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!
A VAV system does both heating and cooling via the air and so the SBEM equations for central AHU aux energy come into play (unlike zone level SFPs only which is what we were talking about further back in the discussion). In particular Single Duct VAV uses Equation 9 (see table 12 of the NCM Guide):
So essentially a VAV system's aux energy will be dependent upon the base heating/cooling room loads because these determine the flow rate required by the system above and beyond the basic fresh air which is why the energy requirement is normally higher.
I have a split system with a separate ventilation AHU supplying and extracting air. AHU has a SFP of 2.18 and a heat recovery of 83%.
So in effect I believe I have the same scenario as you were discussing at the start, but with a Split system instead of radiators.
So... I am going to enter both the supply and the extract to make the notional an more realistic target.
My question is...
What extract rate should I apply to the extract ventilation to make it the same as the national?
Is this correct:
Supply SFP = 2.18 (as stated above)
Extract SFP = 0.6 (as in NCM MG)
Extract remote form room
HR of 83%
Extract rate of?? Where do I find the extract rate to the NCM activity type??
The notional building will match whatever extract rate you enter for the actual building BUT it will continue to use the NCM fresh air requirements to size the supply. This basically leaves you with 2 options:
1) enter the extract rate as per your real design but potentially have a weird imbalance between the supply and extract which is especially annoying if you are supposed to have a balanced system
2) enter a flow rate for the extract which matches the NCM supply rate to force a balance* (which is the method I have been adotping for balanced systems)
Of course there is option 3 which is to do what IES suggest and input the combined supply and extract SFP into the supply only bit which conveniently means you can ignore this whole sticky business. Unfortunately this always results in being penalised because the notional building only sees it as "supply only" and uses an SFP of 0.9W/l/s which is crazy low for a balanced system.
In your case it sounds like you are actually trying to do 1 and 3 and double counting the extract.
CP
* to do this you need to run the model first then highlight all the rooms on that particular system and pull out the combined NCM air flow rate from vista. Then divide that by the combined floor area of the same rooms to get an averaged l/s/m2 figure which you can then feed back in as the extract rate.
Looking at the draft version of the NCM modelling guide for Part L 2013 (page 20). It would appear that the notional building will keep the 0.9 W/l/s for zonal SFP.
So the confusion may roll on for a few more years. If anyone has a contact at BRE now would be a great time to sort this!
RossThompson87 wrote:Looking at the draft version of the NCM modelling guide for Part L 2013 (page 20). It would appear that the notional building will keep the 0.9 W/l/s for zonal SFP.
I don't interpret that as the intended SFP for a balanced supply and extract because that is talking about the supply component only.
The wording hasn't changed much BUT I still maintain that the guidance in the document is quite clear but that it is IES's advice on the issue which is causing the confusion and which I believe is logically flawed.
The highlighted sentence couldn't make it much clearer: The notional building is supposed to add the extract fan energy on top of the supply. So it should be supply 0.9W/l/s and extract 0.6W/l/s (so 1.5W/l/s for a balanced central system). The only way to force the notional building to do this is to check both supply and extract boxes in your actual building.
However, IES says you should only check the supply box and put your combined SFP into it. This means the notional building thinks your building is supply only and doesn't add the extract component, unfairly penalising you.
I even have an email from IES where they basically admit that my interpretation is probably just as valid as theirs (after much deliberation at their end) but they don't actually go as far as saying their advice is wrong because para 64 can be read in two different ways.
This issue is giving me a bit more grief. I've inherited a model from another consultant that favors a different work around! Changing things could make a big difference to the design, budget etc.
I've had the following response from my accreditation body. This suggests that the 0.9W/l/s in the modelling guide is for local mech vent.
Hopefully the EPC conventions group can help clarify things. It would be good if we could get a response from someone at BRE that produces the modelling guide?
Ross
I think that this is an issue that basically occurs because of the software's limitations. As the software will never allow the user to enter a radiators heating system with mechanical ventilation ( it is always naturally ventilated ) the only thing that the user can do is to assign the mechanical ventilation on a zone level. However, this is not the purpose of this software area. Assigning a mechanical ventilation system on the zone level assumes
"Zonal ventilation type – A mechanical ventilation system separate from the heating or cooling system (i.e., zonal ventilation) can be added here. If the selected HVAC system does not already include ventilation, the zonal ventilation radio buttons become active. This would be possible, for example, with radiators or under-floor heating. You need to select either: Natural or Mechanical supply & extract, according to whether there is a mechanical ventilation system present in the zone to provide fresh air." (i-sbem manual)
The software has this option for small, local AHUs, serving one or a few areas, independent from the heating or cooling, that provide fresh air only. This explains the fairly low SFP of 0.9, which is achievable for these small systems, which only include a few ducts, if any.
