Realistic rates for natural ventilation ACH

[JamesPa]

What's a realistic value for air changes per hour in an older house (1930, solid wall) which has been fully double glazed (without tricks vents), has solid floor downstairs and carpet on floorboards up, and has extensive retrofitted iwi and no operational chimney.  The 'standard' assumptions (for ashp sizing) say anything between 1 and 3 depending on room, but the only way I can make my calculated heat loss equal actual measured consumption is to assume 0.5 throughout, ie the figure that the standard assumptions give for a post 2006 house.

 

To be honest, without mechanical ventilation (which my house does not have), I am struggling to imagine how the ventilation rate could be as high as the assumptions claim it is, but presumably they are based on something!

 

Has anyone any idea whether 0.5ACH (or even anything sub 1) is realistic for a house like the one I describe, and if so do the standard assumptions in practice overestimate natural ventilation by a substantial amount in many cases?

 

Any facts or other evidence that anyone has would be welcome.

 

(Apologies to anyone who has already read this in the insulation topic, I had IT problems this morning and managed to mis-post this twice!)

[JohnMo]

If you have no trickle vents and no mechanical ventilation and you do not have mould issues, you are obviously getting ventilation from somewhere so you have to assume a reasonable house leakage rate. So I would say the lowest figure would be something like 0.5 - 1.0.

[JamesPa]

That's helpful thanks.  

 

I'm trying to rationalise my gas consumption with (my own) loss calculations,  as my most likely choice of ASHP (once I resolve the extensive issues I am having with my local planning authority) is sized quite low on the assumption that the figures from the gas consumption are correct. 

 

If 0.5-1.0 is a believable value for ACH, then

 

(a) I'm prepared to believe it, as we do get a very small amount of mould in some specific places (which I need to deal with) and

(b) it establishes a level of consistency between the measured and calculated heat loss, which is comforting

[marshian]

On 14/10/2023 at 14:38, JamesPa said:

I'm trying to rationalise my gas consumption with (my own) loss calculations,  as my most likely choice of ASHP (once I resolve the extensive issues I am having with my local planning authority) is sized quite low on the assumption that the figures from the gas consumption are correct. 

 

If 0.5-1.0 is a believable value for ACH, then

 

(a) I'm prepared to believe it, as we do get a very small amount of mould in some specific places (which I need to deal with) and

(b) it establishes a level of consistency between the measured and calculated heat loss, which is comforting

 

I did heat loss calcs with https://www.heat-engineer.com it was £12 for a one off survey and I had exactly the same issue as you  - the ventilation rates suggested for the age of the property and room use resulted in a suggested annual gas usage that was way in excess of my actual

 

Background - It's not a leaky house now, it probably was in the 80's when it was built (Single glazed wooden windows and doors, 35mm of loose fill loft insulation, no cavity wall insulation and a 3 ft void under the suspended floor with an air brick every five.) it's only got three trickle vents, SUDG on all windows and doors, cavity wall insulation, 100 mm of loft insulation under the boarded section and 200 mm everywhere else and the ground floor has been insulated with 75mm PIR between the joists.

 

However I have a PIV unit which is currently set to push in 20 litres per sec which with the volume of the house is equal (in theory) to 1 air change every 4 hours or 0.25 ACH so accepting I will have some other leakage and an increase when I use extractor fans in kitchen and bathrooms or even enter or exit the house I used 0.5 as my ACH per hour rate for all rooms which got my suggested heat loss per hour and annual energy usage to much closer to actual

 

Sure bathrooms and kitchens have a higher air change when you run an extractor but no one has extractors running 24/7 so why would the expectation be that the ACH are 2 or 3??

