IES-VE User Forum

Hello chaps

I've never posted on here before I felt it might be worth getting some discussion going based upon a query I have sent to tech support and I'd like other opinions.

My issue involves the advice that is on the following FAQ:

http://www.iesve.com/support/knowledgebase/faq/2729

“…When a space has mechanical supply the defined SFP should reflect the total Supply and Extract fan power divided by the supply flow rate. This is defined in the Non-Domestic Building Services Compliance Guide.

The local mechanical exhaust is used to include for Exhaust from Toilets, Changing rooms etc. rather than to be used for general extract.”

So this suggests that, for a room with radiator heating and central mechanical supply and extract, I should only be checking the "Is there mechanical supply in this room?" box and inputting the full system SFP in there.

The problem is that the notional building then only uses 0.9W/(l/s) for it's equivalent SFP since it is only activating the supply bit (I checked this through the results in vista). This means I am getting hammered on aux energy in comparison to the notional building since no central system I have ever seen can achieve anything close to that.

I would make the additional point that the the NCM Modelling Guide 2010 appears to suggest that an additional allowance should be made for the notional system extract fan power on top of that defined for the supply. This is given in clause 60 and appears to act as an addundum to clause 58 (supply) ands clause 59 (extract).

The only way to get IES to include the additional 0.6W/(l/s) for the extract fan is to check the "Is there local mechanical exhaust in the room?" box as well but that then requires a flow rate to be input which I can only assume is the design flow rate. The problem here then becomes that the design flow rate is often several times higher than the NCM defined supply side which creates an energy balance which is hugely biased towards the extract. This would be fine except that the notional supply SFP is defined as 0.9W/(l/s) and the notional extract SFP is defined as 0.6W/(l/s) which means the actual building is at a huge disadvantage if it is a balanced system.

The only way I have been able to produce what I feel is a fair result is to check both boxes so that the notional building has an allowance for extract fan energy but then input extract flow rates that approximate the NCM supply rates, thus equally balancing the energy. I have no idea if this is acceptable but it feels like it's complying with clause 60 of the NCM Modelling Guide 2010 even though there appears to be no official guidance on that "user defined" extract flow rate box.

Thoughts anyone?

Complex Potential wrote:The only way I have been able to produce what I feel is a fair result is to check both boxes so that the notional building has an allowance for extract fan energy but then input extract flow rates that approximate the NCM supply rates, thus equally balancing the energy. I have no idea if this is acceptable but it feels like it's complying with clause 60 of the NCM Modelling Guide 2010 even though there appears to be no official guidance on that "user defined" extract flow rate box.

An update:

I put the above methodology to IES to ask if it would be acceptable as appears to be suggested by paragraph 60 of the NCM modelling guide:

"
Hello

I think I have found the bit on your website on the FAQs where it clarifies the methodology IES recommends:

http://www.iesve.com/support/knowledgebase/faq/2729

It does clearly state that

“…When a space has mechanical supply the defined SFP should reflect the total Supply and Extract fan power divided by the supply flow rate. This is defined in the Non-Domestic Building Services Compliance Guide.”

The problem is that if you input the total supply and extract fan power into the supply box only the notional building only uses 0.9W/l/s (I have confirmed this via vista) which does not account for clause 60 of the NCM Modelling Guide 2010 where it clearly states that the notional building should add the extract fan energy to the supply fan energy. The only time IES does this is if the extract box is also ticked which then raises the question about defining the extract flow rate and the huge imbalance that can cause.

The only option I can see is to check both boxes but then manually match the NCM defined l/s/m2 flow rate within the user defined extract flow rate box but I have no idea if this is allowable since it is not explicitly mentioned within the NCM Modelling Guide 2010. "

After much discussion I got this back:

"
Dear _____,

Firstly I would like to apologise for the delay in getting a response to you, the issue has generated much discussion between our consultancy and development teams. We feel that the advice given in our FAQ is consistent with the intent of the functionality of the mechanical supply and exhaust options within the compliance framework, and corresponds both with advice given by BRE on this issue and guidance given in the manual for the SBEM tool.

However, the NCM Modelling guide is the ultimate authority and not the SBEM manual. The wording given relating to input of fan power for these types of systems is unclear and your interpretation of it, while differing from the advice given in our FAQ, is perfectly reasonable. Given that there is currently no overall authority from which we can receive a definitive ruling on this issue, the decision on whether it is correct to apportion fan power for a balanced ventilation system between the supply and exhaust inputs rests with the individual assessor. It is certainly possible to argue that doing so complies with both the NCM modelling Guide and the Non Domestic Buildings Services Guide definition of SFP whilst at the same time giving a more reasonable target in the Notional Building. Given the ambiguity caused by the wording of the Modelling Guide, we may update the FAQ to make clear that while we believe our interpretation of the guidance is correct, a reasonable case can be made for splitting the fan power between the supply and exhaust options."

Hi,

I have spent a while going through the documentation and I'm still not much clearer on what to do.

My colleague recently contacted BRE directly to ask them how to model the common situation of a room with radiators for heating and mechanical ventilation. We got the response below:

Based on your description, the closest approximation of your system for modelling it in iSBEM is to select the HVAC system as “Central heating using air distribution”, and then input the appropriate seasonal heating efficiency and SFP for the central mechanical ventilation.
Alternatively, you may wish to consider using instead one of the dynamic simulation software packages that have been accredited by DCLG for building energy calculations.

