CIBSE Loads - Intermittent heating profiles & preheat margin

[gazzat5]

In CIBSE Loads, how does the option 'Adjust for intermittent heating using profiles' work?




If the heating is off and is set to come on, how long does it take for the system to reach temperature?

There surely must be a time limit like 1 hour to keep the heating load to a sane value (rather than expecting instant temperature reached and causing excessive heating load), but the results only show a one hour snapshot ie 'January 12:00'.

Do i need to set the time setting for the profile to the time the heating is first turning on to get heating results with preheat margin included? And if so, again, what is the time taken to achieve setpoint temperatures?

Feature request: show heating power required to reach temperature in 1hour, 2 hour, 3 hour, 4 hours, 6 hours similar to hevacomp's output.

Thanks in advance for your response(s)

[gazzat5]

Thus far i have this:

Hi gazat5,

Thank you for your email.

Tick this box if you have intermittent heating i.e. the heating is ON/OFF at certain times rather than on continuously with setback and setpoint temperatures. Essentially, this just accounts for the boiler pre-heat period required.

In the ApacheCalc interface the number of hours operation is derived (if you tick the box ‘Adjust for intermittent heating using profiles?’) from the heating profile for a particular month and week day. You specify these in the box labelled ‘Time settings for profiles’.

I hope this helps.

Regards,

James

James,

Yes but how is the heating power derived? If the modulating heating profile goes from 0-1 instantly at say 7am, then how is the heating sized to cope with the load – will it try to bring the room to temperature instantly, in 15 minutes, in 1 hour, what?

Does this depend entirely on the profile so if it ramps from 0-1 over an hour it would be sized to take an hour to reach temperature? And if it ramps from 0-1 instantly the heating is [grossly over]sized to do this?

Cheers,

gazzat5


Hi Gazzat5,

This will prefrom an adjustment to the heating loads based on the hours of plant operation and thermal of the building in line with the method laid out in CIBSE Guide A. If you wish to use this method you need to ensure that the time you select is one for which the heating plant is set to be “ON" otherwise, if the heating plant is “OFF" there will naturally be no heating load. A common mistake is for users to assign a 9-5 with lunch occupancy profile to their heating plant, with lunch occurring at 12 o’clock. With the default time for the profile being 12o’clock the heating loads are reported as 0. In reality it would be very unlikely for the heating plant to be turned off at lunchtime.

I hope this helps.

Regards,
James

[VirtualEngineer]

Hi guys

Just a few thoughts, hope it helps.

In Apache Sim you can change the heating systems from unlimited to a specific heating capacity in kW.

Using this, I sometimes run a calculation to determine the heat load with an unlimited capacity heating stem.Then if this seems oversized, I input a specific capacity in kW and look in Vista Pro to see if the heating system can reach the room's set point.

With regards to the operation of profiles and the heating system, you can see if your heating system is sized to reach set point within an hour, as per the profile, by looking at the room set points in Vista Pro. If the set point is reached within an hour then the heating system is sized to achieve this load.

Alternatively, I have seen a few companies just use heat losses and add a percentage margin for heat up. Not really sure about that though.

Thanks

VEng

[gazzat5]

Continuation of IES support ticket

Hi James,
I hope you are well.

I think I need to read up on the CIBSE Guide A /adjustment for intermittency.

I’ve considered trying sliding profiles but I don’t think they would achieve what I’m looking for as they would reduce the loads while in the middle of the ramp up.

So how to calculate the required Extra heating capacity needed to bring the spaces up to temperature in a specified amount of time?

Cheers,
gazzat5

Official replies from IES

Hi Garreth,

Apologies for the delay in getting back to you.

For this I would recommend using ApacheSim in conjunction with CIBSE Loads.

CIBSE Loads and ApacheSim are certainly different calculation routes but they are available to compliment the assessment of each. Both have their strengths and weaknesses, so using them together will help your design process through the various design stages. For example the loads route allows you a greater flexibility to quickly change external design conditions to understand the maximum load that may be required. Think of it like generating an absolute worst case, but this worst case just may not always arise which would be a condition the simulation route would help you experience in more detail. The industry trend now is to have a far greater focus on plant optimisation and not just the maximum load for the peak condition. ApacheSim will allow you to understand how plant needs vary so you can make this optimisation work for your building (i.e Ramped start ups). At the same time it only naturally allows you to see the maximum load your systems/building will be managing through the year which is indeed a condition you could you use to inform your loads sizing.
Take for example cooling, in ApacheSim because of the variation in period we can assess, the peak load may be due to either air temperature or solar gains but not necessarily the peak of the two conditions together. Issues like seasonal effects are conditions we can assess in Sim but not Loads.

So both have their abilities but it also depends on what stage of the design you are at and indeed what type of building and its complexity. Earlier in the design you may use Loads to give you a quicker understanding of the building’s needs but then use this to start to inform your design toward developing your energy model through the ApacheSim route. It’s all a matter of perspective at the correct time, which is why as engineers we are employed to communicate findings.
My suggestion is to use both to help you as much as possible and not view the Loads and Sim routes as explicit competition. My hope is that through both tools the industry can use them to reduce the outdated trend of significantly oversizing plant which results in poor plant performance to the detriment of its occupants. I’m sure you think the same way which is why you are using the VE in your professional capacity.

I hope this helps you with your modelling as you go forward.

Regards,

James

[gazzat5]

It looks like you can calculate what the heating load would be for a set preheat margin using dynamic simulations. Whether the same can be said for CIBSE loads is still unclear.

For example, i was testing a model using dynamic simulation and came across the following results using a two-stage heating setpoint with 'system extended hours'. This seems to create a trapezoid shaped heating setpoint with a preheat slope of 2 hours.

Therefore a preheat margin of 2 hours must be applied to the heating loads.
See the below image