Mechanical Ventilation Systems with Trim Cooling

[bootsam]

I tend to treat it as a 'constant volume system' with either 'fixed' or 'variable' fresh air, depending upon the application.

[bootsam]

aaah...is see, i get your drift now. Mech Vent. Because air delivered at 18W or 22S is tempered air. I'd guess the profile for the air supply temp should cover this. Its not air conditioning as i'd interpret it.

(i also use Hevacomp as well as IES, i can get confused sometimes. This issue doesnt crop up as i am only using it as lvl4.)

[bootsam]

You can set the supply air as 'Temperature from Profile' so i guess i'd set that at 30 or 29. Then the next box is the deltaT box, The 'Air supply temp difference'. I'd put in 7deg for an outside 29 and supply of 22. Thats how I interpret this. I may be wrong but I hope i am not :eek

[bootsam]

I see. But you can apply the same conditions using the auxiliary ventilation via the air exchanges tab. The only difference i can see is that you have no control over the external temp. You can still specify the supply air condition. However i note you can only have 1 air condition. Which is awkward if you supply at 18 winter and 24 summer. Maybe a weighted ave supply temp may be appropriate here.

Sorry if i am no help to you but this discussion is helping me

[PCully]

Less of the Glasgow bashing! we're actually snow free at the moment which is a nice change (more on the way at the end of the week).

I'll see if I can get one of the consultants to comment on this and hopefully give some you some confidence in what you are doing.

Phil

[PCully]

Hi,

I asked one of our consultants who is experienced with the L2 regs and have copied his comments below:

You are correct in saying the problem doesn't arise until trying to carry out a UK Compliance check (either Part L or Section 6), as the NCM generally only allows air at external conditions to enter the building.

The thread creator states it perfectly, "This isn't a proper air conditioning system, so to treat it as an 'Air Conditioned' space within Part L is misleading, but neither is it a simple 'Mechanical Ventilation' system as this contains no cooling allowance."

As usual, it's a best-fit situation - in this scenario I would use the 'mech vent' cooling/ventilation mechanism. This will avoid excessive cooling loads experienced if it were set to 'Air conditioning', and allows for an Aux energy load entry.

Having said that, this method creates a thermal balance which is very different from that created by a tempered air supply, and unfortunately due to the NCM conventions nothing can be done about that - the best-fit is still fairly ill-fitting.

I hope this sheds some light on the matter.

Phil

[Jonnymac]

Can you elaborate when you say

"As usual, it's a best-fit situation - in this scenario I would use the 'mech vent' cooling/ventilation mechanism. This will avoid excessive cooling loads experienced if it were set to 'Air conditioning', and allows for an Aux energy load entry"

Particularly "mech vent" cooling/ventilation mechanism" ? Is this the Aux vent system, i.e. a secondary system?

[PCully]

I believe it refers to setting the Cooling/Vent mechanism to Mechanical Ventilation - on the Ventilation tab of the System Wizard and on the Cooling tab of Apache Systems itself.

Phil

[ColinR]

Hi,

I know this thread is ancient but I find this problem comes up regularly and trying to explain it to clients can be frustrating.

I wonder if selecting a "mixed mode" operation would suit? this would give a small cooling load and only during the Summer months but not excessive amounts so that the space is cooled right down to 24C, I think the set-point becomes 27C. Has anyone used this method for trying to satisfy the gap in the NCM conventions?

Cheers,

C