Heat loss calculation check

[JamesPa]

18 minutes ago, SBMS said:

Can MCS factor this in?

From my personal experience it seems that the MCS 'rules' are interpreted with differing degrees of latitude by different installers.  Sizing my own installation turned on:

• allowing for fabric upgrades which I know have been done (because I did them) but were not visible - some installers would and some point blank refused.  This made a difference of 4kW
• assuming an ACH value substantially lower than the default  - again some installers would and some point blank refused - this made a difference, on top of the above, of 3.5kW

If your installer wont factor in something that matters, my advice is to find an installer that will.  The actual text of MIS3005-D, the governing document - which is worth reading, is very much open to flexible interpretation.  Following it to the letter and refusing any interpretation that the 'letter' leaves open probably gives maximum protection for the installer, which may not what you, as the customer, wants!

Edited Friday at 21:05 by JamesPa

[SBMS]

1 minute ago, JamesPa said:

From my own experience it seems that the MCS 'rules' are interpreted with differing degrees of latitude by different installers.  Sizing my own installation turned on:

• allowing for fabric upgrades which I know have been done (because I did them) but were not visible - some installers would and some point blank refused.  This made a difference of 4kW
• assuming an ACH value substantially lower than the default  - again some installers would and some point blank refused - this made a difference, on top of the above, of 3.5kW

If your installer wont factor in something that matters, my advice is to find an installer that will.  The actual text MIS3005-D, the governing document, is very much open to flexible interpretation.  

Thanks @JamesPa this is useful. 
 

May I ask what values your installer agreed to regarding your ACH? Did they use a higher ACH but allow you to factor in MVHR? Or did they just allow you to adjust the ACH value to accommodate the heat recovery and if so, what value did they use?

[JamesPa]

12 minutes ago, SBMS said:

Thanks @JamesPa this is useful. 
 

May I ask what values your installer agreed to regarding your ACH? Did they use a higher ACH but allow you to factor in MVHR? Or did they just allow you to adjust the ACH value to accommodate the heat recovery and if so, what value did they use?

I don't have MVHR, I have a 1930s house with (originally) solid walls, that has been subjected to various fabric upgrades at various times including double glazed windows that have no trickle vents and solid flooring downstairs.  MCS ACH = 1.5-2.5 based on age.

 

I ended up with two installers that provided satisfactory quotes.  One assumed ACH = 0.5, the other a bit higher (0.8 I think).  0.5 gives a total loss that is consistent with the measured loss (based on analysis of half hourly gas consumption over 2 years), 0.8 ACH works out a bit higher than the measured heat loss, but still resulting in the same design choice of Heat Pump. 

 

I understand that decent MVHR has a recovery rate of 80% so I cant see why one would assume an ACH above (say) 0.25 unless its expressly mandated, specifically for heat loss calculations (which is what they are supposed to do)  in the British Standard (to which I don't have access) referenced in MIS 3005-D.

Edited Friday at 21:17 by JamesPa

[SBMS]

2 minutes ago, JamesPa said:

I don't have MVHR, I have a 1930s house with (originally) solid walls, that has been subjected to various fabric upgrades at various times including double glazed windows that have no trickle vents and solid flooring downstairs.  MCS ACH = 1.5-2.5 based on age.

 

I ended up with two installers that provided satisfactory quotes.  One assumed ACH = 0.5, the other a bit higher (0.8 I think).  0.5 gives a total loss that is consistent with the measured loss (based on analysis of half hourly gas consumption over 2 years), 0.8 ACH works out a bit higher than the measured heat loss, but still resulting in the same design choice of Heat Pump. 

 

I understand that decent MVHR has a recovery rate of 80% so I cant see why one would assume an ACH above (say) 0.25 unless its expressly mandated, specifically for heat loss calculations (which is what they are supposed to do)  in the British Standard (to which I don't have access) referenced in MIS 3005-D.

Does sound like I’ll need to sit down and discuss with the installer what they would be prepared to accept. 

[JamesPa]

12 minutes ago, SBMS said:

Does sound like I’ll need to sit down and discuss with the installer what they would be prepared to accept. 

