Hi,
we are modeling the HVAC system for the baseline building of a residential building. Since heating type in the proposed design will be electric, system 2 applies (1 PTHP shall be modeled for each space / thermal block). Regarding ventilation, separate fans shall be modeled in order to supply the same amount of outdoor air as in proposed design in each space.
Our doubt is related with the way these fans shall be considered in regard to heat recovery requirements. Thus:
1) If one fan per space / thermal block is considered, baseline system does not have heat recovery;
2) if we consider a single ventilation system for all spaces / thermal blocks, baseline system will have heat recovery (the flowrate of that ventilation unit would be higher than the threshold).
Given this, and since we could not find any evidence in the standard or users manual on how to consider these fans, which way do you think we should follow?
Regards,
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 28, 2017 - 4:22 pm
The first one is correct. Each of those systems are intendced to be stand alone.
Ricardo Sá
Director of SustainabilityEdifícios Saudáveis Consultores (503 910 767)
85 thumbs up
November 29, 2017 - 5:10 am
Thank you Marcus.
In this case, since the proposed design will include preheat coils, we need to model those also in baseline with the same control. However, which system shall be considered to feed those coils? Electric resistance?
Please note that proposed building has electric heating (heat pump).
Regards,
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
December 15, 2017 - 10:25 am
If the proposed has electric resistence preheat then yes the baseline should use the same fuel source for preheat.
Ricardo Sá
Director of SustainabilityEdifícios Saudáveis Consultores (503 910 767)
85 thumbs up
December 27, 2017 - 10:22 am
If the proposed has electric preheat (e.g.: heat pump, COP=3,2), the baseline system preheat shall be:
1) electric resistance (COP=1);
OR
2) heat-pump (COP equal to proposed);
OR
3) heat-pump (with minimum ASHRAE 90.1-2010 efficiencies)?
Regards,
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
December 27, 2017 - 1:24 pm
The standard is not exactly clear on this. #1 would inflate the savings and is not called for by the standard. Certainly #2 is the most conservative. I think #3 is the one that makes the most sense. If the building is all residential then it should probably be a PTHP.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
January 2, 2018 - 8:18 am
On a side note...is "pre-heat" defined? I'm thinking in terms of a DOAS system where the outdoor air is tempered to have about room temperature on the supply, and the actual "heating" is done by decentralized equipment like radiant hydronics or fancoils. I'm certain the "pre-heat" in these DOAS would require #3. IMO heating is heating is heating. I would check the fuel source of the predominant heating source, right?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
January 4, 2018 - 11:15 am
We did a little more digging and I am changing my response above. I think this should be the option #2. There is a sentence in the User's Manual that says that the intention is that preheat coil should not contribute to the performance rating. So it basically should be identical to the proposed system design.
Preheat does not appear to be defined. We would not define the DOAS you describe Jean as preheat. LEED Interpretation #10285 provides examples of hybrid systems and example #2 the system doing the tempering is not preheat and factors into the system selection. We agree that heating is heating except for differences in efficiency. In the case of a DOAS the baseline would not have one, and all heating would be provided by the PTHP. The preheat system should be the same fuel source, efficiency, and type as the proposed.
Eduardo Vidal Kume
energy and building physics expertATP sustain
1 thumbs up
October 12, 2021 - 10:15 am
I would like ask if Ricardo´s first afirmation for LEEDv4 ASHRAE 90.1 2010 for the Baseline Building is correct:
"1) If one fan per space / thermal block is considered, baseline system does not have heat recovery"
So, if using Baseline Systems 1 to 4, where the individual HVAC system are decentralized per thermal space, even if the system falls into one of the cases from Table 6.5.4.5, they could be modelled without heat recovery? I would interpret that they fall into 6.5.6.1 exception "h) Where the largest source of air exhausted at a single location at the building exterior is less than 75% of the design outdoor air flow rate. " Is this interpretation accepted by USGBC?