Some "Part F" Calculations: Where Have I Gone Wrong?

[Kate]

Due to total ignorance, but seeing a reference to them on here, I looked up the Part F regulations on the OPDM site.

If I have this correctly, the Regulations require a Minimum Ventilation Rate of 0.3 l/s (litres per second) of fresh air per 1 sq m (square metre) of total floor space in a priivate dwelling.

My smallish semi is approx 6m x 5m over two floors, for a total of 108 sq m. This gives a MVR of 0.3 x 108 = 32.4 l/s.

Now, in one hour this represents 32.4 x 3600 litres of air.

The Specific Heat of Air at 0 degC is 1000J/kg-K (joules per kilogram-Kelvin) at constant pressure, and its density is 1.29 kg per cu m.

So in one hour 32.4 x 3600 x1.29/1000 kg of fresh air are required to enter the property. The 1000 comes from the conversion of litres to cubic metres. This is 150.5 kg of air.

For the sake of argument, if the external air temperature in the winter quarter is 0 degC, we need to warm this air to say 20 degC.

So, 150.5 x 20 x 1000 J are required each hour, which comes to 3.01 MJ.

But this air requires warming 24 hours a day, as the ventilation required is 24 hours a day.

So in one day we will need 3.01 x 24 MJ, and over the winter quarter this translates into 3.01 x 24 x 90 = 6500 MJ, or in familiar units,

1805 kWh of energy.

Over the winter quarter I currently consume 6000 kWh in gas. I will be required now to increase that to 7805 kWh, at an extra cost (at

*todays* rates of £38; much greater next year.

It kind of makes the Insulation and Boiler-efficiency regs look kind of tame.

Now, even I can spot the approximations in this calculation; but I get the impression that all these wonderful regulations are beginning to trip over each other.

In addition, anyone living in an open-plan house with this kind of air-flow rate will need to wear fur-lined boots and a warm top, and set the gas-fire to "stun".

Any comments?

[Ben Blaukopf]

6*5*2 == 60, not 108. So 18l/s, not 32.4

1008 kWh

£21, and that's £21 more than having NO ventilation at all. I assure you that you do!

It's an extra 15% consumption in energy, which will be more than compensated for by the insulation and efficiency regs.

Ben

[Kate]

Apologies!

It should have been 6 x 9m; I got it right on the back of the envelope, and transcribed it wrongly.

It represents a breathtaking two complete air-changes per hour.

Brrrrr.

[Ben Blaukopf]

I did wonder if it might be 6*9, but not sure that counts as "smallish".

One air change every two hours.

Ben

[Kate]

It is when it's full of junque ;-)

Again you're quite right; I shouldn't do these things on the fly. It's still a lot of MJ, though, and we're double-glazed and cavity-walled; the 'sammy snakes' (draught excluders) come out for the winter season too. My road, at the top of a hill, is consistently 4 deg C cooler than my friend in the suburbs 3 miles south.

[meow2222]

What I always find bizarre is that if you start with this vent level, you're then expected to add draughtproofiing and THEN ventilation! Nuts or what.

It seems those drawing up these regs have lost track of what theyre meant to be doing somewhere along the way. Having lived quite happily in a couple of buildings a very long way from compliance really illustrates this.

NT

[Kate]

(Erronenous floor area corrected)

I've been thinking a bit more about this.

The regs speak of this air flow-rate at 50 Pa pressure difference. This is pretty small, roughly 4.5mm of water gauge if my numbers are right.

I've been looking at vacuum cleaner specs on the web, and found a Karcher cleaner (A2251ME) that goes on to say "Ideal for picking up coarse dirt, liquids and fine dust All-round bumper strip 64l/s air flow rate 190mbar vacuum 18l container capacity 1200W max motor rating"

So, If we put Mr Karcher outside the house, with the suction pipe inside the house with any gaps stuffed up with rags, and a simple water-gauge doing the same, we can close all the windows and doors, turn on the Karcher, and see what we get.

All other things being equal, I should get 9mm of water gauge if the house exactly meets the regs; a higher reading implies lower ventilation than the regs require; less than this and we already meet the regs for flow-rate. Different houses will require different wg readings.

If the house meets the regs anyway, why bother with all this paraphernalia of expensive vents, etc?

I realise a QA system might involve a bit more than a Karcher and a water-gauge, but you get the idea.

[Roger Mills (aka Set Square)]

According to the Barlo heat loss program, the ventillation component for a 'room' of 9metres x 6metres x 2.5 metres high with one air change per hour and an internal/external temperature difference of 20 degC would be 891 watts. According to my calculations, 0.3l/sec per square metre is about 0.5 air changes per hour (assuming a ceiling height of 2.5 metres). So, for 2 floors, the heat required would be the same as for one floor with 1 change per hour - namely 891 watts.

The energy required per day would thus be 891x24/1000 kwHr = 21.38

I've no idea where your figure of 1805 kWHr comes from - I haven't attemped to follow the calculation - but I would be more inclined to believe Barlo's figures! Incidentally, the usual recommendation for most rooms is 2 changes per hour (which would push this up to 85 kWHr) - so the Part F requirement is fairy modest.

[Kate]

Many thanks for the interesting reply.

I don't know the Barlo program; I just ran up some figures from basics.

The interesting thing is that you've arrived at a figure for 1 day's worth of energy usage; my figure of 1805 kWh was for a winter quarter of 90 days. So taking your figure and multiplying by 90 gives 1924 kWh for that period; reassuringly close to the figure I found.

I was quite surprised at the fraction of total energy consumption needed by the regulated amount of ventilation.

[Hugo Nebula]

On Thu, 16 Feb 2006 11:53:52 +0000, a particular chimpanzee named Kate randomly hit the keyboard and produced:

The principal behind ventilation in recent years is, "build tight, ventilate right". In other words, make sure that heat losses from uncontrolled ventilation is reduced, and that ventilation removes only the stale and moist air.

Parts F (Ventilation) and L (Conservation of Fuel and Power) are due to be changed in April. The document you're reading is still a draft. I must admit I haven't yet got to grips with the new documents yet, so what follows amounts to my experience of previous versions, plus a quick skim of the new requirements.

The figures you quote are for the supply air for whole house mechanical ventilation. In a note to Table 1.2c it says that, "no allowance is made for infiltration as the extract system lowers the pressure in the dwelling and limits the exit of air through the building fabric".

It's anticipated (in Part L) that new dwellings will have to be pressure tested, and they will have to achieve an air flow of no more than 10m³/hr per m² of floor area when the pressure difference is

50Pa. The 3m³/hr you quote in another post is the best that can be achieved by normal building construction. The existing SAP rating for my house (an even smaller 1999 semi) assumes an air change rate of 0.74 changes per hour, which my back of envelope calculation suggests is 0.5 l/s per m² floor area.

AFAIK the 'normal' methods of ventilation (mechanical extract fans in addition to opening windows and trickle vents) will still apply, but increased slightly.

These requirements will apply to new dwellings and extensions only. There's no 'extra' cost to anyone. There's no reason to think that the new requirements for ventilation and insulation taken together won't reduce the heat loss from new houses by reducing infiltration.

[Capitol]

You both seem to be talking about the same numbers ie 2K KWH per winter quarter. The key point is that the data is specified by ODPM, and is therefore worthless. Simply behave as a normal person, fit the required ventilation to please the jobsworths, then close up all the vents until you achieve your own comfort level. The same rules apply in diy as elsewhere, never give any more information than someone needs to know. It only confuses them. If in doubt, operate as the IR, create FUD in the persons mind by asking all sorts of unnecessary questions to create as much work as possible and ignore for the answers.

Regards Capitol