I think so if it makes a material difference. 

 

The key points of MIS 3005 D are that it requires a heat loss calculation to be done with

• specified minimum interior temperatures,
• exterior temperatures dependent on location,
• and 'otherwise complying with BSEN12831-1:2017'

Without referring to the latter I don't know what it says, but it would be surprising if it doesn't allow some latitude for accounting for the actual building.  Unfortunately its £460 to buy. 

 

I doubt many if any installers actually have a copy so they will most likely be relying on interpretations/software produced by others.  Thus they may be (and in my experience are) open to persuasion or may dig their heels in, depending on how confident they are in their own abilities/judgement.  In the end they know that the MCS rules, followed to the letter, are their protection because following makes them essentially bomb proof.  However the better ones also know that pleasing the customer and doing the right thing, with an appropriate interpretation of the MCS rules, is a better way, in most cases, to gain a good reputation.

Edited Friday at 21:31 by JamesPa

[MikeSharp01]

2 hours ago, SteamyTea said:

Half time on the play ATM.

The interval then - all hands on deck in the bar!

[Gordo]

MVHR dose not increase the heat losses as such. As air is blown in it is sucked out at a measured / adjusted rate to be left "slightly" pressurise in the house. If the house is not air tight enough it wont work properly. From memory a maximum air permeability of under 4 is recommended., otherwise the ACH would be uncontrollable. 

 

As a matter of interest ACH units are based on the volume of the building whereas the air permeability is based on the surface area. Coincidentally it generally works out at about the same number ie air permeability of 5 is about 5 ACH. very roughly mind.

[Gordo]

18 hours ago, SteamyTea said:

So with a U-Value of 1 [W.m^-2.K^-1], a surface area of 32.9 [m²], the unknown ΔK, but a solution of 881 W.

 

881 [W] / 1 [W.m^-2.K^-1] x 32.9 [m²] = 26.8 [ΔK]

Gee someone has regurgitated an environmental science book

[SBMS]

7 hours ago, Gordo said:

MVHR dose not increase the heat losses as such

It does - if you think about it, @SteamyTeais right. There’s uncontrolled ventilation from air tightness. Then, no matter how airtight your house is, adding MVHR always ADDS more ventilation and therefore more heat loss. It can’t remove the heat loss from

the uncontrolled ventilation so it necessarily always adds to heat loss. That being said, the idea is to be so airtight that we need mvhr to provide minimum ventilation, and that controlled ventilation is heat recovered so overall it’s efficient. But it does have to increase the heat loss somewhat. 

[MikeSharp01]

32 minutes ago, SBMS said:

It does - if you think about it, @SteamyTeais right.

Yes BUT it depends somewhat on how you look at it. Assuming you need .5 ACH from all sources (Natural leakage + Controlled Ventilation) the smaller you make the Natural leakage with great air tightness and more of that controlled ventilation ACH you can get the MVHR to package the more you are saving. Looking at it this way it does not add to the losses, 0.5 ACH would be a total loss if all of it was natural, but does what it says on the tin and recovers a significant proportion of the heat embodied in that air thereby saving you money and the planet some energy. So, in that sense, it does not add to the losses you needed 0.5 ACH anyway it just recovers a chunk of the energy from it such that it will always be a saving just in the slightly different dimensions of money & energy. 

[SteamyTea]

21 minutes ago, MikeSharp01 said:

Yes BUT it depends somewhat on how you look at it

Yes, but that is because controlled ventilation (MVHR) is already added to uncontrolled ventilation (holes) to get to the total ventilation.

 

I think, in reality, it makes very little difference.  Most people with MVHR get them commissioned at the BR requirements, then turn them down.  I think that the efficiency of the unit goes up when the flow rate is lower.

The volumes also make a difference i.e. large house or small house to practical flow rates.  I am sure that if my bedroom (second smallest room in the house) was as large as my living room (largest room in house) I could reduce the ACH.  This is because the human contribution to the air i.e. water vapour and CO₂ levels are more diluted in the larger volume.

