Fresh air, building regs 10 l/s per person

[Andy W]

I have been to see a customer today regarding installing some casettes and wall mounts and he is asking for building regs of fresh air of 10 l/s per person, I have spoken to my wholesaler and they are able to offer an 8 l/s fan to the fresh air inlet of the cassette but is this enough.

If the air flow of a cassette is giving 381 l/s on medium fan speed how much or what percentage of fresh air on to the casette spiggot is required or am I looking at it totally wrong, I seem to remember that 25% fresh air was what we used to base things on some years ago.

How would you put fresh air in a room containing a wall mount, what size duct and fan for what size rooms, are there any tables for selection and would you operate the fan from the light switch when the room is occupied.

[The Viking]

Ohh, enter the dark side.......

Generally the easiest thing to do is to fit a lossnay type of unit. You can even get ones that looks almost like wall-mounted a/c units.

If you speak to the supplier of the cassette, they should be able to provide you with data of how much fresh air you can put through the cassette (be warned, if it's freezing outside you will have to pre heat the fresh air).

[TRASH101]

we used to introduce fresh air independantly whenever possible for these reasons. (are you sure its 10 l/s? we always designed on 8 l/s)

1 the indoor fan is not fixed speed therefore minimum fan requirement of make up fan must be 8l/s when ac is off.

2 the volume of air increases the heat/cool load because it will invariabley exceed 8 l/s.

3 you will get call outs saying the unit wont switch off or cool or heat when they are feeling the make up air draught.

4 although installation cost may be higher due to an electric heater battery its a lot less hassle in the long run to use a standard independant fresh air make up system.

5 there is probabley reasonable economic gains to be had for the end user if you need reasons to argue the case against direct duct connection.

I do concede that if you ran the ac on high fan you will find in most cases that you could pull around 64 l/s without a boost fan.

hope this helps.

[Andy W]

Thanks TRASH101, yes it is definately 10 l/s, it is the building reg for offices, I would prefer to put an independant fresh air system in for the above reasons, I have 5 different rooms to do so it would be a matter of calculating ? the requirements of each room and sizing a fan or multiple fans to suit the total air demand but how would you control multiple rooms, if it was a single switch you would have to rely on a person to switch it on, a time clock would need to be set or over ridden, from a light switch, which light switch?, lots of questions, some of the a/c guys must be dealing with this daily.

[Brian_UK]

Andy, considering your question of how to switch on/off the various rooms.

Why not use PIR detectors to switch the fans? This way they will only run when the room is occupied.

[frank]

Thanks TRASH101, yes it is definately 10 l/s, it is the building reg for offices, I would prefer to put an independant fresh air system in for the above reasons, I have 5 different rooms to do so it would be a matter of calculating ? the requirements of each room and sizing a fan or multiple fans to suit the total air demand but how would you control multiple rooms, if it was a single switch you would have to rely on a person to switch it on, a time clock would need to be set or over ridden, from a light switch, which light switch?, lots of questions, some of the a/c guys must be dealing with this daily.
Hi Andy
the maximum external fresh air you can introduce into a standard cassette is 10% of the cassette's rated volume, without affecting the duty of the cassette. The fresh air has to be introduced into the fan suction side and most manufacturers supply a special spacer that fits between the cassette and the grille. it isn't any good just connecting it to the side of the cassette above the ceiling as this is the discharge side of the cassette, therefore it won't be conditioned. These knockouts are for adding small branch ducts to serve grilles in awkward shaped rooms or adjacent small offices.
If your cassette is rated at 381l/s then you could add 38.1l/s. This would only be enough for 3 people as far as Building Control would be concerned.

Obviously, to do the job right, you need to have a separate ducted fresh air input system, with a common fan and grilles into each room. Don't forget to allow for balancing dampers on each branch so that you can regulate the air volumes into each office, and allow for a silencer on the main duct so that they are not driven crazy by the fan noise coming out of the grilles ;).

In winter, when the ambients are low, around freezing, you will need to consider if and electric or LPHW heating coil will be required, as you will be dumping very cold air into each office.

Also, for summer, you will have to increase the duty of your cassette to allow for the heat gain from the fresh air input.

If you post the occupancy of each room, I'll size the vent system for you

You can add an additional pcb to most @good' makes of AC system which will do the switching for you. If the fans FLC is larger than the relay rating on the pcb then you can always add a contactor or such

[The Viking]

5 rooms you say...

How many persons per room? (To get an idea of the required flow rate)

I would still opt for a Lossnay type of unit. A larger one would probably cope (depending on your required flowrate), you would duct it in to each room and in the supply duct you would fit VCDs (Volume Control Dampers) to balance the air between the rooms.

Depending on what A/C system you go for, the Lossnay unit could be part of the control circuit.
Or just controll it by a time switch.

[Andy W]

Thanks guys, when I am back in the office tomorrow I will dig the plan out but from memory room 1 1 occupant, room 2 meeting room based on 8 - 10 people, room 3 2 occupants, room 4 1 occupant. room 5 general office 10 occupants.

A fresh air duct system with branches off and grilles fitted in the suspended ceiling seems the best way to go, I just hope who ever else quotes for the job does the same or else it can make my quote look expensive.

[The Viking]

I just hope who ever else quotes for the job does the same or else it can make my quote look expensive.


Ah,
But that's where you will get the advantage if you have quoted a Lossnay type unit (YES, I do know that there are different manufacturers, but I can't remember what they call their heat exchangers).

When you point out that 90% of the heat from the extract air will be transferred over to the supply air (or coldness if during summer months), all very energy efficient and green. If an electrical heater needs to be fitted then it will be much smaller/less used/cost less to run.


Good luck.

(Sorry for ranting about these units, but they are the dogs shiny bits for what you need, whatever brand you go for, and as I said if you are installing a VRV/VRF system, it can be integrated in to that as well)


For Daikin's units: VAM (http://www.daikin.co.uk/airconditioning-products/ventilation/default.jsp)
Toshiba's units: VN (http://www.toshiba-aircon.co.uk/bus_air_to_air.htm)
Mitsubishi Electric's: LOSSNAY (http://www.mitsubishi-aircon.co.uk/)(but you have to navigate your way to the Lossnay bit, no direct link option, sorry)

(You see, I did find some others! But I bet there are more that's not listed above)

[TRASH101]

hello Andy

as to the control of the ventilation you really coould do with either seperate timer set to building occupancy or using a local occupancy sensor. I personally would put one common system and design your ductwork on no more than 4 m/s (keep the noise down) and a low discharge velocity at your grills. Dont forget to check on natural ventilation i.e. windows with trickle vent, wether they open or not etc. this could drastically reduce the cost.

the vam/ lossnay cross flow re cuperators are smashing but expensive. I have used the vams before and they seem pretty good.

[John MacK]

I'd agree with The Viking, I've used the VAM units before and they are just what you need, or Hitachi EconoFresh units might do for the bigger rooms.

[TRASH101]

If you got space for a cassette is there space for a ducted unit? This way you can introduce the make up air to the back of the unit and equipment cost offset may get you that vam in your quote too:)

[Andy W]

Some real good points there guys, thanks very much for taking the time and trouble.