DSM: Secondary Circulation

[RossThompson87]

Hi,

I was just wandering how people were approaching entering details of DHW secondary circulation in Apache Systems.

The pump power is fairly obvious.

How should the loop length be established? The SBEM technical manual (section 4.3.2) uses the following equation if there is no user input:

Loop length = sqrt(Area served)* 4.0

Can this be used in DSM or should I be measuring the pipes off the DHW drawings?

The Circulation Losses (W/m) could be manually calculated based on the temp difference etc. which isn't too complicated.
However in a typical building the pipe sizes and insulation thickness constantly varies. So it would be hard to do anything meaningful unless you spend ages on it.

Ross

[Complex Potential]

I always read the loop length directly off the CAD drawings. If you have a DWG you can install a small plugin called Bomlengths.lsp which allows for a command which quickly adds up all the lengths of any selected lines. I believe this is what an auditor would expect you to have done for DSM for a construction phase EPC.

If you are at an early stage or the drawings isn't available then the SBEM equation would be a good backup.

As for the W/m, I simply base it on an average thickness estimation which is often about as good as you can do I think.

[RossThompson87]

Hi,

I am guessing you set the 'DHW Delivery Efficiency' to 1 if you go down the route of entering both the storage and secondary circulation losses?

Otherwise you would surely be accounting for the same loss twice.

Ross

[Complex Potential]

Yes indeed. It is one method or the other but you should avoid using both together because you will double count, as you say.

A bit of useful trivia for you: the DHW Delivery Efficiency is defined as the base annual DHW demand divided by the total annual DHW heating load and so you should be using this if you decide to take on the calculation of the delivery efficiency yourself. Sometimes this can be handy in cases where you feel the NCM DHW demand assumptions are completely unfair to your building (especially when you consider that the notional building is using an eye-watering 95% delivery efficiency) but otherwise, inputting the component parts and allowing IES to work it out is easier.

On the same topic, I've never actually found anything in the NCM Modelling Guides that specifies if the delivery efficiency should be based upon NCM demand or your "real" building demand. As far as I can tell, either is allowable but can generate some hugely different results

[RossThompson87]

Hi,

The way the notional building accounts for this seems very unfair. The 5% delivery losses only take effect during the occupied times.

Adding the storage losses, secondary circulation and pump power adds a constant energy consumption to the actual building.
The pump power also takes the much higher fuel factor for electricity and the notional ignores this.

This would explain why manually calculating the delivery efficiency gives improved results and in my opinion a vaguely fair comparison to the notional building.

Is it worth making a correction in the manual delivery efficiency calculation to account for the pump using electricity?

Thanks

Ross

[Complex Potential]

Hi Ross

I think it's just the pump power that gets added as a constant figure but you are right that this seems unfair when compared to the notional building.

IMHO the notional building should also include a pump power in addition to the 5% since it is normally going to be assigned to a different carbon factor than the DHW. Unfortunately this is not specified in the modelling guide and so it does not happen.

I don't think there's anything in the rules saying you can't manually account for the pumping within the delivery efficiency calc and so it's probably worth a go if you had absolutely no other options to achieve compliance. Just be careful that you account for the carbon factor differences fairly.

CP