Unless I invite @Pocster and his friends around for one of his parties, then full flow is necessary.

 

Edited 21 hours ago by SteamyTea

[SBMS]

12 minutes ago, MikeSharp01 said:

Yes BUT it depends somewhat on how you look at it. Assuming you need .5 ACH from all sources (Natural leakage + Controlled Ventilation) the smaller you make the Natural leakage with great air tightness and more of that controlled ventilation ACH you can get the MVHR to package the more you are saving. Looking at it this way it does not add to the losses, 0.5 ACH would be a total loss if all of it was natural, but does what it says on the tin and recovers a significant proportion of the heat embodied in that air thereby saving you money and the planet some energy. So, in that sense, it does not add to the losses you needed 0.5 ACH anyway it just recovers a chunk of the energy from it such that it will always be a saving just in the slightly different dimensions of money & energy. 

Completely agree @MikeSharp01 and this is exactly how I look at it from a design point of view - ie what is my ventilation and heat loss strategy. I think I was mentioning it from how I think the MCS installer might look at it, and from a fairly procedural view: whatever natural ventilation exists + mvhr ventilation = more heat loss. Of course it’s right that if you start out to minimise natural ventilation and control it with mvhr the overall heat loss will be much lower. 
 

Jeremys spreadsheet was great for this as I realised how much of a difference controlled, heat recovered ventilation makes. Tweaking his ACH figure and reducing the MVHR recovery to zero was a bit of a shock first time I tried it. It is by FAR the biggest contributor to heat loss and it surprises me that there’s a singular focus on fabric heat loss and tightening of u values when so much heat loss is through uncontrolled ventilation. it’s why I stopped with a 200mm cavity and beads because I really was in the realm of diminishing returns - better to spend money on airtightness and MVHR by far.
 

And also that it’s a challenge to get the MCS installers to properly factor this in (especially when it’s pretty easy from a modelling perspective). 

[G and J]

I’ve learnt a huge amount over these last months, planning our build and with my nose firmly buried in buildhub.

 

If i think back to when I started I recall that I could never have believed how important airtightness is to the overall thermal performance, so if I put myself in the shoes of an engineer trying to dimension a heat pump, without the benefit of an understanding of the massive affect of airtightness, and with an eye on the history of heat pumps and the very public moaning over their efficacy, then I can see me falling into the trap of oversizing simply because I was trying to do the right thing for the customer.  So I sympathise with both sides of this.
 

It feels like early days in the UK building industry as far as airtightness is concerned.  I imagine for many in the trade it’s another stupid rule imposed by idiots who don’t have a clue.  I imagine in a good few years it’ll be in the industry’s DNA.  Until then it’ll be a wrestle sometimes to get the right result.

[SteamyTea]

3 hours ago, G and J said:

It feels like early days in the UK building industry as far as airtightness is concerned

My neighbours were having new windows fitted and I started chatting to the fitter.  I mentioned airtightness and the site managers said "I can't sleep in a stuffy house, I need to open the bedroom window".

He was totally missing the point, and seemed intellectually incapable of understanding that you can fit controlled ventilation and open a window if you want to.

 

There does seem to be some confusion about airtightness and MVHR/controlled ventilation.  The way I think of it is that airtightness is to do with energy usage and ventilation is about internal air quality.

They are not the same thing in isolation, but can be combined to make a better house.

Edited 16 hours ago by MikeSharp01
Change outrightness to airtightness

[G and J]

4 minutes ago, SteamyTea said:

"I can't sleep in a stuffy house, I need to open the bedroom window".

That’s where I was at the start of the process. 
 

The architectural technician who drew up our plans was also of that mindset, but by that time I was converted and so I quietly stood my ground.  MVHR and a truly airtight house would never have been suggested to us.

[Kelvin]

Insulation is easy to understand at a basic level which generally is more is better (forget about diminishing returns for a moment) But that’s as much understanding your typical builder/trade has. Even then they don’t get the importance of fitting it correctly. Air tightness is almost other worldly to most of them alongside controlled and uncontrolled ventilation. I could tell from my conversations with whichever trades were on-site that they really didn’t understand it. The number of times I heard them saying ‘a hoose needs tae breathe’ Consequently I banned anyone on-site from drilling holes in anything. 

