Tas, IES or Hot3000

[fostertom]

Which is best, pros and cons?

(At risk of duplication http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5193 but wishing to consult both forums)

[Mark Siddall]

PHPP ;-)

[Nick Grant]

You wind up merchant Mark!

I have to agree though.

Alan and I have been working alongside various engineers who are using TAS and IES. The results vary hugely but not because of the 'engine'. Is all down to assumptions. It should be possible to get IES and TAS to produce better results with more realistic internal gains and suitable fudges for thermal bridges but life is too short. PHPP allows instant what if scenarios that would take hours in IES.

Don't be fooled into thinking that models that use minute by minute temp and radiation data will necessarily be more accurate than a simple degree day model. Even anual and monthly models give quite similar results.

PHPP will be demonstrated on the AECB stand at EcoBuild (times to be agreed) so if someone wants to turn up with IES on a laptop we can show and tell.

Nick

[Nathan Williams]

Is this because PHP already has the assumption built in so you do not need to think about them? IES/TAS have a much wider scope of application than PHPP and therefore the options available are much greater and more flexible. IES/TAS are in a different league to PHPP - PHPP is a much more specfic product, orientated towards a particular level. Are you not assuming that the IES/TAS results are wrong because they are not the same as the PHPP?

[Nick Grant]

Nathan you are right, I was being simplistic.

PHPP assumes a low energy building operated at a constant average temperature and does not attempt to model over-glazed buildings with high levels of thermal mass. That is 100% of the new build projects that I am likely to work on so PHPP suits me fine.

I am not saying the IES results were wrong in terms of maths, only that it seems common for users to assume higher internal gains when calculating heat requirements and this results in conclusions such as the building has no heat requirement so less need for insulation or HRV. I think this is probably the main explanation for the huge gulf between design and reality that is the norm for many so-called low energy buildings.

PHI have put a lot of effort into determining realistic net internal heat gains and you will have seen the video interview between Pete Warm and Wolfgang Feist discussing this. For design reasons you can calculate IHGs in PHPP but for certification you have to use the defaults where they exist to prevent cheating (zero heat building heated by electrical goods).

The writers of PHPP use dynamic models and would be bemused by a discussion as to which is best.

I can see a dynamic model having an edge for very glazed high thermal mass designs but then I have not seen such a design that actually works and is comfortable so I'd rather play it safe and stick with what I am confident with. This means reducing the heat flux (in or out) to a level where the modelling can be quite simple.

I would also see a role for dynamic models in checking summer overheating in larger non-domestic buildings.

Or have I missed something?

Nick

[Nathan Williams]

I think dynamic thermal modelling has a role in any low energy design - although having confidence in the results is very important (rubbish in = rubbish out etc). I was first introduced to IES about 12 years ago at university and have been using IES for thermal modelling for the last 5 years. I did purchase PHPP recently but have never had the time to sit down and go through it. I would of thought that as long as the IHG's are laid out, then it should be easy to use the same data in the IES/TAS model.

The results will always be different. A study was done some years ago that looked at various different design software (it included TAS and IES and a number of other academic packages but not Hevacomp and Cymap) and modelling the same simple building across all the different software showed that some of the results differed greatly - top to bottom of the spread was something like 50%. Which one is right...?

I think in someways the choice of software comes down to user preference. In IES you can physically model shading to the building (for instance), therefore on one project we worked on the design of the brisoleil  - not using arbitary numbers for the shading factor but the actual phyiscal size of the shading fin and its geometry with the solar path etc. Also in IES you can look at temperature variation within the room, stratification and so on. The last point is the flexibility for user interaction - for instance you can model the house shut up during the day, the temperature rise in the space, and then the effect of the occupier coming home after work and opening the window - this is real life.

IES/TAS can be a little bit like "cracking a nut with a sledgehammer" for some appliactions, but personally I think it has greater scope than PHPP.

[Mark Siddall]

Hi Nathan,
I've heard that some dynamic tools don't allow for psi-values to be enters (you have to enter Y-value instead - which a a bit sloppy in a low energy building.) Can you enter psi-values in IES?

Mark

[Nick Grant]

Nathan

In PHPP you would not worry about modelling temperature stratification as that is dealt with by the design philosophy which addresses radiant temperature asymmetry and glazing temperatures that could lead to downdrafts etc.

In terms of modelling what happens if you leave the building unventilated in summer on a specific day with a specific user behaviour - well no PHPP doesn't do that. What it does do is say the building will overheat if the ventilation (day and night separate) is insufficient WRT the gains and mass. It's a crude tool calibrated by dynamic models but it seems to work in practice.

