As a trade engineer, I often find myself working on technologies that have been specified inappropriately, installed incorrectly or commissioned improperly, and are often installed with preset factory settings, which can be a problem. Here’s why...
Iwas recently called to a newly built property where the homeowners reported that the bedrooms were often only around 14oc, even with the heating on.the house had good levels of insulation, tested airtightness, an air source heat pump, underfloor heating and mechanical ventilation with heat recovery (MVHR), as well as a log burner in the lounge. All of the technologies were installed well and the controls were properly commissioned. On my visit the rooms were all the correct temperature, so it was one of those occasions when no fault can be found because no fault is immediately evident.
Luckily, the homeowners had been monitoring the problem. It seemed that the rooms were only getting cold on some nights or on certain weekends. I checked the rooms’ thermostats and found no issues with the schedule. I also checked the hot water demand, as it’s common for hot water to take priority over space heating. However, even if that had been happening (which it wasn’t in this instance) it would not have accounted for such a rapid and dramatic drop in room temperature. room temperature? This theory would have made sense if the floor was getting cold in the room with the log burner, but it did not explain why the bedrooms were cold, as they had their own thermostats in a separate part of the house.
SOLVING THE PROBLEM
The lounge in this property was open plan to the kitchen. There was a combination plant room and utility room, which was directly behind the kitchen. What was happening was that heat from the kitchen, the log burner, the bathroom and the utility room (where laundry was done) was combining to push the air temperature going into the MVHR unit above 24oc. This is a crucial detail, as most MVHR units with automatic summer bypass will trigger the heat exchange bypass when the room temperature reaches 24oc. This means that cold (external ambient temperature) air from outside will be delivered to all the air inlets, usually including bedrooms. This behaviour is designed to bring cool air into a property from outside on hot summer days, but the temperature that will trigger it is preset in most MVHR units and of course will be triggered on any day when the air the system extracts is 24oc or above.the bypass temperature can be changed on many MVHR units, but sometimes it does take the addition of a digital controller or commissioning software on a computer to do this.
As summer overheating was not a risk on this property, solving the problem was simply a matter of changing the bypass temperature. However, it does make me wonder how many other properties have the same issues, not only causing them to lose all the benefits of heat recovery, but also resulting in additional heating costs to compensate for cold air brought in by their MVHR unit.