NECB - MURB Reference Heating Type

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Hello,

I've got a question about the NECB Reference Building heating system type
for Multi-Unit Residential Buildings. Specifically what should it be when
you have water-loop heat pumps connected to a fossil fuel boiler?

Table 8.4.4.7.-A of the 2015 NECB states that Reference Building HVAC
systems for Multi-Unit Residential areas are:

"Where the proposed building or space is heated as well as being cooled with
an air-cooled unitary, packaged terminal or room air conditioner (or heat
pumps), or fan coils, the reference building or space's HVAC system shall be
modeled as being identical to that of the proposed building or space;
otherwise, the reference building or space shall use through-the-wall
systems."

I'd consider water-loop heat pumps to be in the "through-the-wall systems"
category so the cooling would be air-cooled DX. It doesn't describe the
heating source for "through-the-wall systems" but 8.4.4.9.4 says that the
energy type of the reference building's heating system shall be identical to
the energy type of the proposed building's heating system. The code
considers heat pump to be an energy type so I think it should be an
air-source heat pump. I'm just not sure about the auxiliary heat - electric
resistance or hot water?

I tried running this in CAN-QUEST but got some errors. When I run a
4-storey office building with WLHP, the reference building is coming out as
air-source heat pump with electric resistance heat so I'm leaning toward
using that. I can see there being an argument for hot water auxiliary heat
though based on the proposed building having a fuel-fired boiler (to avoid
fuel switching).

Thanks,

Aaron

Aaron Smith, P.Eng

Principal, Mechanical Engineer

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Aaron Smith's picture
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Hi Aaron,

The way I would interpret it is the Reference system will be identical to
your Proposed system (i.e. WLHP connected to fossil fuel boiler). If you
had a air-source, water-source (i.e. river, lake) or ground-source, then
you would have to worry about auxiliary heating (per Section 8.4.4.13.2).
The "through-the-wall" system only applies in the otherwise case (i.e. when
your system is not one of the ones listed).

This is my interpretation - I'd be interested to hear others.

Chris

On Sun, Feb 17, 2019 at 10:10 PM Aaron Smith via Bldg-sim <

Chris Hadlock's picture
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Hi Chris,

Thanks for your response. It would make sense if the Reference Building were also a WLHP but based on AHRI definitions I think a WHLP is pretty clearly not:

a. an ?air-cooled unitary ? product,

b. a packaged terminal air conditioner or heat pump , or

c. a fan coil.

A WLHP is water-cooled unitary product.

If they simply said ?unitary? or ?air-cooled or water-cooled unitary? then it would makes sense to me that you model an identical system but they specifically state ?air-cooled unitary?.

Aaron

Aaron Smith, P.Eng

Principal, Mechanical Engineer

LEED? AP BD+C, BEMP, BEAP, CGD

M&R Engineering Ltd.

Aaron Smith's picture
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One of the interesting aspects of this is the link to unitary products for instance would include certifications for water-cooled equipment consistent with a WLHP system as you mentioned Aaron ? if the code just said ?unitary? it would be clear - it?s only that the Table 8 reference uses the word ?air-cooled? instead of applying to all unitary equipment so that a WLHP system seems to be left out of the matrix.

1) Central CHW & HW w/ FCU ? use identical

2) Air-cooled unitary ? use identical

3) PTHP ? use identical

4) Other ? use through-the-wall baseline

It?s strange to me that the WLHP option which I?d consider to be ?in the middle? of the options above as sort of an option 1.5 is excluded and then you are left in ?through-the-wall unit purgatory? where the baseline system is partially defined. i.e. if the fully applied system with boilers, chillers, and FCU is on one edge of the range and PTHP are on the other end of the range, why wouldn?t WLHP fit in there?

That said, it seems to be how it reads. So to your question, you mentioned 8.4.4.9.4 and that your sample compliance building used air-to-air heat pumps with electric resistance heat, which makes sense that a TTW unit in practice is going to be heat pump and/or electric resistance heat. A furnace option would not be part of a TTW unit either. If the baseline is supposed to make sense that is ? I think an argument could be made that on an annual basis the WLHP system takes primary heat from the boiler more than the compressors for cold climates, but I?m not an expert in NECB so you?d be best positioned to comment there whether it calls for a hybrid system or not.

It would be pretty contrived to have a baseline that used an air-to-air heat pump as primary and then had auxiliary hot water ? in practice any building that went to the trouble to add the hot water system?s cost would tend to use it as the primary system. The combo that makes practical sense is for the auxiliary heat to be electric resistance.

David

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
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David Eldridge's picture
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Hi Chris,

I don?t think a WLHP can really be a through-the-wall system ? I believe that implies that the cooling is air-cooled DX. Unless you were thinking the unit would reject heat outdoors in the summer and use the heating loop as a source of heat in the winter? I don?t think that can be modeled though.

I think modeling a PTHP will satisfy both Table 8.4.4.7.-A, 8.4.4.9.4 and 8.4.4.9.5 if you say both Proposed and Reference cases have a ?Heat Pump? energy type and ignore the fact that one happens to be WLHP and the other is ASHP. I don?t think you consider a WLHP to have two energy types (electricity and natural gas) ? it is one energy type (heat pump). I think that is the closest you are going to get. Refer to 8.4.4.20 where they say if your energy type for SWH is air-, water- or ground source heat pump, the energy type of the reference building?s service water heating system shall use an air-source heat pump. I think that provides a pretty strong case for ASHP. I think it should have electric auxiliary heat as well based on my findings from the office building test case in CAN-QUEST.

I appreciate the comments.

Regards,

Aaron

Aaron Smith, P.Eng

Principal, Mechanical Engineer

LEED? AP BD+C, BEMP, BEAP, CGD

M&R Engineering Ltd.

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Hi David,

Thanks for your comments, much appreciated. I agree that WLHP systems are somewhere in between 1 and 2 and the baseline should realistically utilize some kind of central heating and/or cooling system. Based on my recent reply to Chris, I?m going to go with the same system you suggest - PTHP with electric resistance auxiliary heat. I?ll post up a reply if I learn anything more definitive.

Aaron

Aaron Smith, P.Eng

Principal, Mechanical Engineer

LEED? AP BD+C, BEMP, BEAP, CGD

M&R Engineering Ltd.

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