I can't say I agree with this response as 'Central heating using air distribution' uses equation 9 in the NCM modelling guide which is for 'zones where the ventilation system also provides heating / cooling' and this is not the case.

One thing I would warn against is entering both the supply and exhaust details on the zone level. This seemed like a good idea at one stage and gives a fair comparison to the notional building. However if you are doing an EPC the rating is terrible due to all the air being exhausted without heat recovery!

It seems like you can't win.

I feel a bit sorry for IES getting bombarded with these questions as they have to follow the NCM modelling guide in the same way that we do.

Maybe somebody has a contact at BRE that can confirm why 0.9W/l/s was choosen for the notional zone level SFP. Because this doesn't seem a fair benchmark for AHUs that are commercially available.

Ross

As my previous post says, the NCM modelling guide is actually a bit ambiguous about that 0.9W/l/s and my take on it is that it should only apply for supply only systems with radiators and that the additional 0.6W/l/s should be added if you have a supply and extract system and my discussions with IES appear to suggest that this method could be considered valid.

As to the BRE's response to your question, I do not agree with it either and it would appear to show a fundamental lack of understnading on their part.

Your warning about the heat recovery is confusing though: The check boxes at room level only change the aux energy and have absolutely no impact upon the heating loads or heat recovery (daft as that sounds). So I would suggest that your EPC is being made worse for the simple reason that it is measured against the reference building which does not add additional fan energies for checking the room level boxes like the notional building does. When it comes to EPC, mechanical ventilation and air conditioning are punished far more than when assessing against Criterion 1 of Part L2A.

Hi,

Yeah I think I've confused you. By ticking the box for 'local mechanical exhaust' and entering some parameters it will increase both the BER and TER.

I believe the increase in BER due to adding the exhaust to be independent of the heat recovery effectiveness.
Maybe mentioning heat recovery was a bit of a distraction.

On a recent project we compared the the two options of entering the full SFP on the supply side, or splitting it between supply and exhaust and got very different EPC results. Maybe just from different flow rates.

I'd have to look into it again to be sure of the details.

Ross

In theory, it should only be the different flow rates that make the difference. If you match the extract flow rate to the NCM aux supply number you should find that the two EPC results come out the same.

I'd be interested to know if it works that way in practice however.

Hi,

I have done some experimenting and you are right it is just the exhaust flow rate that effects the EPC.

The problems we experienced on previous projects were caused by entering the real building's exhaust flow rate rather than mimicking the NCM flow rate.

I think your method of splitting the SFP and using the NCM exhaust flow rate has to be the best one. It gives a reasonably fair comparison the notional building (I'm still not entirely happy that notional exhaust SFP is 0.6W/l/s for remote fans) and an accurate EPC rating. The BRUKL report also makes sense.

The IES FAQ recommendation of putting the full SFP on the supply side gives an unfair comparison to the notional building and contradicts line 60 of the NCM modelling guide.

The extract fan energy will be n addition to the fan energy for the supply ventilation

Hopefully this can be changed, so we can defend ourselves if the work gets audited! From a software point of view I think the tick boxes and options could be made clearer.

Maybe the first question to ask the user is 'is this a central mechanical ventilation system?' then if it is the notional exhaust flow rate could be handled automatically and there would be no confusion.

Ross

It's heartening to know of someone else who has looked into this and come to a similar conclusion, so thank you for that.

I still have the original email from IES which essentially admits that "my" method could be viewed as just as valid as theirs which I'm keeping in case of audits. If IES have no problem with it I could probably send you a copy.

Fan powers

Hi thank you for your discussions I have been having similar issues. I am trying to model a central supply and extract AHU (VAV single duct) and using the notional building SFP of 1.8 w/ls (which I thought was the notional building value) . This send my aux twice as high. If I amend the sfp to 0.9 it matches the Notional building. 0.9 to me is not a realistic SFP for a central AHU? Have they just split extract and supply SFP in half from 1.8 to 0.9!

The next project I have has VRF system with Central AHU for ventilation which will then have to be modelled at zone level and I will have the same issue as you guys that I will have to input at 0.6 and 0.9 to reach the notional Aux levels. What can I say to the client regarding having to have a central AHU that has a supply SFP of 0.9!

Hi,

To the best of my knowledge the notional SFP on the system level with VAV single duct should be 1.8W/l/s.

It may have been other factors that were causing your Aux energy to be twice as high.

A good learning exercise may be to model just one room to get the exact same emissions as the notional building. I personally learned a lot from trying this.

You will find that the notional building has the best available; power factor correction, ductwork leakage, metering and monitoring, pump type etc. Making it very difficult to get near its auxiliary energy emissions even with excellent SFPs.

So even with the same SFP you could be a factor of two worse.

On the zone level though things are a bit messy, I think Complex's potentials method of entering both the supply and extract is your best way forward. On a typical room the notional building may allow you 0.9W/l/s for your supply fan and 0.6W/l/s for your (remote fan) extract. So you would only need to tell your client to hit 1.5W/l/s which isn't too bad.

Hope this helps

Ross