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

[joth]

Recently moved into our renovated house, temperature is great everywhere, around 20ºC, except the master bedroom that sits at about 21-22ºC all day and shoots up to 23+ every night. I want to get ahead of this before the summer overheating risk really starts!  Full build context below. Our goal would actually to be to keep it cooler than the rest of the house, e.g. 19ºC

 

The bedroom + ensuite have a supply + extract duct so I was hoping these would make a balanced pair and not need much additional ventilation.  Even if it's purge ventilated all day, as soon as we try and sleep. it rapidly overheats.

 

Thanks to @SteamyTea I have the formula for the rate an MVHR  can move energy:

  P = SHC x Mass Flow Rate x ∆T.

 

SHC of air at 20ºC = 1.012 J/(g.K)

Mass of air = 1.2041  (kg/m3)

Flow rate of the entire house air supply = 151 m³/hour

- Lets assume 1/5th of this can be sent to Master bedroom. (A bit optimistic as we have 7 supply vents, but, whatever).

 

Plugging all this into a good old Google search box:

 

 

So for every 1ºC of air temperature difference, the MVHR can remove 10 watts of heat generation per room. That seems far too low! Where have my calcs gone wrong?

 

2 adults sleeping will put out about 80W each (and a bit more after a couple glasses of wine).

 

So  the MVHR supply, at 30m³/h, needs to be 16ºC colder than target temperature to keep the temperature constant.  That seems completely insane! Where have my calcs gone wrong?? Our poor Q350 still thinks we're in "winter heating season" so is recovering all the heat it can, pumping 20.5ºC air into the room. No wonder it overheats.

 

Unless I can set a different supply temperature of flow rate for the bedroom vs every other room, it seems sure to overheat. 

OR I need to put the whole MVHR into bypass mode, and have every other room freeze overnight.

OR I need to install a secondary ventilation system to circulate the air from this room to the rest of the house more actively.

(Leaving the bedroom door open is not a preferred option, as we want to keep pets out, and also it opens to a hallway atrium with a too much glass that will let light into the room at 4am.)

 

Does this reflect what other MVHR owners find? How do you manage this?

 

Thanks for reading this far ? 

 

 

 

@Dan F may also be interested in this, as I know you are also planning an isolated supply/extract for bedroom+ensuite.

 

 

 

 

Full build Context:
151m2 house, 2 occupants. Passive house enerphit standard (25kWh/m2/yr).   Zehnder Q350 MVHR

House is being maintained 20-21ºC via solar gains and (as needed) ASHP to ground floor UFH. Upstairs has no heating system.

Master bedroom is on first floor, with rockwall beneath the floor boards. It has North-East aspect, so some morning light but little in the way of solar gains or other heat sources. not huge, about 30m³ volume but some of that taken with cupboards.

  The attached Ensuite has UFH mat, which just operates 1 hour each morning.

Warm loft above it.

Edited May 11, 2021 by joth
Remove time from formula!

[SteamyTea]

2 minutes ago, joth said:

P = SHC x Mass Flow Rate x Time x ∆T.

Think I typed E for energy, not P for power.

Remove the time and you get power.

 

(I think, out at moment so only half a brain on it)

[joth]

23 minutes ago, SteamyTea said:

Think I typed E for energy, not P for power.

Remove the time and you get power.

 

(I think, out at moment so only half a brain on it)

Thanks! yes I removed the "multiply by time" from my own calcs (hence why the Google box resolved to W not J) but forgot to remove that term from the formula - fixed.

 

 

[Temp]

I'm kinda surprised your body energy raises the temperature so much. Have you tried monitoring the temperature in there while sleeping in other rooms? 

 

[SteamyTea]

36 minutes ago, Temp said:

I'm kinda surprised your body energy raises the temperature so much.

Probably caused by the 'thermal mass', you know, that thing that keeps the house at the right temperature, at the right time, and saves everyone money.

Probably cures illness and poverty as well.

[Dan F]

11 hours ago, joth said:

@Dan F may also be interested in this, as I know you are also planning an isolated supply/extract for bedroom+ensuite.

