DHW Delivery Efficiency

[PGABENG]

I have a question regarding Direct Hot Water Delivery Efficiency.

In the IES software, the DHW delivery efficiency can be input in 2 ways.

1. Add storage volume, Storage Losses, Loop length, w/m loss and pump power etc and set delivery efficient to 1.
2. Calculate the DHW delivery efficiency.

Can anyone offer any guidance on how to calculate the delivery efficiency?

Why do I ask.

We often work on projects that have been started by another consultant and often in EDSL TAS, the TAS software does not include an option to add storage, loop length, storage losses etc etc, only the delivery efficiency can be added, as far as I am aware TAS is just as accredited as IES so why the difference?

One thing I am noticing as a trend is that our hot water systems (when calculated in IES) are always worse than the notional. I believe the notional uses a delivery efficiency of 95% which is just not achievable in the real world, in order to get 95% multiple point of use gas fire heaters would be needed and this is not commercially viable given the amount of money that would be required to do this, also using this sort of system would negate the use of Solar thermal cells.

I think the notional is unfairly stacked against what is achievable in the real world but this is not a fault of IES. In the TAS BRUKL documents I often see the hot water energy consumption in the actual is less than the notional which is rather annoying as I can’t seem to get my hot water below the notional.

So who is right? IES or TAS?

[Complex Potential]

Hi PGA

The delivery efficiency is (I think) the ratio of

Annual DHW Demand / Annual DHW Boiler Load

The DHW Boiler load is the base demand plus whatever losses are incurred for storing and moving the water to the taps etc. You can work out the annual storage losses and pipe losses manually as long as you know the pipe lengths, storage tank size and insulation thicknesses.

As for which method is valid; the short answer is that they are both valid under the dynamic simulation method. Level 5 analysis allows for manual inputs or software specific capabilities depending on preference. The same would be true of utilising IES Radiance for daylight sensors instead of the simple NCM method. Different software has differing functions and capabilities and the NCM Guide recognises that.

The fact that IES includes a function to dynamically model the DHW losses and Tas does not is purely coincidental to the NCM Guide.

Of course, if you opt to not use the IES detailed inputs and instead do your own manual calculation then you should always have that calculation filed away for reference in case you are challenged about its validity.

The 95% thing is a problem and I agree that it's not achievable for most central return loop systems. Playing Devil's advocate, I would perhaps argue that the notional building is sometimes used to drive innovation and technological development. By setting close to an impossible target they are forcing the industry and designers to push the boundaries as far as possible. The BRE might say that they only set the targets and it's up to us to figure out how to hit them.

But for now, for the vast majority of projects I expect to have to make up on the DHW in some other part of the design.

As for Tas showing DHW consumption lower in the actual building, I cant say I've spotted that myself, but it sounds worthy of further investigation. Since Tas only allows for manual input of the DHW delivery efficiency, any number input at less than 95% should be returning the actual DHW consumption as higher than the notional. Perhaps I'll look into it and update this thread if I spot what's happening.

CP

[PGABENG]

Thanks CP

[DesignCO2]

Very useful response CP, thank you. I tend to agree with your sentiments regarding the Notional building performance i.e. just because the Notional is performing to an unrealistically high level of efficiency, it does not mean that the Actual design will be able to get close to this figure. The fact the Notional building assumes a 95% delivery efficiency as well as gas-fired boilers is simply a mechanism to reduce the TER, and as you state, BER improvements will likely need to be made elsewhere (lighting, Aux, PV etc.). Centralised DHW generation and distribution is best to be avoided for buildings with excessive circulation runs and limited DHW usages (according to the NCM room type templates anyhow).

On a related note, would someone like to take the brave step of defining what the correct definition is for the secondary circulation loop length? I'll go first, the total length (flow and return) of all DHW pipework from the dedicated DHW generator to all taps/showers/fittings etc. within the building. If anyone has any other evidence or thoughts to the contrary please could you share.

Many thanks.

[DesignCO2]

To add some further clarification on this issue, see page 80 of the latest ISBEM User Guide (v.5.2b April 2014)

"Further Guidance: The secondary circulation pipework length refers to all the pipework i.e. flow and return."