Heat loss query

[CC45]

Evening all,

 

Spent the evening taping the vcl & just finished off the evening doing some heat loss calcs and the calc surprised me.

 

Background:

 

Ceiling u value 0.09

Walls 0.15

Glazing 1.4

Timber frame, well sealed I'd say.

 

Floor area 16m2 with volume of 41m3 (it is a high ceiling)

 

MVHR.

 

My calc suggests fabric heat loss of 180watts with the ventilation losses of 620watts.  I assumed 2 airchanges an hour with -1 outside and 21 target, these two factors are responsible for the ventilation heat losses so are they reasonable assumptions?  The calcs work out at a heat loss of 51w/m2 of floor area.

 

I'm being rather ocd with taping etc so I'm hoping for a good airtest so if I achieve between 1 and 2 air changes an hour.  I havent allowed for the mvhr or any heat rising from below (floors will have sound insulation which will reduce the heat rising from below).

 

So do I need to suck it up or are some of my assumptions out?

 

Thanks,

CC

[Jeremy Harris]

The usual ventilation rate is between 0.4 and 0.6 air changes per hour with MVHR, plus the MVHR heat recovery efficiency needs to be accounted for.  I think that some parts of the UK have a minimum ventilation rate of 0.5 ACH, but we run our system at about 0.43 ACH and it seems to be about right.  Even on full boost we can't achieve 2 ACH, I think we can get to about 1.5 to 1.6 ACH maximum on full boost.

 

The fabric losses seem about right to me (I've not checked them) as they are in the right ball park for a low volume, high surface area to volume ratio building.

 

The ventilation loss you've assumed is throwing your calcs out a heck of a lot, I think, especially when you factor in around 80% of more heat recovery at a more reasonable ventilation rate of around 0.5 ACH.

[CC45]

Thanks JSH for replying - it is the ventilation losses that are throwing me - Ive never done these calcs for a highly insulated, pretty airtight house before so I'm prob going ott.

 

How do the ACH you quote above relate the the blower test figures?

 

I will go back tomorrow night and rerun the calcs for that room with a ventilation temp difference of 5degrees and one ACH, while leaving the fabric heat losses the same .  My spreadsheet works room by room rather than whole house calcs as yours does.

 

More to chew on.

 

Thanks.

 

CC

[Jeremy Harris]

The blower test is a bit extreme, as it simulates you having a fairly large hole in the wall with a wind speed of just over 9m/s (a bit over 20mph, or the lower end of force 5) blowing at it.  It's designed to test air leakage, not give a figure for the normal ventilation rate.

 

So, if you have a blower test result that's around the 0.6 ACH at 50 Pa region (that's the PH upper limit) then natural ventilation without MVHR would be very much less than the blower test figure almost all of the time.  Because dynamic pressure is proportional to the square of wind speed, the pressure difference at low wind speeds is a lot less than the 50 Pa that the blower test uses.  A 5 mph wind speed would give a dynamic pressure of just under 3 Pa, for example, so if the air leakage was linearly proportional to pressure differential (a reasonable assumption, I think) then 0.6 ACH at 50 Pa leakage would drop to around 0.036 ACH at 3 Pa.   On a fairly still day like today (here, anyway) where the wind speed outside is around 1 mph, the dynamic pressure is just under 0.1 Pa, so less than 0.2% of the pressure used for the air test and correspondingly less than 0.2% of the air leakage rate.

 

The very low normal air leakage from getting down to PH levels of airtightness is what allows MVHR to work well, as the natural ventilation rate will be extremely low most of the time.  If the MVHR was providing a ventilation rate of 0.5 ACH, say, and the house natural ventilation from leakage was around 0.036 ACH, then about 93% of the ventilation will be provided by the MVHR.  In practice, the average natural ventilation rate for a house this well sealed will be a lot lower that 0.036 ACH, unless the house was in a very exposed location.

 

If you have sealed the house well, then to all intents and purposes you can assume there is no natural ventilation and that it's all provided by the MVHR.  This is a reasonable assumption because the MVHR flow rates will always significantly exceed the natural ventilation flow rates, as long as the doors and windows are closed.

 

So, if you use a high MVHR background ventilation rate of 0.5 ACH, together with the MVHR efficiency (say 80% to allow for a few losses), then the equivalent ventilation heat loss ends up being determined by the effective cold air ventilation rate of 0.5 x 20% = 0.1 ACH .

 

 

[CC45]

Fantastic, I put in 1ACH & 80% efficiency into my calcs and it made a massive difference.  Figures look a heck of a lot better.

 

Many thanks JSH.

[Jeremy Harris]

Good news!  I had a feeling that doing this would make a big difference.  If you're fitting MVHR then it's unlikely that you'll need to ventilate at more than 0.5 ACH in practice, so the losses could be even lower.