Qahv Secondary Side Control - Mitsubishi Electric QAHV-N560YA-HPB Data Book

4. System Design

4-2. QAHV Secondary side control

4-2-1. General description and purpose of secondary side control
Secondary side control is a control method to raise the hot water temperature in the tank by using another heat exchanger
added between the tank and the existing heat exchanger.
By incorporating the secondary side control to the QAHV, the need to assemble a pump control on-site is eliminated, and
the system construction in cases where water exceeding the QAHV water quality standard is used is now easier.
(1) System diagram
Secondary side control system
[1] Unit heating circuit
System overview
The system using the secondary side control is roughly split into the following three elements: [1] Unit heating circuit, [2]
Secondary side circuit, and [3] Hot water supply circuit. The hot water heated in the unit heating circuit [1] and the cold
water from the lower section of the tank are heat-exchanged in the secondary side circuit [2], and then the high-tempera-
ture water flows into the hot water storage tank. The heated hot water in the hot water storage tank is supplied to the
customer via the hot water supply circuit [3].
Control target
Based on the data of the flow rate sensor and temperature thermistor installed on the secondary side, the output of
secondary side pump is adjusted by QAHV (adjusts by 0-10 V output).
Purpose
Adjusting the pump output on the secondary side maintains the target outlet hot water temp. and prevents acute rising
of water inlet temp. to QAHV.
Overview on pump operation in different operation modes (○: operated, ×: stopped)
Hot water storage mode
Defrost operation mode
Anti freezing operation
At the end of hot water storage mode, before and after defrost operation (*)
(2) Difference between systems with and without secondary side control
Diagram
Water quality standard
Calcium hardness (mg CaCO3/ℓ)
Hot water supply temperature range
Control target
MEES19K001
[2] Secondary side circuit
Temperature sensor
T
Flow sensor
Operation modes
Without secondary side control
Unit
6.5 ≤ pH ≤ 7.5: 90 or less
7.5 ≤ pH ≤ 8.0: 50 or less
55-90°C
[3] Hot water supply circuit
Primary side
○
×
○
○
With secondary side control
Tank Unit
Unit, secondary side pump, and
Unit
secondary side flow adjusting device
Secondary side
○
×
○
○
Secondary side
Tank
heat exchanger
pH ≤ 250 or less
Unit: 55-90°C
Secondary side: 55-80°C
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