february 03, 2010 01:10pm

Supplementary heating

It's a long, long story, but Y Foel was eventually built without any real supplementary heating system.

We do have a roughly 1,100W towel radiator in the bathroom. The idea was to provide a top-up by recovering heat via the MVHR extract in the bathroom.

However, in practice this does not work, for the following reasons:
(NOTE: I am aware that the numbers and calculations are simplified, and possibly a little naiive.)

There are 4 extract points in the house. The bathroom vent extracts roughly 25% of the total extract volume. Even with the bathroom door open the transfer of heat from the bathroom to other areas is minimal.

Therefore, the surplus heat energy extracted is diluted at a ratio of 1:3.

The bathroom gradually heats up but much of the energy is absorbed by the immediate surrounds and some is lost to the outside world.

If left to reach a stable state the heating effect via the MVHR would increase of course, but I am describing practical, useable operation.

So, for arguments sake let's consider a scenario where the inside temperature is 18°C. If the radiator is left on for 2 hours the temperature in the bathroom rises to 26°C. 75% of the extract air is at 18°C and 25% at °26C. After mixing the temperature of the air entering the MVHR's heat exchanger is roughly 20°C. At a heat recovery efficiency of 90% (NOT the overall MVHR efficiency with electrical and thermal losses) the supply air is warmed to roughly 18°C.

Even when the ventilation rates are increased the net gain is insignificant, because most of the heat from the radiator stays within a radius of about 2m. There are neither sufficient extraction volumes nor sufficient other mixing and circulation rates to raise the temperature throughout the house within a useful time period.

The above scenario is a simplified version of one experienced last November. The result is that even after several hours the air temperature does not show an increase in those parts the house farthest away from the bathroom.

As an experiment the other extract vents were closed and the cover of the bathroom extract was removed. The supply air temperature increased to > 20°C. However, further tests with a heater directly conected to an extract showed that ventilation rates of ~ 1 ACH were required before a temperature increase could be fealt within a reasonable time frame. However, the RH fell so much that the discomfort was unacceptable (sore eyes and dry skin).

This is an area where I believe the theory departs considerably from actual useability. At least for us, supply air heating on a cold day is not at all suitable. Use of a single heat source in an enclosed space is also counter productive, particularly if that source is on an upper floor and the main living area is below it.

Posted By: MarkTiramani

37914 comments | Comment