[EnergyPlus_Support] Heating demand not passed from DHW loop to Boiler loop [3 Attachments]

["Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>]

Hi,

First, this is going to be a rather long post (sorry!), you might be better off reading it on UnmetHours directly here.

Short story

I'm trying to model an indirect water heater, where a Boiler:HotWater operates in constant flow, variable temperature, trying to meet a setpoint in a water tank, that in turns serves WaterUse:Connections.

It seems that I cannot get the DHW loop (secondary) to pass its demand back to the Boiler loop (primary). I've attached a graph of what I'm experiencing: the DHW loop temps are NOT satisfactory, while I'm only at 30% capacity of one of my two boilers... There is Unmet Demand Rate on my DHW loop but not on my Boiler Loop.

My IDF file is also attached.

What am I missing?

Thanks,

Julien

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Details

In reality

Piping arangement

This is a primary/secondary loop arrangement. The boiler has a dedicated boiler pump, ON/OFF, constant flow, interlocked with boiler operation (typically, the pump actually starts before the boiler, and a flow switch will allow the boiler so start its ignition sequence).

The boiler feeds the tank via an internal heat exchanger: the boiler water is in the heat exchanger tubing (I guess this means source side effectiveness < 1) that is sitting in the Service Hot Water (Use Side effectiveness = 1)

The DHW loop has a recirculation pump for priming purposes only (keeping the loop at temperature to avoid drawing cold water for minutes during low load), typically only a fractional pump (less than 1HP) moving about 1 GPM per 20 fixtures (rule of thumb, ASHRAE Handbook of Fundamentals Chapter 14). It's the cold water pushing in that really moves the fluid when there's a draw on the DHW loop (from city mains pressure for low rise buildings, or for high-rise a gravity tank, booster pumps...)

There's usually a mixing valve after the tank to temper the DHW to a reasonable level, say about 120°F.

Control Sequence

There is an aquastat in the water tank that is the main variable controlling the boiler. Here is the control sequence as I understand it:

• The boiler has a Setpoint, an offset and a differential. Typical values are respectively 125°F, 5°F, and 5°F
• The boiler cut-off is at Setpoint+Offset. Typical value: 130°F. (Read: if tank reaches 130°F, boiler cuts off)
• The boiler cut-in is at Setpoint-Differential. Typical value: 120°F
• In between, the firing rate is adjusted by PID control to meet the tank setpoint.
• There are additional settings such as anti-cycle (a delay in minutes to prevent short cycling) that I'm not concerned with yet.

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In EnergyPlus / Efforts so far

Plant Loops

I have attached the drawing of my plant loops as I figure it should be.

• The boiler pump is a Pump:ConstantSpeed while the DHW pump is a Pump:VariableSpeed with a minimum flow rate corresponding to the recirculation rate (technically only the recirculation rate should have an electric consumption here, but I'm not going to care).
• The boiler is a Boiler:HotWater.
• The storage tank is a WaterHeater:Mixed with zero capacity, set to have a Setpoint of 130°F and a deadband of 10°F, and set to have a Source Side Flow Control Mode of IndirectHeatPrimarySetpoint

Controls

My initial thoughts:

• The Water Use Connection has a target temp of 120°F (doesn't really matter)
• The DHW Plant Loop has a SetpointManager:Scheduled of 120°F on the DHW Plant Loop Supply Outlet
• The Boiler Plant Loop should have a SetpointManager:DualSetpoint with a low of 130°F and a high of 180°F on the Boiler Plant Loop Supply Outlet. The PlantLoop should have a 'Plant Demand Calculation Scheme of Dual Setpoint'
• The Boiler Plant Loop should have two availability managers: an AvailabilityManager:HighTemperatureTurnOff and an AvailabilityManager:LowTemperatureTurnOn both set to the Storage Tank Use Outlet Node with respective values of 130°F (cut-off) or 120°F (cut-in)

Problem:

I cannot get the DHW loop (secondary) to pass on its load requirements to the Boiler loop (primary), as shown in the graph attached. I was expecting the boiler loop to vary the temperature coming out of the boilers so that the tank gets heated enough, but it doesn't.

Efforts:

Well, I've tried pretty much everything. Hard sizing equipment capacities, flow rates, changing setpoints, etc. The only way I can get it to meet the DHW demand is to put only a scheduled setpoint on the boiler supply side set to a constant 180°F, but this is definitely not what I want...

Can someone help me please?

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

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Posted by: Julien Marrec <julien.marrec@xxxxxxxxx>

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