zone level SFP & heat recovery

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swilson
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zone level SFP & heat recovery

Postby swilson » Mon Nov 25, 2013 4:16 pm

I am modelling a system using radiators and HRUs.

As such I am specifying the ventilation at room level.

If the overall SFP of the HRU is 1.8 this is how I have been setting up the rooms.

Supply or extract only I enter the SFP as 1.8.

If a room has both supply and extract then I put supply and extract as 0.9 each in fear of duplicating the SFP and having an actual calculated SFP of 3.6.

Is this correct?

Also, as some of the rooms have supply only or extract only but are connected to the same HRU, which room do I appy heat recovery to?

The room the heat is being recovered from (extract)
The room the heat recovery is being delivered to (supply)
or both?

Thanks

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Complex Potential
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Re: zone level SFP & heat recovery

Postby Complex Potential » Tue Nov 26, 2013 4:03 pm

Hiya

Often you have supply in one part of the building and extract in another. Personally I like to think of these as one big supply and extract system and will set them all as such with the extract rate being the averaged (l/s)/m2 for the floor area of rooms applicable.

According to the IES knowledge base you are supposed to input the entire supply and extract fan power of 1.8W/(l/s) into the supply only side if you have a supply and extract system, so go ahead and do that if you want but it will give a poorer result than the below method.

I personally do not agree with IES on this point and say you should apply both supply and extract as either 0.9/0.9 or some other proportion based upon the pressure balance the system is trying to achieve. I actually have an email from IES which stops short of saying I'm right, but concedes that I may not be wrong ;)

As for the HR, you can apply that to the system as a whole which technically would apply to all of the rooms regardless of whether they are on the supply or extract side.

CP

swilson
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Re: zone level SFP & heat recovery

Postby swilson » Tue Nov 26, 2013 5:18 pm

Hi CP

I think you have basically confirmed what I thought was right ( or at least doing it an acceptable way)

So if you have a negative pressure system do you stick a higher SFP on the extract side or vica versa for a positive pressure system?

I saw you thread on this matter but I wasn't 100% sure on if my post was slightly off topic.

So correct me if I'm wrong but if I was doing it the "IES way", I would enter yes in the box for both supply and extract but then an SFP of 1.8 for supply and 0 for extract.

Or do you enter supply at room level and tick no in the mechanical exhaust box?

(either method seems counter-intuitive)



On a side note I can't understand why IES uses the term "mechanical exhaust" as opposed to "mechanical extract"

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Complex Potential
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Re: zone level SFP & heat recovery

Postby Complex Potential » Tue Nov 26, 2013 5:28 pm

Hi

Yep, if you have positive or negative pressure the supply and extract SFPs will be unbalanced. This should drop out when you divide the running currents of the fans by the higher of the two flow rates.

If doing it the IES way you would check the supply only box and enter the full SFP in there, leaving the extract side unchecked.

As for IES's choice of terminology... we can let them off, they're based in Scotland. :lol:


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