Modelling MVHR in Honeybee Energy Model

I am carrying out an overheating assessment in Honeybee using the Pollination Rhino plug-in, using scripting in grasshopper to process the energy model according to the TM59 overheating procedure.

One of the requirements with TM59 methodology is the inclusion of MVHR in the energy model if it is being proposed in the design. The type of unit I am looking to model is the Compact P - Nilan unit: Compact P - Nilan | England & Wales Distributor, which is a fairly typical system for new build residential Passivhaus projects in the UK now.

From the website:

"Compact P is an energy-efficient total indoor climate solution for all types of low-energy buildings, single-family homes, flats and small office areas in commercial leases with a ventilation requirement of up to 275 m³/h .*
Compact P recovers the energy from the extracted air using a highly efficient counter flow heat exchanger. The remaining energy that is not utilised by the counter flow heat exchanger is used by the heat pump to produce hot water, and to further heat the supply air.
The heat pump has a reversible cooling circuit, which means that, in the summer, the unit can cool the supply air by up to 10 °C. Due to the low air exchange, the cooling does not function as an air conditioning system. On cooling, the supply air is dehumidified, which gives a more pleasant indoor climate than is possible with an ordinary ventilation unit without a heat pump."

What is the best way to model this type of HVAC system in Honeybee? Ironbug seems like the best candidate here, but it would be good to avoid doing so to keep things simple if possible.

Hi @milog ,

My short answer is going to be “yes Ironbug can definitely model this.” Even though this system seems to have a lot of parts, none of them seem more complex than what Ironbug already offers.

However, I am struggling a little to understand what exactly this unit does since it seems like they started with something simple that did only MVHR without anything else. But then they added extra stuff to it (like a heat pump? and then a domestic hot water storage tank?) and they still kept calling it a MVHR unit. At this point, it almost seems unrecognizable as MVHR or, at the least MVHR seems like a misnomer.

Granted, I know this type of situation happens in mechanical engineering a lot (eg. why are they still called “chilled beams” when they provide space heating). But I could use some clarity about what you want to be modeled here. For example, is my understanding correct that this unit does NOT do space heating but it can make domestic hot water (DHW)? If so, are you ok not modeling the DHW part of the unit for now and so you’re only modeling a fan with an enthalpy wheel?

If so, making a minimal Ironbug sample is pretty straightforward. In not, we can still make an Ironbug sample but I will have more questions and it will take a little more time.

Hi Chris, yes I agree the description of the system is a little misleading. The system is essentially a compact unit which provides ventilation, space heating/cooling via supply air as well as DHW production The manufacturer states that the unit has a reversible cooling circuit which is able to cool the incoming air, however due to the low air exchange it does not act as a typical air conditioning system.

For the purposes of our modelling you are correct in that we can ignore the DHW system as what we need to model for this particular system is the air flow supply rates and the air flow temperatures to see how effective this system is at controlling overheating during heat waves/testing against future weather files.

I appreciate the statement *the unit can cool the supply air by up to 10degC" is a little vague. I am trying to find out more information from the manufacturer to understand how the system operates under different external conditions, for example in hot humid weather etc. I will try and get my hands on detailed test data for temperature data under different conditions as I have not found much through the website.

Hi @milog ,

I’ll admit that I don’t really understand what you or the manufacturer mean when you say that it does “space heating/cooling via supply air” but I’m going to take this to mean that there’s no heating or cooling coil within this ventilation air stream and the unit is only doing MVHR with a fan and an enthalpy wheel. Here’s a sample of an IronBug HVAC that’s just doing MVHR: (64.7 KB)

… and you can see that this translates to a single air loop with the constant volume fan, a heat exchanger representing the enthalpy wheel, and a setpoint manager that is targeting 21.1 C:

I can also see that this make things a bit more bearable in the room (albeit only by a small amount). Here are the temperatures when the Room has no MVHR unit:

And here is what they are like with the unit:

Hope that helps and just let me know if there’s something I am misunderstanding about this system.