sharing.