[JamesPa]

1 hour ago, marshian said:

 

I did heat loss calcs with https://www.heat-engineer.com it was £12 for a one off survey and I had exactly the same issue as you  - the ventilation rates suggested for the age of the property and room use resulted in a suggested annual gas usage that was way in excess of my actual

 

Background - It's not a leaky house now, it probably was in the 80's when it was built (Single glazed wooden windows and doors, 35mm of loose fill loft insulation, no cavity wall insulation and a 3 ft void under the suspended floor with an air brick every five.) it's only got three trickle vents, SUDG on all windows and doors, cavity wall insulation, 100 mm of loft insulation under the boarded section and 200 mm everywhere else and the ground floor has been insulated with 75mm PIR between the joists.

 

However I have a PIV unit which is currently set to push in 20 litres per sec which with the volume of the house is equal (in theory) to 1 air change every 4 hours or 0.25 ACH so accepting I will have some other leakage and an increase when I use extractor fans in kitchen and bathrooms or even enter or exit the house I used 0.5 as my ACH per hour rate for all rooms which got my suggested heat loss per hour and annual energy usage to much closer to actual

 

Sure bathrooms and kitchens have a higher air change when you run an extractor but no one has extractors running 24/7 so why would the expectation be that the ACH are 2 or 3??

That's really interesting.  If the piv unit is making any material difference (which I presume it is otherwise why do you have it?) then it implies that tha ach it pushes is material in relation to the natural value.  Thats another data point tending to confirm a low natural value rather than a high one. 

 

And your comment about kitchens/bathrooms is spot on, it just doesn't make sense to assume higher values for these rooms unless there is a specific construction difference.

 

The mcs rules fortunately do allow some flexibility, but most of the mcs heat pump installers I have encountered insist on using the default, despite my protestations and evidence from my actual consumption.

[marshian]

48 minutes ago, JamesPa said:

That's really interesting.  If the piv unit is making any material difference (which I presume it is otherwise why do you have it?) then it implies that tha ach it pushes is material in relation to the natural value.  Thats another data point tending to confirm a low natural value rather than a high one. 

 

And your comment about kitchens/bathrooms is spot on, it just doesn't make sense to assume higher values for these rooms unless there is a specific construction difference.

 

The mcs rules fortunately do allow some flexibility, but most of the mcs heat pump installers I have encountered insist on using the default, despite my protestations and evidence from my actual consumption.

 

Don't know how to break up your post to deal with each point so formatting is going to be a bit meh - sorry in advance

 

PIV unit was purchased and fitted because after I installed the PIR under the floor it sealed up the house a bit to much - for the first time in 15 years we got condensation on the inside of the SUDG windows and humidity levels were much higher than before - we thought we'd left that behind when they replaced the single glazing (we didn't get ice on the inside like we used to get with SG but wet windows were a new thing for us)

 

Before PIR living room needed to be 21 or higher to be "nice" after 18 is fine and anything over 20 is way too hot

 

Bathroom rates - I get that they need to be higher but as I said it's not a 24/7 thing - two of us have showers humidity before 60 humidity peaks with extractor on at 80 - after 20 mins with extractor on it's 70 and after an hour with PIV (Extract off) it's 60 again

 

I have tried occasionally forgot to turn on extract and it's not far off same result with just PIV - just the fall off is slower and takes a little longer

 

I'm sure the mcs rules just use worst case - I don't believe using worst case is representative of every house - I'm comfortable with using 0.5 ACH because I know my base line is 0.25 and it's a controlled base line 😉

 

I've been changing rads for bigger ones (T11 to T22's of the same size) insulating where I can (the PIR underfloor work was a horrid task) but I only had to do it once) and doing a detailed heat loss calc to make sure the house is prepared for a ASHP but I'll probably buy another gas boiler that modulates down to 3 kW (current gas boiler min is 10 kW) before I go ASHP

 

In autumn the house will warm up nicely with a 45 deg flow temp - we will see how much higher the flow temp needs to go in the middle of winter but I doubt it will be as high as previous years where we sometimes ran the boiler in the high 70's

[SteamyTea]

11 hours ago, marshian said:

I have a PIV unit which is currently set to push in 20 litres per sec

Is that measured or just something generic from a switch position and a manufacturers/installers chart?

