SAP/PHPP comparison

[Nick Grant]

Just spotted the AECB SAP/PHPP comparison report on the home page:

http://www.aecb.net/Comparison_of_PHPP_with_SAP.php

[fostertom]

Very clarifying. Do we agree with this analysis? Anything to add?

Even if it seems obvious, would anyone care to spell out the difference between these two approaches, and full thermal modeling by Tas, EIS etc?

[Nathan Williams]

I think this is an unfair comparison. SAP is not, and was not set out to be a design tool, it is merely the statutory approach to confirming compliance with building regulations part L for dwellings. You do not size your radiators, boiler etc etc based on the SAP calculation. In this PHPP is probably a much greater tool (although I have a copy I have not had the time to go into it).
Personally I use IES modelling software to carry out designs and the differences between SAP and IES in terms of the energy consumption are considerable. We are currently involved in some low energy housing for a housing association where we have modelled the buildings using IES, but also needed to carry out the SAP calculations for the Code for Sustainable Homes paperwork. The main differences are in the heating and hot water energy consumptions - the hot water depends very much on the profile of use which SAP overestimates.
When SAP was first introduced, the idea was to provide a standard rating system for a dwelling which could be calculated manually by anyone. Although the process has moved on since then with accreditation processes now required, the standardisation principle still applies. SAP was not provided to provide real life figures, merely a way of comparing buildings, ie one is better than the other, not that one consumes 10% less energy than the other.
I think what is needed is that other software, such as PHPP should be accepted as an option by building control to confirm compliance with part L, along with acceptance in C4SH etc.

[Nick Grant]

Hi Nathan

Agree with your plea for BC to accept other approved models. Such a pain to design with PHPP then shift headspace to do SAP/CSH. Either the latter is just done because we have to or if the client believes in it, it becomes a complex game of trying to score points without losing the efficiencies gained through rational design.

[Alan Clarke]

Hi Nathan
I found this a very worthwhile comparison to carry out, and in fact found that there isn't all that much fundamental difference between PHPP and SAP.  Both are simple degree-day models, though being aimed at different uses they have different emphases.  Being originally for assessing existing stock, SAP focusses on controls of heating systems in pooorly insulated houses, whereas PHPP goes into more detail on passive solar gain and MVHR systems. Also, both are tools to show compliance with some sort of an energy or carbon standard.

In practice compliance tools become design tools by default. I don't mean for sizing radiators etc, but for the basics of the building fabric - which as far as I am concered is the important bit. SAP is used for demonstrating the higher levels of the code, and therefore the tool you have to use to design your fabric to these low energy standards. It would be nice to be able to use PHPP for this, but I expect the best we can hope for is for SAP to adopt a lot of the detail contained in PHPP.

Alan

[Alan Clarke]

Very clarifying. Do we agree with this analysis? Anything to add?

Even if it seems obvious, would anyone care to spell out the difference between these two approaches, and full thermal modeling by Tas, EIS etc?

SAP and PHPP use what is called the degree-day method, where the steady-state heat loss of the building is multiplied by a factor for how cold it is and for how long during the heating season. There are a number of implementations of this basic principle, but they are all pretty simple, and provide instant results.  Gains from internal heat sources, or solar gain, are lumped in either monthly or over the whole heating season.

Working on energy surveys I have found this is actually gives pretty good results for heating energy consumption, well within the range of unknown factors such as actual internal temperature and air leakage.

Full thermal modelling tracks the energy movements through the building hour-by-hour taking into account thermal mass and conductivity.  This takes a lot more computation, so you are generally restricted to doing "runs" for a praticular set-up, rather than just tweaking the design and seeing the result change instantly.  The theory is that this approach gives much better consideration of the specifics of thermal mass etc, which should help with designs further from the norm.  However what TAS seems to me quite poor at is giving insight into what is actually going on in the building, whereas PHPP does break down all the heat losses and gains in a fairly transparent way, which helps identify when there is an error in the model, and also helps guide the design more quickly than putting lots of "runs" through a black box analysis.