 

[SteamyTea]

1 minute ago, G and J said:

MVHR and a truly airtight house would never have been suggested to us.

[JohnMo]

3 hours ago, SteamyTea said:

mentioned airtightness and the site managers said "I can't sleep in a stuffy house, I need to open the bedroom window".

He was totally missing the point,

Many a similar conversation during and after our build. One said that I would green mould on the walls within a week of moving in. No joined up thinking or understanding you could actually have a ventilation strategy, had never heard of MVHR, dMEV, would only ever install intermittent fans - why anything else?

[SteamyTea]

3 minutes ago, JohnMo said:

No joined up thinking or understanding you could actually have a ventilation strategy

Yes, seems rife in the building industry.

Another example is integrating PV into the roof.  It is still seen as an after market add on (though the new houses I pass on the way to work have it).

And how often do we read on here about people not putting enough insulation under their UFH, usually claiming that it meets building regs or the architect approved it.

[MikeSharp01]

I think is a simple enough, as, if you could reduce the uncontrolled airtightness to 0 (zero) then any ventilation would all need to be controlled windows open or not. MVHR is a way of controlling the ventilation (MV) while recovering 90% (ish) of the heat in the air being expelled from the house to allow the house to breath. Airtightness as you say @SBMS is important. here are the figures from the PHPP for our build.

 

First with the achieved air tightness of 0.2 (n₅₀)

And then as the Passive house worst case allowable of 0.6 (n₅₀)

 

And just for fun I set the figure the PH equivalent of the Building Regs figure. (0.2 n50 = 0.3 ^{[From our certificate]}  of 8 m³/hr/m²  from Building Regs. So 8 /0.3 = 26.6666 => 26.6666 * .2 = 5.3333 which is what I entered – clearly at that point we would not have a passive house but see what it does to the Heating Load – almost doubles it.

 

Heating load goes from 8.268 * 139 = 1149.25W to 1957.54W still only need small ASHP but hey I am saving all that energy and cost. I am not sure if I can wind the efficiency of the MVHR down to 0 and see what fully uncontrolled ventilation would cost - lets see, well the PHPP spreadsheet will work it out, but a little message says - 'please get a new designer', and the figure is 20.384W/m²  pops out. So 139 x 20.384 = 2833.38W 

 

 

Edited 13 hours ago by MikeSharp01
Units error

[SteamyTea]

1 hour ago, MikeSharp01 said:

2833.38W/m²

That should be W not W.m^-2

[MikeSharp01]

1 hour ago, SteamyTea said:

That should be W not W.m^-2

Yes you are correct - I will get that edited.

[SteamyTea]

10 minutes ago, MikeSharp01 said:

will get that edited

Can you get all my mistakes edited as well. Go back to at least 1970.

[Gordo]

“Build tight ventilate right” goes the saying. Still I stand by my  tmy quote that MVHR doesn’t increase ventilation losses if the envelope is reasonably air tight. Ventilation overal is not a huge % of the overall heat load. Years gone by with poor insulation and air tightness is was less significant proportion. The 24hr electric running costs are significant tho especially if you have a limited battery size.

[SBMS]

41 minutes ago, Gordo said:

MVHR doesn’t increase ventilation losses if the envelope is reasonably air tight

Not being overly pedantic but it does increase ventilation losses, all things being equal. It’s only ever adding to the ventilation, never taking away so how can it not

increase ventilation losses? Obviously we design it in because we build an airtight house that therefore needs controlled ventilation. But if you have two identical houses with the same ACH50 score and one has MVHR and the other doesn’t, isn’t the one without MVHR more energy efficient?

 

42 minutes ago, Gordo said:

Ventilation overal is not a huge % of the overall heat load.

Only if airtight and using MVHr. Otherwise It certainly can be. If a house is well insulated and poorly ventilated, then ventilation can easily account for the majority of the heat loss.