How much ventilation for a bedroom? (aka our MVHR is far too efficient)

[joth]

55 minutes ago, IanR said:

The master bed volume is around 106m²

Can I check those units and values?

Assuming 106 m3 then that's a substantial volume (vaulted ceiling?) -- about 3x our master room, so all else equally I'd estimate it'd take 3x as long for 2 people  to heat up the air by 1ºC. It takes 2-3 hours for our room to get unbearable, so with a 3x larger room, I can easily see it getting through the night OK using daytime only purge.

 

57 minutes ago, IanR said:

When we go to bed the doors are shut, but roof lights are left just open (50mm - 100mm), and the bedroom windows are left tilted open. I don't believe the bedroom windows being left open overnight has any greater effect to the room temp than if they were closed and the MVHR was on bypass, but my wife likes the windows open.

 

Obviously this will depend a lot on the outdoor temps overnight (worst case it'd actually be heating up the room by leaving the windows open) but for us, windows open makes a radically large difference. If it wasn't for the nearby train line, and wanting closed windows with triple glazing and additional sound insulation to block that out, this would be the solution (if a weird one to have to use in heating season)

 

 

1 hour ago, IanR said:

I hadn't appreciated this was a "whole year" event, I'd assumed as the post was from May it was related to summer over-heating.

 

Doesn't that point to something other than just the occupant's energy contributing to the temp rise? Much of the occupant's energy would be offset by typical heat losses and once you factor in the transfer of energy to the room's walls, ceiling and floor it doesn't "feel" as if occupant heat alone could cause much of a temperature rise.

 

Well this year it was still unusually cold here in early May   and my post was based on the very cold month + of experimentation leading up to that date. But yeah since then the experiment of sleeping in another room / house on a couple occasions has pretty much pinpointed it as a body heat issue. Also, the temperature graphs I have make it clear the temperatures sits stable when empty, but then starts to climb  the moment the room is occupied.

 

 

We also have a central atrium with a skylight that I now have automatically open when stack venting is required. This works like a champion for the house as a whole, but there's just not enough free airway from the bedroom to the atrium to balance out. I'm going to investigate putting a "chimney" like duct out of the bedroom feed up to a high point in the atrium near the skylight, to allow some passive circulation of air too (and also provide a better outlet for the forced-air circulation from the fancoil)

 

 

Thanks for the replies, very helpful to know how others manage things in a passive (like) house

 

 

[IanR]

39 minutes ago, joth said:

Can I check those units and values?

Assuming 106 m3 then that's a substantial volume (vaulted ceiling?) -- about 3x our master room, so all else equally I'd estimate it'd take 3x as long for 2 people  to heat up the air by 1ºC. It takes 2-3 hours for our room to get unbearable, so with a 3x larger room, I can easily see it getting through the night OK using daytime only purge.

 

 

Yep "²" was a mis-typed Alt Code and lack of glasses to notice, should have been "³". Approx 6m x 6m, with a sloping ceiling from 2.2m to 3.7m

You'd have to take more into account than just air volume to calculate the length of time to heat the room up ie. heat losses, heat transfer to floor, ceiling walls and other heat inputs.

I don't see any increase in temp over night. In Winter, if MVHR was not in heat recover mode I'd see a drop in temp. Even in heat-recover mode, when it's below freezing outside I'll see a 0.5°C drop. (both in the bedrooms and across the whole house)
 

39 minutes ago, joth said:

Obviously this will depend a lot on the outdoor temps overnight (worst case it'd actually be heating up the room by leaving the windows open) but for us, windows open makes a radically large difference. If it wasn't for the nearby train line, and wanting closed windows with triple glazing and additional sound insulation to block that out, this would be the solution (if a weird one to have to use in heating season)

Yep on the odd occasion the overnight temps outside do not drop below 20°C, active cooling would be the only option to keep temps down. I do this very occasionally, but only from a chilled 200l buffer, and via the MVHR, not the UFH. I've not had to have the ASHP on overnight.

Just to be clear I do use cooling via the UFH occasionally at other times, in the living areas, during the day and evening, but not overnight. (and not at all so far this year.)

Edited July 8, 2021 by IanR

[IanR]

45 minutes ago, IanR said:

You'd have to take more into account than just air volume to calculate the length of time to heat the room up ie. heat losses, heat transfer to floor, ceiling walls and other heat inputs.

 

You've also got to account for an air change every hour (or perhaps every two hours), which should be at a lower temp than the air in the room, unless something is warming it up...

 

Pretty sure you have an incidental heat input into the room.

[joth]

 

 

4 hours ago, IanR said:

You'd have to take more into account than just air volume to calculate the length of time to heat the room up ie. heat losses, heat transfer to floor, ceiling walls and other heat inputs.

Yes, that's why said "all else being equal" (or at least attempted to, sorry about the typo).

 

 

3 hours ago, IanR said:

You've also got to account for an air change every hour (or perhaps every two hours), which should be at a lower temp than the air in the room, unless something is warming it up...

 

This is exactly what the calculation in my original post is accounting for. 1 ACH is shifting 10W per °C difference in temperature between the supply air and the room temperature, which is effectively negligible for removing the 160W of body heat. Even in bypass mode in winter it's  rarely going to exceed 150W so only just keep up (and of course, we'd not use bypass mode in winter, so in practice it's only around 10W  in winter and a bit more in summer, on cool nights at least. Hence why with very efficient heat recovery, the overheating issue is actually going to be worse in winter than summer)

 

 

3 hours ago, IanR said:

Pretty sure you have an incidental heat input into the room.

 I'm increasingly  sure I don't

 

 

[Dan F]

5 minutes ago, joth said:

 I'm increasingly  sure I don't

 

Good job you don't have a minibar!