Alan

Alan

[fostertom]

That's very interesting Alan, especially in view of Bentley's (Microstation + Generative Components) interest in Tas - see http://communities.bentley.com/forums/thread/14108.aspx , which must have as its intention exactly that kind of design-tool usefulness.

[Michael George]

Full thermal modelling tracks the energy movements through the building hour-by-hour taking into account thermal mass and conductivity.  This takes a lot more computation, so you are generally restricted to doing "runs" for a praticular set-up, rather than just tweaking the design and seeing the result change instantly.  The theory is that this approach gives much better consideration of the specifics of thermal mass etc, which should help with designs further from the norm.  However what TAS seems to me quite poor at is giving insight into what is actually going on in the building, whereas PHPP does break down all the heat losses and gains in a fairly transparent way, which helps identify when there is an error in the model, and also helps guide the design more quickly than putting lots of "runs" through a black box analysis.

Alan

Alan

Hi Alan, Can you elaborate a little about what you see as the disadvantages of using Tas. I use it regularly and would be intested to know what you mean.

Have attached a basic output file. Its generated by the software so easy to produce. What's missing for me is internal temperatures, but these are also easy to output.

[Alan Clarke]

I have to admit I haven't used TAS for a long while, and the output does look better than when I did. Is it possible to break heat loss down into different elements, eg walls, windows etc?  This is something that a degree day model calculates explicitly, so you can see straight away what the big issues are, and say whether the thermal bridges are a problem, or whether there is much scope for an improved glazing spec etc, without having to try a range of options and wait and see what the final result is. My fear is that because full thermal modelling lets all the heat loss paths and thermal mass interact at each time step, it is impossible to disentangle the results in a simple way - but I would be interested to be proved wrong!

The other aspect is just speed - and doing a relatively quick-and-dirty model asap will be better than not doing anything at all as there isn't the budget for full TAS analysis, say when no one wants to spend much money before they've even got planning permission. I don't know how quick you can run through a whole variety of options in TAS, but I know with a spreadsheet you can set up a watch-window on the results you are interested in and get instant feedback to running U-values up and down, or adjusting the window area, etc.

Alan

[fostertom]

There's a deep trend in computing to cut the intelligent designer out of the loop.

We would like computers to feed back instant results of design changes - as Alan says 'tweaking the design and seeing the result change instantly'

However the trend is to rely on a 'black box' coupled to an iteration machine. The internal inter-relationships, becoming ever more sophisticated, are not shown to the designer. The 'designer's' role becomes one of defining a starting scenario, and the target to optimise towards, then setting the machine 'to doing "runs"' - not just one at a time, but endless combinations of variants, and iterative sequences of same, so eventually the perfect optimised design pops out automatically! - which may be nothing like the 'designer' ever imagined!

This may not please us designers, but design/simulation, like everything else, will become commoditised and automated for the mass market - but there will still be the high-value niche providers of design done the traditional way.

See Jan edition of Develop 3D, article on p27 http://www.develop3d.com/downloads/

[Michael George]

Alan, sorry for the delay in replying.

Is it possible to break heat loss down into different elements, eg walls, windows etc? 

Yes it is, I’m working on a publication at the moment which will discuss these data

This is something that a degree day model calculates explicitly, so you can see straight away what the big issues are, and say whether the thermal bridges are a problem, or whether there is much scope for an improved glazing spec etc, without having to try a range of options and wait and see what the final result is. My fear is that because full thermal modelling lets all the heat loss paths and thermal mass interact at each time step, it is impossible to disentangle the results in a simple way - but I would be interested to be proved wrong!

It’s also possible to model thermal bridging now, though it has to be done as a percentage of wall/roof etc.

The other aspect is just speed - and doing a relatively quick-and-dirty model asap will be better than not doing anything at all as there isn't the budget for full TAS analysis, say when no one wants to spend much money before they've even got planning permission. I don't know how quick you can run through a whole variety of options in TAS, but I know with a spreadsheet you can set up a watch-window on the results you are interested in and get instant feedback to running U-values up and down, or adjusting the window area, etc.

Alan

For me, it is quicker in Tas, though I am not proficient in other spreadsheet based software. I suspect you are right, in that the latter is quicker if we were to have ‘a race’ : )