15
System operation strategy
This section describes the operation strategy in various conditions .
If a preheater is installed, the unit can add electrical heat to the T1 incoming outside air, in
order to reduce defrost situations, and increase the supply air temperature .
• Preheat is applied after the T1 sensor .
• If outside temperature is < -3°C or supply air is < 16,5°C the preheater will switch on
with 10% power .
• The power will increase/decrease 10% for each 60 seconds depending on the T1 or T2
temperature .
Preheating setpoints are fixed and cannot be changed .
In cold conditions where T1 is below -3°C and exhaust T4 is < +2°C the condensed water
could built up as ice in the heat exchanger .
In order to prevent this, following sequence is initiated:
• Apply additional heat with preheater if mounted .
• The supply fan speed will decrease speed until minimum RPM is reached .
• After 10 seconds the supply fan will stop completely, while the ongoing exhaust fan
supplies warmer air into the heat exchanger component, to remove potential ice .
• When T4 yet again is > +8°C the supply fan will start at minimum RPM, and then
increase speed until the original required speed is regained .
• This procedure is repeated as long as necessary .
• If T1 is ≤ -13°C for more than 4 minutes and 25 seconds, even with defrost mode active,
the unit will stop all operation for 30 minutes, and reattempt previous operation
condition . If electrical preheat is present, this total off-mode is disabled .
The defrost operation will create an underpressure inside the house . This may cause
smoke from e .g . a fireplace to be drawn back into the house . If fireplace mode is enabled, the
unit will stop all operation in 4 hours instead .
Setpoints for defrost cannot be changed .
When defrost is active HRC 3 will show “dEF” in the display .
When defrost has shut all off, the display will flash the T1 temperature
See more in the “User’s guide” section on page 26 and page 27 .
Introduction
Preheat
Defrost / fireplace
function
Bypass cooling
and summer mode
097647 •
Version 1
.0 • 24
.09
.2019
en