 

Not isolated supply/extract, no. Rather just planned to avoid a door undercut on the master bedroom given the supply (bedroom) and extract (en-suite) rates are the same. 

 

I don't see how your set up is any different to any standard MVHR install though. is it?

 

[joth]

9 hours ago, Dan F said:

 

Not isolated supply/extract, no. Rather just planned to avoid a door undercut on the master bedroom given the supply (bedroom) and extract (en-suite) rates are the same. 

 

I don't see how your set up is any different to any standard MVHR install though. is it?

 

 

Exactly!  I don't see how our situation is that different, I know others happily keep bedrooms a degree or two cooler than the rest of the house so trying to figure out  if there's anything specific we're missing that is resulting in ours being warmer rather than cooler than the rest of the house.

 

 

FWIW the bedroom door does have a smaller than average undercut, just as you are planning, I thought this would be fine as the bedroom + ensuite would makes its own balanced pair but it is not enough to keep the room at a comfortable temperature, but it doesn't seem so. I don't think widening the door gap would help though, as leaving the door open a crack does not seem to help much - there's just not much airflow through that door because the bedroom+ensuite are already acting as a pair.

 

20 hours ago, SteamyTea said:

Probably caused by the 'thermal mass', you know, that thing that keeps the house at the right temperature, at the right time, and saves everyone money.

 

I know this is not seeking a reply, but for avoidance of doubt the issue with Thermal mass is it requires a temperature gradient to put any energy into (or out of) it. So if the whole house is already at or slightly above target temperature, the room has to overheat by a couple more degrees before it'll get much effect in moving energy into the mass (or through the fabric of the building). This is probably why, we see the room temp shoot up but then "level off" at about 23ºC each night.

 

Also, there's always going to be a limit to how much can be stored, and depends on being able to dump all that stored energy during the (hot?) day while the room is unoccupied.  If I'm unwell with a fever and have to stay in bed for several days I don't want to be overheating because the nearby thermal mass has "filled up"!  I'd rather the system can deal with moving out the heat  perpetually, to stay in equilibrium.

 

My maths so far [which I still hope is wrong!] show that MVHR is only able to move energy at a fraction of the rate a human generates it, so is doomed to overheat without a designed in secondary pathway to carry energy away.

 

 

[Temp]

13 minutes ago, joth said:

trying to figure out  if there's anything specific we're missing that is resulting in ours being warmer rather than cooler than the rest of the house.

 

In our case one bedroom is silightly hotter later in the day because it has a west facing roof and window. It's why I suggest sleeping in another room to see if the heat source is definitly from you.

[Bitpipe]

16 minutes ago, joth said:

My maths so far [which I still hope is wrong!] show that MVHR is only able to move energy at a fraction of the rate a human generates it, so is doomed to overheat without a designed in secondary pathway to carry energy away.

 

I don't think that's much of a surprise - the specific heat capacity of air is low so you need a lot of it moving from A to B to transfer meaningful heat (or cool).

 

If you've ever been in an American home that uses central air heating, the ducts are huge and the flow rates considerable.

 

MVHR is designed to ventilate, not heat or cool. It supplies/extracts air at a sufficient rate to keep rooms fresh and remove stale / damp air. It's normally barely noticeable unless in boost or purge mode.

 

The heat recovery is transferring the majority of the heat in the extracted air to the incoming supply to avoid wastage. 

 

I would only expect MVHR to have a meaningful heating / cooling effect if the house had extremely high levels of insulation and airtightness - effectively acting as a trim function.

 

I would focus on how the bedroom is heating up in the first place. What is its orientation? How big are the windows and are they shaded in the daytime? What's underneath it?

[Temp]

21 hours ago, joth said:

SHC of air at 20ºC = 1.012 J/(g.K)

Mass of air = 1.2041  (kg/m3)

 

Perhaps I missed where you did the conversion but you need both in kg.

 

Eg SHC of air is 1012 J/kg.k

[ProDave]

So the issue is a bedroom that is so well insulated, it warms up over night with the heat from the occupants.  We found that upon first occupying our new house.