If you are going to EcoBuild it would be interesting to meet up on the AECB stand and perhaps run a comparison. If you have an hour free we could model a simple building and try different window frames, glazing, insulation, shading, orientation, ventilation etc and see the effect on energy demand and summer overheating.

Nick

[fostertom]

V gd discussion of PHPP vs 'universal' modellers. I've no doubt that PH is a miracle of well-developed pragmatic usefulness, within its limits and assumptions, which are at present the best-practice mainstream - glad to leave that to the experienced wisdom of our PH specialists. I'm v interested in exploring what lies beyond PH - because believe me PH is only one step along the way, not the end of time.

Therefore, leaving PHPP aside, Tas, IES or Hot3000 - which is best, pros and cons?

[Mark Siddall]

Tom,
I must confess that I don't understand your premis of "beyond PassivHaus." Once you've tackled the fabric and ventilation (as far as PH) then you're are left to concentrate upon DHW and electrical efficeincy - these are the most cost effective next step. At this point in time this is beyond even dynamic modellingsta as little is truly understood about behaviour (if it ever can be). Reducing the internal elec loads is obviously a big step, and this is considered when internal gains and the primary energy use.

Mark

[Nick Grant]

Tom

we jumped straight into banter but worth stepping back and saying what you want to use the software to help you model.

Nick

[Mark Siddall]

Good point

[Nathan Williams]

Sorry late replies...

Mark - no you can not put in the thermal bridges very accurately, in fact the u-value calculator is not brilliant either and this is one of the bad points of IES. In terms of the level of detail you currently look at in relation to bridges and junctions, the model is a little bit more basic. To accomodate you would have to adjust u-values to take into account thermal bridges. For residential I use another program which links the geometery from IES into a SAP program which has a better U-value. Thermal bridges can be entered on that but this is a manual entery.

Nick - I think this touches a little bit on Marks point. The level of detail in the thermal modelling does not match the level of detail in relation to PHPP. For instance, although shading, orientation, ventilation, glazing etc can be modelled, different frames become difficult. For instance, when looking at solar control glazing, I have to look up the glazed units on Pilkington to obtain the various short/long/shading coefficents etc and then adjust the parameters of the glazed unit within the model to get the same details. However I am guessing that if a material is not in the PHPP database you would need the details to enter in a a user defined material anyway. Coming back to frames, there is a selection for frame types however you are better off looking at the manufacturers window unit data and entering that data into the materail database rather than trying to "custom build" the unit within the software.

I am working on a project at the moment with an AECB architect who has already put the buildings through the PHPP software, so hopefully I will have the opportunity to compare. Alternatively if anyone has a small simple project they want to compare then please let us know - I think it is useful to compare a "real life" project.

[Nathan Williams]

Does HOT3000 have UK support or energy files for UK locations? I do not have an experience of HOT3000 but looking online it seems to be based on the Canadian market.
IES/TAS are both supported in the UK and have weather files for various locations through the UK, as well as around the world. I have had very limited indirect experience of TAS, however speaking to someone who has used TAS for decades(!) and used to work on mainframes(!) with the software, they advised that it was more to do with user interface than the actual software.

[Mark Siddall]

Hi Nathan,
Thanks for clarifying the thermal bridging issue. This worries me a little though. In a PH failing to account for the thermal bridging could suggest the building performs twice (or more) as well as it would in PHPP. Similarly with glazing, and the shading factor from the frames, this can have a major impact.

I appreciate that you could introduce fudge factors: One technique that I've heard about is to create a section of wall (1m wide/deep by X meters long.) You can than create a limited area of wall with an adjusted U-value (similar to what you suggest but localised to the detail in question rather than developing a whole building Y-value. But fudging the performance of the glazed area is far more problematic, esp if you can not readily tweak the frame dims.

Two more window related queries: In what detail can IES model windows? You suggest that window frames can be done, though it is demanding of effort. I assume that you can model all the mullions and transomes so that you can reflect the elevation (shading/reduced gains and additional heat loss). What about spacer bars? Can it manage the thermal bridging from these? Also is IES subtle enought to handle the shading from window reveals and window heads?

And one on DHW: Can you model the losses from the pipework to the internal environment? (Impacts upon primary energy use).

It'll be very interesting to see how you IES model compares to your architects PHPP calcs.

Cheers,
Mark