"64. For zones where the ventilation system does not provide heating or cooling (but can include heat recovery), the fan power density is the product of the fresh air supply ratefor the activity type from the NCM Activity database and a specific fan power of 0.90 W per l/s."
So, in order to overcome the limitations of the software (in assigning a central mechanical ventilation plant in a, for example, radiators system) we have to use this option which is however designed for a different purpose. And this is what is causing the problem. Unfortunately, there is not another way to model such a system.
I usually advise assessor to leave the SPF to the default value, however I appreciate this is not a satisfactory option when the building in question is a new-built that needs to comply with Part L. But calculating the SFP for a centralised system enter in this software area will never achieve a figure of 0.9, as you mentioned this is nearly impossible.
These cases have troubled me a few times but I was never able to come up with an idea of a convention that would cover most cases and also cover both the centralised air distribution system entered as local and the actual local system.
Effectively the proper way to resolve such issues is through the software, by upgrading it to allow more HVAC system types and both natural and/or mechanical ventilation options. This appears as a long term plan though, it does not appears as something that could be done soon.
I will take your query with me in the next EPC Conventions Group meeting and I will suggest that we think of a way to address this issue, under the current software arrangements, as this is a problem which we have come across before and we will in the future again.
I'm starting to lose what little faith I had in the whole NCM modelling system. Problems like these have existed in the NCM for several years and there seems no drive to address them. Instead we just get increasingly detailed and complicated EPC conventions which don't even apply to Part L.
My latest bugbear is we now have buildings designed to the 2010 regulations requiring EPC's that must be produced using the 2013 methodology and due to the large change in fuel CO2 factors they no longer meet their EPC design targets. This is especially true for buildings which relied upon PV to meet their CO2 / EPC targets.
All the while, whoever it is who writes the NCM requirements and checks the various software packages for compliance is hidden and doesn't seem interested in engaging with the modelling community to hear our ideas for improving the process.
Sorry, I'm just moaning, but I find the whole thing infuriating and with each new version of the NCM we seem to introduce new problems while never addressing the pre-existing ones.
I completely agree, its very frustrating. Setting up EPC conventions is helpful but do these also apply to Part L models too? I imagine almost all IES users perform both tasks on the project so we need consistency.
I often get the response to these sort of issues along the lines of 'do what you like, as long as you can justify it with evidence'. This seems fair enough but with issues like this SFP one you can make a case for lots of different inputs that give vastly different results.
Design and build contractors seem increasingly keen to use one modeller up to a certain stage and then pass it on, then get understandably upset if the pricing needs to change.
A lack of response from DCLG/BRE is just going to mean more time wasted in meetings picking through all the differences because we have to find work arounds for very common situations.
Hi all, on a related note, has anyone noticed that room level Supply and Return SFP's are not shown in the latest version of the BRUKL (2014.1.0.0 Part L 2013)? My scenario is that I have a split system with fresh air supply and return defined at room level. I have run the calculation with the AHU defined as either 'centralised balanced AC or mechanical ventilation system' as well as 'Zonal Supply System with remote fan,' and in both scenarios the SFP is not shown within the BRUKL. Fortunately, where I have rooms that have an extract rate only and SFP defined, this SFP is shown in BRUKL!
For what it is worth, whenever I define room level supply and return (to AHU) ventilation, I apply the combined supply and return SFP within the IES 'supply' box. My understanding is that this dialogue box is incorrectly named by IES and instead should be called 'Supply and Return' rather than 'Supply' only. Note that 'Return' to an AHU would be regarded as different to a separate local 'Extract' air stream. I have had this explained to me by IES, though only they would be able to comment as to why renaming has not occurred. Despite this, I agree with the sentiment that we are then forced to compete with a Notional building that assumes a combined supply and return SFP of 0.9W/l/s. My view on this is that just because the Notional building assumes a seriously low SFP figure here, it does not mean that we have to come close to this, it is simply one route by which IES reduces the TER. This is similar to lighting controls, we all know that it is not sensible to have PIR's in ALL building room types, so just because the Notional does this doesn't mean we have to. Again it's a way of reducing the TER.
Really this ventilation SFP issue needs to be clarified officially once and for all by the powers that be.
I've not spotted the BRUKL thing yet but I'll keep an eye out for it now thanks mate.
I still maintain that following the official advice from IES and having to compete with a total system SFP of 0.9W/l/s in the notional building is plain wrong (as I said above, the NCM Modelling Guide clearly states that the 0.6W/l/s should be added to the notional building). I'm convinced this is a software issue and to test my hypothesis, I have done some tests using our friend on the other side of the fence, Tas.
I set up a radiator heating system using LTHW and gas boilers with mechanical supply and extract with heat recovery in Tas to see what would happen. This is exactly the situation where IES needs room level SFPs defined and all the problems start.
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).
So there you have it my friends, make of it what you will.
