Defining fan powers - something seems wrong

[VVladinovsky]

Hi,

I have been following the discussion and I equally share your frustration with 'illogical and unbeatable features' of the notional building.
I would like to enquiry about the extract flow rate. I have been using the method of splitting the overall SFP into SPF for supply and SFP for extract. However when I have entered the design (real) flow rates the auxiliary energy skyrocketed and made the building impossible to pass. So I kept the extract flow rates on default 0.8 l/s.m².
This appears to be contradicting with the EPC convention. The EPC convention requires to enter values from Local Mechanical Exhaust and Ventilation rates table or real flow rates - which both make the performance of the building worse than the software default value.
As Complex Potential suggested I could enter the NCM extract flow rates.
Can anyone please advice if this approach gives more realistic assessment and how would you justify the use of it if audited?

Thanks,
VV

[Complex Potential]

Hi VV

The advice IES gives is that for a central supply and extract system, you should input the full system SFP (supply power + extract power divided by the larger of the two flow rates) into the supply side and leave the extract side unchecked.

This essentially means that the actual building is basing its aux energy on the NCM flow rates for both the supply and the extract. In other words, a balanced system which is exactly how it should be according to the NCM modelling guide. Fine.

The problem arises when you then look at the notional building which is only applying the supply component of 0.9W/l/s because it doesn't think there is any extract which puts you at a disadvantage.

However, by checking the extract side box and setting the flow rate to match the NCM you have essentially an equivalent set up to the "official" IES advice but with the added bonus that the notional building is then adding the 0.6W/l/s it is supposed to include for the extract side.

To an auditor, I would argue that it is a logical work around for what is essentially a software limitation IMHO. I even have an email from IES somewhere that admits this method is probably just as valid as theirs.

CP

[JohnLloyd]

Complex Potential - I completely agree with what you've written there.

I don't usually do the Extract tick-box work around in my models because of the potential for it to be questioned in an audit, but I believe it is a valid option as in my opinion the Notional building in IES should be including the additional 0.6 W/l/s by default.

Perhaps IES are reluctant to change these things as when their software is accredited it must be within a specified range of SBEM results? If they're currently in that range I can understand why they don't want to fiddle with fundamentals of the existing NCM set up.

[Complex Potential]

Perhaps IES are reluctant to change these things as when their software is accredited it must be within a specified range of SBEM results? If they're currently in that range I can understand why they don't want to fiddle with fundamentals of the existing NCM set up.

Perhaps, but I was raising this long before the recent (and rather daft) requirement that DSM and SBEM be within a certain range of each other.

I suspect that either:
1) it's a bit of a pain to change in the base VE Compliance code, or
2) it would mean IES having to openly admit that their software has contained a rather embarrassing oversight for several years, or
3) they are simply apathetic about the whole issue because it's subtle enough that 99% of users and auditors will miss it.

I'm betting on a combination of all 3.

CP

[RossThompson87]

Hi,

I was just wandering if anyone knew how other software packages approach this.

I've only really worked with IES. Does iSBEM, Hevacomp, TAS etc. work in the same way with similar advice?

The more I think about it the advise from IES doesn't really justify the

extract fan energy will be an addition to the fan energy for supply ventilation

clause in the modelling guide.

Ross

[Complex Potential]

Good question Ross, and the answer is, no they don't, at least in the case of Tas.

Further back in this very thread (at the bottom of page 2 in fact) I posted results from a test I did using Tas as a comparison to IES and it basically shows that their notional building adds on the 0.6W/l/s for the extract side with no problems as an addition to the 0.9W/l/s for the supply. Please scroll back and have a read and let me know what you think.

CP

[RossThompson87]

Hi CP,

Jumping back to what you posted:

When it comes to defining the air side configuration for that system Tas asks for, among other things, the Extract Fan SFP and the Fresh Air (supply) Fan SFP separately. Tas then generates the individual HVAC systems with individual components that can be interrogated for both actual and notional buildings.

And guess what...? For both actual and notional buildings the extract and supply fans are shown separately on the schematics and the SFPs for the notional building are 0.9W/l/s for supply AND 0.6W/l/s for extract with all fresh air flow rates matching the actual versions (assuming demand control is set correctly and you don't set the fresh air as heated which, incidentally, is something else you cant do in IES).

What do you mean by 'actual' fresh air flow rates? Is that the ones derived from the NCM database? Or the real building flow rates i.e. what you may enter yourself from a schematic.?


Ross

[Complex Potential]

The flow rates are all just based on the NCM figures in both actual and notional buildings. As you can see from the screenshots below, Tas does not ask for a separate flow rate input for the extract side unless it's a dedicated extract only system such as a toilet or changing room, which makes everything a lot more logical.

Supply and Extract


Extract Only

[Terence]

Hi All,

Think I need to chip in here. The SBEM Documentation contains the following.



This is clear as day and our implementation is as per the above guidance. Mechanical Supply SFP in the VE interface is the Mechanical Supply & Extract as in iSBEM. Local Mechanical Exhaust is for additional extract fans like toilets etc…

To confirm we went back to iSBEM this week and from running some tests we confirmed the VE-DSM is operating exactly the same way.

We have always been of the opinion that the solution implemented in the VE is correct and we have not said otherwise. It has also not been shown to us, by the relevant authorities, during software approval or afterwards that our implementation of this is incorrect. If it were we would certainly review.

We feel that the end users are responsible for their inputs; even if the documentation is unclear as to what to input it is not our role to force users towards our interpretation. Users are free to stray from the official guidance, we cannot police this.

[Complex Potential]

Thank you Terence

Everything you've said is technically correct, the best kind of correct However...

I'd like to point out that those "relevant authorities" have accredited two separate pieces of software that appear to have implemented these methodologies in completely different ways, and so their credibility is questionable in my eyes. This is also leading to the notional building in Tas being more lenient than in IES which is something I thought you would be more concerned with frankly.

So this comes back to an NCM which states the notional building should be using 0.9W/(l/s) as a total system SFP for mechanical supply and extract. This is verging on impossible to achieve.