 

Part of the issue is the calculated decrement delay was in the order of 13 hours so on a typical day with maximum heat around mid day, the peak internal temperature from that occurs in the small hours, no doubt adding to the bedroom warming overnight observation.

 

Our solution is simple.  Open the bedroom window and en-suite window a little at night and let the cool night air in.

 

It only becomes a problem when the night temperature does not drop below 20 degrees, something that does not happen very often up here.

[joth]

6 hours ago, Temp said:

 

Perhaps I missed where you did the conversion but you need both in kg.

 

Eg SHC of air is 1012 J/kg.k

Thanks! The beauty of using the Google search box calculator is it automatically performs simple unit conversions like this (and verifies the resultant units are what you expect)

Paste either these in a search box and you should see it work:

 

1.012 (J /g)/K * 1.2041 (kg/m3) * 151/5 m³/(3600s) * 1K

 

1012 (J /kg)/K * 1.2041 (kg/m3) * 151/5 m³/(3600s) * 1K

 

 

[joth]

6 hours ago, Temp said:

In our case one bedroom is silightly hotter later in the day because it has a west facing roof and window. It's why I suggest sleeping in another room to see if the heat source is definitly from you.

Yes, thanks I meant to reply on this one: definitely planning to perform this experiment but waiting on installing curtains for the other rooms ?

 

[joth]

7 hours ago, Bitpipe said:

I don't think that's much of a surprise - the specific heat capacity of air is low so you need a lot of it moving from A to B to transfer meaningful heat (or cool).

 

 

yeah I knew well enough it wouldn't be sufficient for dealing with summer overheating and crazy solar gains, but honestly hadn't considered it's an order of magnitude too wimpy to deal with a single sleeping human. Wish I'd done the maths in advance.

 

 

6 hours ago, ProDave said:

So the issue is a bedroom that is so well insulated, it warms up over night with the heat from the occupants.  We found that upon first occupying our new house.

 

Part of the issue is the calculated decrement delay was in the order of 13 hours so on a typical day with maximum heat around mid day, the peak internal temperature from that occurs in the small hours, no doubt adding to the bedroom warming overnight observation.

 

Our solution is simple.  Open the bedroom window and en-suite window a little at night and let the cool night air in.

 

 

This is really helpful to confirm that you see the same issue and have had to make a solution for it.

 

Unfortunately this solution doesn't work well for us - we are in town with  a train line one side and sometimes busy road the other, and part of the inspiration for MVHR and passive-house design was the great sound proofing that insulation and the 3G windows bring. (And it does work great for that, modulo the overheating)

 

Time for me to start designing secondary ventilation I think!

 

We do have a fan coil in the loft, but issue is we need bedroom cooling even when the rest of the house is borderline needing heating on. It seems mad to enable active ASHP cooling during the heating season. (And, it's not that easy to do this dynamically with our heat pump). So need to figure another plan. Ideally I'd have a water to water heat pump that could cool upstairs while pumping the heat down into the the ground floor UFH

 

First, I think  I'll see if I can get some stack ventilation working through the room and out to the hallway vaulted ceiling.

Edited May 12, 2021 by joth

[Dan F]

 

29 minutes ago, joth said:

Time for me to start designing secondary ventilation I think!

 

We do have a fan coil in the loft, but issue is we need bedroom cooling even when the rest of the house is borderline needing heating on. It seems mad to enable active ASHP cooling during the heating season. (And, it's not that easy to do this dynamically with our heat pump). So need to figure another plan. Ideally I'd have a water to water heat pump that could cool upstairs while pumping the heat down into the the ground floor UFH

 

Can you do something to adjust the theshold at which the MVHR switches bypass mode on/off?   The only downside of this is that in spring/summer when its still quite cool at night outside your ground floor rooms would cool down more than you'd like and might require heating in the morning that otherwise wouldn't be needed.

 

Other option is to add some minimal MVHR cooling for first-floor only, which would mean you wouldn't need to turn on MHVR bypass when still cool/cold outside, but still be able to create enough temperature difference in MVHR supply air to get rid of the watts you emit overnight.  It probably wouldn't need to run continuously but could come on occasionally based on room temperature. 

