Our project is a residential building with no central DHW system, instead, each dwelling unit has its own gas instantaneous boiler (all apartments have the same model), there are 5 apartments by floor, and it is a 40 story building. Therefore we are talking about 200 DHW systems.

In order to keep things simple, the modeling methodology was the same from previous projects, to just model one equipment and the flow would be estimated from the Service Hot Water Fixtures Table from the 1.4 Service Water Heating Tab, consistent with WEp1. (flow=annual hot water cosumption/DHW schedule minutes). This methodology worked just fine for several projects. However there was never a project were the flow was incredibly high, we are talking about 60 gpm for baseline and 50 gpm for proposed (most are 6 gpm or some). So basically the flow exceeds the equipment capacity and when the flow is over 30 gpm the energy consumption remains the same, also the energy cost.

We've been doing some workaround the matter and came up with three approaches:

1.- Model 5 different DHW systems/5 different loops, were each system is one kind of apartment, this way we would be modelling 1 complete floor, and then, manually, multiply the consumption/cost by 40 floors. Make additional narratives explaining the modelling protocol.

2.- Model 1 DHW system that with a flow that account for a whole floor, and same as first approach, multiply by 40 floors. Make additional narratives explaining the modelling protocol.

3.- Multiply the DHW system capacity by the number of heaters, but this doesn't sounds right.

Which would be the most accurate, or are there new ideas here?

System data is as follows: Fuel: Gas instantaneous, Equipment capacity 0.09 MBtu/hr, HIR 1.08. Modeling software: eQuest.

Thanks in advance!

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Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Tue, 01/22/2019 - 16:08

Sounds like there is something wrong with the water calcs. If the system as designed can't keep up with the flow then either the system is not properly designed or the water calcs are not accurate.

Jorge Lopez de Obeso

Architect / Environmental adviser

eosis Consulting

LEEDuser Basic Member

20 thumbs up

Tue, 01/22/2019 - 17:48

Yes, well, the water calcs are not entirely wrong but we are assuming that all the flow goes through only one equipment, when actually is going through 200 systems. This is why i think approach 1 or 2 are the most effective way to replicate real behavior. Instead of modeling 200 times the same equipment and the same flow. Just modeling 1 system, and manually, multiply it by 200.

David Eldridge

Energy Efficiency Ninja

Grumman/Butkus Associates

LEEDuser Premium Member

64 thumbs up

Tue, 01/22/2019 - 18:56

The problem is that your method seems to be providing all of the flow at one time, when the actual use in the building will not be simultaneous. The DHW designer is assuming there will be diversity in the use of the fixtures. i.e. the shower isn't used at the same time as the lavatory faucets. You'll need to make some equivalencies in eQUEST either way. I think it would be fine to aggregate everything together, or model by floor and multiply, or by unit and multiply - any of these are fine. (other than if there is thermal loss to the space, then one per unit would be most accurate.) Keeping the modeled capacity the same as actual capacity is desirable from a review perspective, since the schedules are estimates anyway make these the estimated cumulative effect of fixture diversity and flow rates. Instead of the time frame you use now based on gpm, use an equivalent full load hour approach to scale the duration of flow based on the capacity of the heaters.

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Tue, 01/22/2019 - 19:47

Do all the apartments have the same occupancy? I think you should model a single system that would represent each level of occupancy, then extrapolate across the building based on the number of units with that expected occupancy. The location of the systems should not matter and this end use does not really have any interactive effects within the model. If each floor is identical then your per floor strategy would work as well.

Jorge Lopez de Obeso

Architect / Environmental adviser

eosis Consulting

LEEDuser Basic Member

20 thumbs up

Tue, 01/22/2019 - 22:01

yes, all apartments have same occupancy, I will follow strategy #2, thanks!, btw, heater is located in a specific non conditioned room, non adjacent to any conditioned rooms nor regularly occupied, so thermal loss/gain to space isn't really something to worry about.

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