[JamesPa]

10 hours ago, marshian said:

 

Don't know how to break up your post to deal with each point so formatting is going to be a bit meh - sorry in advance

 

PIV unit was purchased and fitted because after I installed the PIR under the floor it sealed up the house a bit to much - for the first time in 15 years we got condensation on the inside of the SUDG windows and humidity levels were much higher than before - we thought we'd left that behind when they replaced the single glazing (we didn't get ice on the inside like we used to get with SG but wet windows were a new thing for us)

 

Before PIR living room needed to be 21 or higher to be "nice" after 18 is fine and anything over 20 is way too hot

 

Bathroom rates - I get that they need to be higher but as I said it's not a 24/7 thing - two of us have showers humidity before 60 humidity peaks with extractor on at 80 - after 20 mins with extractor on it's 70 and after an hour with PIV (Extract off) it's 60 again

 

I have tried occasionally forgot to turn on extract and it's not far off same result with just PIV - just the fall off is slower and takes a little longer

 

I'm sure the mcs rules just use worst case - I don't believe using worst case is representative of every house - I'm comfortable with using 0.5 ACH because I know my base line is 0.25 and it's a controlled base line 😉

 

I've been changing rads for bigger ones (T11 to T22's of the same size) insulating where I can (the PIR underfloor work was a horrid task) but I only had to do it once) and doing a detailed heat loss calc to make sure the house is prepared for a ASHP but I'll probably buy another gas boiler that modulates down to 3 kW (current gas boiler min is 10 kW) before I go ASHP

 

In autumn the house will warm up nicely with a 45 deg flow temp - we will see how much higher the flow temp needs to go in the middle of winter but I doubt it will be as high as previous years where we sometimes ran the boiler in the high 70's

With the exception of the piv and underfloor insulation I have followed a similar path.  I'm now at tha stage of enthusiastically wanting to fit an ashp, had potentially found an installer who would exploit the full flexibility of the rules to do a sensible job at a sensible price, but my local authority is currently making it either impossible or too risky.  So I'm doing battle with them. 

 

If I can sort the la issue I now hope to install in 2024.  Along the way I've had installers who want to install 2 16kW units in my 8kW house, and surveyors who cheerfully ignore all the fabric upgrades and double count losses between rooms when doing the whole house sizing.  If I didn't have an engineering background I dread to think what I might have been suckered into buying.

[marshian]

2 hours ago, SteamyTea said:

Is that measured or just something generic from a switch position and a manufacturers/installers chart?

 

It's stated output from manufactures installation and operation documentation - it's a Nuaire unit

[marshian]

7 minutes ago, JamesPa said:

With the exception of the piv and underfloor insulation I have followed a similar path.  I'm now at tha stage of enthusiastically wanting to fit an ashp, had potentially found an installer who would exploit the full flexibility of the rules to do a sensible job at a sensible price, but my local authority is currently making it either impossible or too risky.  So I'm doing battle with them. 

 

If I can sort the la issue I now hope to install in 2024.  Along the way I've had installers who want to install 2 16kW units in my 8kW house, and surveyors who cheerfully ignore all the fabric upgrades and double count losses between rooms when doing the whole house sizing.  If I didn't have an engineering background I dread to think what I might have been suckered into buying.

 

Under floor insulation for me is the biggest improvement in the house in terms of energy reduction - I'm absolutely sure it did the lions share of reducing my annual gas consumption from 15,000 kW to 8,500 kW - however it's built just off a flood plain and has a sizeable void under the ground floor with a lot of ventilation - the CH pipes also run under the floor and the main flow and return 22mm pipes were lagged together which seems nuts from an efficiency point of view

 

2 16kW units for an 8kW heat loss is madness - it sounds like we will have a oversizing heat pump epidemic and consequences for energy wastage like we did with condensing gas boilers.