 

Edited May 12, 2021 by Dan F

[tanneja]

Would installing a ceiling fan in the bedroom mean enough perceived cooling?

[Bitpipe]

As heat rises with convection, I'd think that heating downstairs while cooling upstairs would be difficult to achieve.

 

I would second trying a fan to see if the air movement generates a sufficient cooling effect.

 

51 minutes ago, tanneja said:

Would installing a ceiling fan in the bedroom mean enough perceived cooling?

 

[joth]

16 hours ago, Dan F said:

Can you do something to adjust the theshold at which the MVHR switches bypass mode on/off?   The only downside of this is that in spring/summer when its still quite cool at night outside your ground floor rooms would cool down more than you'd like and might require heating in the morning that otherwise wouldn't be needed.

 

 Yeah I've turned it from "Adaptive" to "Fixed" temperature profile, and *think* I've forced it into "summer" mode (it's rather ambiguous, as normally it won't start summer mode until mean outdoor temp hits 15ºC minimum, and we're still only at RMTO= 10ºC, but in the app there is a "start summer now" button I pressed, but it doesn't give much feedback it actually did anything when RMTO < 15)

 

I then set the "Cool" temp profile to target 19ºC, and as it's always at about 21ºC indoors, this means it's pretty much always in 100% bypass mode, just adds in a little heat recovery overnight when the outside temp really drops

 

Downside is we can regularly have >22ºC upstairs yet <20 downstairs, which together with the ground floor having a lot of porcelain flooring means there's still times the UFH might want to come on,  but yet  the MVHR is in bypass mode, which is obviously nuts for energy efficiency but we have to get the place comfortable before really starting to optimize this.

 

But for all that, even with the supply temp now at 13ºC overnight and the bedroom supply vent opened right up, the room still rose from 21 to >23ºC last night, pretty much proving that it doesn't matter what we do with the MVHR temperature, it's the low flow rate that makes it insufficient for dealing with body heat.

 

 

1 hour ago, tanneja said:

Would installing a ceiling fan in the bedroom mean enough perceived cooling?

 

Yeah this is another avenue to look at, although it does feel a bit like failure to start down that path, I'll try that when other options are exhausted. 

 

 

35 minutes ago, Bitpipe said:

As heat rises with convection, I'd think that heating downstairs while cooling upstairs would be difficult to achieve.

 

 

Indeed. It was just aimless musing really, but the thought was an indoor to indoor heat pump that constantly collects the gathered heat from upstairs and relocates it back down into the ground floor slab.

 

 

[RodW]

In a PH with an insulated ceiling between the GF and bedroom and no heating on the FF I would expect the temp in the bedroom to be 1-2 degrees lower than the GF.  Have you looked at the walls/floor with a thermal camera to see if heat is leaking in from an unexpected source?  I once looked at a PH certified flat where one of the bedrooms was unusually warm and that was due to missing insulation in the wall between the airing cupboard and the bedroom and a poorly lagged DHW tank/piping.

[joth]

1 hour ago, RodW said:

In a PH with an insulated ceiling between the GF and bedroom and no heating on the FF I would expect the temp in the bedroom to be 1-2 degrees lower than the GF.  Have you looked at the walls/floor with a thermal camera to see if heat is leaking in from an unexpected source?  I once looked at a PH certified flat where one of the bedrooms was unusually warm and that was due to missing insulation in the wall between the airing cupboard and the bedroom and a poorly lagged DHW tank/piping.

 

Thanks!  yeah done a few informal experiments with the thermal camera (challenge right now is getting the room cool enough that anything shows up), and also done the experiment with not sleeping in the room for a few days, and overall there's no issue with excess heat getting into the room from elsewhere.

Your comment about insulation in the ground floor ceiling is really only relevant in the winter when the GF heating is on (when that insulation is inhibiting the flow of heat from the GF UFH into the upstairs bedroom). In the summer, it's much more about solar gains which can occur just as much in FF as GF. And given the GF is all open plan it's easier to "disperse" any solar gains coming in from one aspect, but in the FF it all gets boxed into the room its in and has to rely on MVHR to eventually blow it away, unless the doors or windows get opened. 

