Integrating MVHR into the heating and cooling control

MVHR, mechanical ventilation with heat recovery, changes a home’s air continuously: it extracts stale air from kitchens and bathrooms and supplies filtered outdoor air to bedrooms and living areas. The outgoing air gives most of its heat to the incoming air, so the ventilation costs little energy.

It is a valuable system, especially in well-insulated homes, where humidity and CO₂ build up quickly without ventilation. But MVHR often has its own wall controller, with three speeds and a schedule, and nothing coordinates it with the rest. Let’s look at MVHR control integration with the heating and cooling.

What MVHR does, in brief

A heat recovery ventilation unit has three main parts:

• two fans, one supplying fresh air and one extracting stale air;
• the heat exchanger, where the two airflows cross without mixing, and heat passes from one to the other;
• filters, which clean the incoming air and protect the exchanger.

Many units also have a bypass, which lets outdoor air skip the exchanger: useful in summer, when the night air outside is cooler than inside and there is no point warming it.

Control signals

An MVHR unit can be controlled in three ways, depending on the model.

Stepped speeds. Three or four contacts, one per speed, as on a fan coil. It is the simplest control and the most common on domestic units.

0–10 V signal. The speed varies continuously. Airflow can match real demand, with less noise and lower consumption.

Modbus. On more advanced units, the controller reads temperatures, filter status and alarms, and writes speed, bypass and operating modes. It is the most complete connection, with the manufacturer’s register map as the key document, as for the heat pumps covered in the article on protocols.

Humidity and CO₂ as inputs

An MVHR unit always running at the same speed does too little when the house is full and too much when it is empty. Sensors decide how much to ventilate.

• Humidity. In bathrooms and kitchens humidity rises fast with a shower or cooking. A sensor measuring it boosts the unit to high speed for as long as needed, then brings it back to minimum.
• CO₂. In bedrooms and living areas, CO₂ is the best indicator of how many people are present and how stale the air is. A widely used practical reference is to stay around 1,000 ppm: above that, ventilation increases.

The Zigbee temperature and humidity sensors already used for the zones, as we saw in the article on underfloor cooling, also serve the MVHR. CO₂ needs a dedicated sensor.

Summer: bypass and dehumidification

In summer, MVHR can help a great deal, or make things worse.

Night bypass. When it is cooler outside than inside at night, the bypass opens and brings cool air in without recovering heat. It is almost free cooling.

During the day, with hot, humid outdoor air, it is better to ventilate at the minimum needed, with the exchanger active: the incoming air cools a little by giving heat to the outgoing air.

Dehumidification. Many MVHR units have a dehumidification module, sometimes with a coil fed by the system’s chilled water. Coordinated with underfloor cooling, it lowers the dew point and so allows more cooling without condensation.

Winter: frost protection and comfort

In winter MVHR recovers heat, but in very cold weather the exchanger can freeze. Units have frost protection, which reduces airflow or pre-heats the incoming air. The controller needs to know, so it doesn’t mistake reduced airflow for a fault.

Comfort also depends on where the air arrives: a cold supply above a bed is unpleasant. A control system that knows each room’s temperature can reduce ventilation in bedrooms when they are already at the low limit, and increase it in a crowded living room.

Filters

Dirty filters reduce airflow, raise fan energy use and, over time, worsen air quality. Many units report filter status, through a contact or a Modbus register. Others don’t, and then a running-hours counter with a due-date alert is used.

Either way, it pays to bring the alert into the controller along with the others, so it reaches whoever handles maintenance.

The most common mistakes

• MVHR always on high speed because “more air is better”: in winter it removes heat and dries the air, in summer it brings in humidity.
• MVHR switched off because it is noisy or because the client never understood what it is for. In a very airtight home without ventilation, humidity causes mould within weeks.
• A bypass never used, or left open in winter too.

Each of these is easy to spot in the charts once the unit is part of the control system, and easy to correct.

An example of logic

For a home with MVHR, underfloor heating and cooling, a sensible logic is:

1. minimum speed as a base, always on;
2. medium speed if CO₂ in the living room or bedrooms exceeds the threshold;
3. high speed for twenty minutes if bathroom humidity rises sharply;
4. bypass open on summer nights, if it is at least two degrees cooler outside than inside;
5. dehumidification on if the dew point is limiting the floor’s cooling;
6. alerts for filters and unit alarms.

It takes a few lines in a Node-RED flow, and it changes how the house breathes.

MVHR and Brick

Every Brick configuration, as well as heat sources, pumps, zones and domestic hot water, can manage mechanical ventilation. The controller coordinates it with heating and cooling, uses the same sensors as the zones and logs the data in Grafana. In systems with fan coils, the logic combines with the one described in the article on fan coils: MVHR brings in the fresh air, the fan coil conditions it.