 

Anyway, the main issue is exactly as I mused in the original post: in 8 hours sleeping, 2 adults generate plenty enough heat energy to cause overheating that MVHR cannot shift on its own, and PHPP simply does not model this. Opening the doors/windows and/or adding additional fan ventilation is the only solution.  The whole  design process somewhat failed to flag this for us, but it's not impossible to retrofit (largely because I'd already planned for a fancoil blowing into the room anyway, I'm now working on  wiring it up to operate in "fan only" mode as needed most nights)

 

Cheers

 

[Nick Laslett]

@joth Thank you for posting this. I feel your frustrations. It is difficult to know how to model in advance these issues. Please keep us updated, this summer hasn’t been particularly warm yet, hopefully you can get the room cool enough to be comfortable. 

[IanR]

4 hours ago, joth said:

Anyway, the main issue is exactly as I mused in the original post: in 8 hours sleeping, 2 adults generate plenty enough heat energy to cause overheating that MVHR cannot shift on its own, and PHPP simply does not model this. Opening the doors/windows and/or adding additional fan ventilation is the only solution.  The whole  design process somewhat failed to flag this for us,

 

PHPP does account for energy from the occupants, but what were you asking for it to report with regards over-heating? It was a while ago I did mine, but from memory, by default, it reports percentage of time you can expect to experience 25°C or over, with not more than 10% (I think) being the target.

Had you lowered the threshold temperature for "over-heating", and got that down to close to 0%? If not, then it wouldn't have reported out what you are experiencing. 

 

Have you got a cooling unit on your MVHR? Even with one, MVHR will only have a small cooling effect across the house, but without one you're unlikely to see sufficient delta between the inside and outside temperature to have any meaningful effect. The Heat Recovery works in Winter with a 20 degree delta temp and lots of energy in the high humidity exhausted air being transferred into the lower temp, lower humidity incoming air, but the reverse cannot happen in Summer, without a chiller unit.

I don't personally experience rising temperatures in the bedrooms overnight, I mostly rely on stack ventilation to purge the heat out of the house overnight, so all rooms will see a small temp drop as long as the outside temp is lower than the inside temp.

[RodW]

@goth Sorry, for confusing things by referring to winter performance but I haven't come across this issue previously so I mentioned situations where I would expect differences between upstairs and downstairs temperatures.   Rereading the thread made me realise how ingrained my experience is of using nighttime cooling in summer.  My own house is in a rural area and we tend to have 1st floor and roof windows open from May to October, only closing during hot days and opening again in the evening. 

 

 

 

[joth]

14 hours ago, IanR said:

PHPP does account for energy from the occupants, but what were you asking for it to report with regards over-heating? It was a while ago I did mine, but from memory, by default, it reports percentage of time you can expect to experience 25°C or over, with not more than 10% (I think) being the target.

Yes, it does it on a whole building level, not room by room (AFAICT - I am not a PHPP expert).

At the whole-building level we're totally fine - 2 people in a 4 bed house, no overheating experienced (except when there's a heatwave).

The issue is on a per-room basis. PHPP has the "Heating Load" -> "Risk determination of group heating for a critical room", which

a) does not have any allowance fort the largest source of heat in bedrooms, the occupants, and

b) only deals with heating demand, not cooling demand. (The "Cooling load" tab has no such critical room risk calculator)

 

14 hours ago, IanR said:

Had you lowered the threshold temperature for "over-heating", and got that down to close to 0%? If not, then it wouldn't have reported out what you are experiencing. 

 

 

It reports 3% overheating risk at 25 ºC. But again, this is a whole-house risk. What we see is 99% risk for one specific room, even on the days the rest of the house is sat at a nice comfortable 19ºC.  Again, the internal insulation between floors and in the stud walls is actually hampering rather than helping here.

 

14 hours ago, IanR said:

Have you got a cooling unit on your MVHR?

No, and I really don't see it would help: My calculations in the OP say at MVHR circulation rates it would need to be delivering 4ºC air in order to keep up with 160W of heat source, which could only be achieved by having the ASHP running 365 days a year in cooling mode - very much against the ethos of a passive house!

 

 

 

14 hours ago, IanR said:

I don't personally experience rising temperatures in the bedrooms overnight, I mostly rely on stack ventilation to purge the heat out of the house overnight, so all rooms will see a small temp drop as long as the outside temp is lower than the inside temp.

This is really interesting. Can I ask how the bedrooms specifically connect to the stack ventilation? e.g. do you have to open the bedroom windows to have this work ?  And like ProDave, do you find you need to do this in the (occupied) bedrooms in winter as well as in summer?

Or do you have some other way to promote more airflow through the room - besides the standard MVHR outlet and door gap - such that the bedroom windows can remain closed, yet still maintain a comfortable temperature? (or are the bedrooms particularly high volume, which would greatly increase the time it takes to warm up from body heat?  or anything else that would aid this?)

 

 

Thanks

 

 

Edited July 7, 2021 by joth

[IanR]

On 07/07/2021 at 10:44, joth said:

It reports 3% overheating risk at 25 ºC. But again, this is a whole-house risk. What we see is 99% risk for one specific room, even on the days the rest of the house is sat at a nice comfortable 19ºC.  Again, the internal insulation between floors and in the stud walls is actually hampering rather than helping here.

 

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.

You mention a warm loft. I have a similar storage area under a warm roof and its the warmest part of the house all year (caused by several factors). I introduced some extract vents to reduce the temp a bit but it is still hot up there. My MVHR ducting runs through there and the supply air to rooms that have a long run through the storage area can be a couple of degrees higher than the temp of the air as it leaves the MVHR. I've never checked mine, but I'd assume the ceilings below the warm loft are also warmer than the typical house temperature. ie if it's constantly 5 or 6 degrees higher temp in the warm loft that temp has to eventually soak through the 100mm rockwool insulation to the ceiling below.
 

On 07/07/2021 at 10:44, joth said:

This is really interesting. Can I ask how the bedrooms specifically connect to the stack ventilation? e.g. do you have to open the bedroom windows to have this work ?  And like ProDave, do you find you need to do this in the (occupied) bedrooms in winter as well as in summer?

Or do you have some other way to promote more airflow through the room - besides the standard MVHR outlet and door gap - such that the bedroom windows can remain closed, yet still maintain a comfortable temperature? (or are the bedrooms particularly high volume, which would greatly increase the time it takes to warm up from body heat?  or anything else that would aid this?)

 

Our Master bedroom has a large glazed facade facing SW as well as another external wall facing SE, so for probably 8-10 months of the year we require the external venetians to stop any sunlight from warming up the room.  We can generally keep it below 21°C or 22°C during the day without any active cooling.

The living areas of the house do get warmer on bright days, and we tend to prefer to let the temp go higher than is ideal, rather than allowing the ASHP to start cooling, as the stack ventilation in the evening and overnight is very effective at bringing temperatures back down.

We have a central atrium with opening roof lights above, these get opened up, as well as tilting a few windows to allow the warm air to be purged from the house. The master bed window is tilted open, as is the en-suite to this room, and during the evening the doors between bedroom and atrium will be left open a little to promote airflow.

 

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.

The master bed volume is around 106m². Floor is carpet over an insulated slab, which is probably at around 20°C this time of year, I don't have a temp gauge in the slab in the bedroom area, but the closest one that keeps a watch on the solar gain in the living area is recording 21°C. Overnight room temps are generally stable, in the 19.5°C - 20.5°C range. There is a sloping ceiling that goes up to 3.5m, and the temp gauge is in the light switch, so there is a chance it's warmer higher up.

The big factor for me is solar gain. This was predicted, and while I reduced roof lights, swapped to solar glazing and added external blinds to get to a 1.5% over-heating risk at the design stage, I knew what I was letting myself in for with regards my choice to go with lots of SE & SW glazing and some roof lights.

Any time of year when the sun is out I have to mitigate the daytime overheating in the bedrooms. The difference in winter though is that overnight we won't have the roof lights open and bedroom window tilted, but will just rely on MVHR.

Edited July 8, 2021 by IanR