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Fabric and ventilation heat loss calculator


Jeremy Harris
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@JSHarris couldn't see if this was asked before - Row 77 is labelled as a daily heat loss figure but when this value is used in Row 84 (Monthly heat energy input) the formula (in Row 84)  multiplies the Row 77 value by 24 and also by the number of days in the month. I'm just getting to grips with some of the terminology used but this looks to me like Row 77 is the hourly rather than daily figure - would appreciate setting straight on this if poss.

 

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  • 9 months later...

Ok, having another go at these calcs in @Jeremy Harris's heat loss tool prior adopting the willis approach (see blog entry). Just seeing if this approach is correct.

 

Wall u-value in B7 - I've excluded any window /door apertures to get a total area for each external (heatloss) walls and worked out the u-values for each. In most newbuild houses this is a fairly simple thing to do as your wall is pretty much the same all round the house! In my case I have a multitude of differing materials (original cavity-insulated brick and SIP walls) so have worked out the u-value for each wall. I have averaged out each wall's u-value according to their respective total wall area (as a%):

 

Column K: ((area*u_val)*wall_area_%)/area

 

image.thumb.png.4f605d707768f3b4bc1dbd49b4119776.png

Ignore the sums / averages at bottom - just sanity checks!

 

Is that the best approach to take? Certainly gives a better u-value, but it needs to be realistic of course!

 

TIA!

 

 

 

 

 

Edited by oranjeboom
deleted confusing duplicated screenshots!!
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2 hours ago, oranjeboom said:

Is that the best approach to take? Certainly gives a better u-value, but it needs to be realistic of course!

 

Honestly not sure exactly what you are doing but the correct way is to calculate the heatloss of each component (in W/K) and add these to give total heat loss. Then multiply by  the temperature difference you want to design to. This gives the heat demand in Watts. 

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54 minutes ago, A_L said:

 

Honestly not sure exactly what you are doing but the correct way is to calculate the heatloss of each component (in W/K) and add these to give total heat loss. Then multiply by  the temperature difference you want to design to. This gives the heat demand in Watts. 

 

 

Agreed.  This is essentially what my simple heat loss spreadsheet does, with some limitations, as I wrote it for a house that has no thermal bridging (all thermal bridges were pretty much entirely designed out).

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Just noted that my entry included duplicated screenshots - now deleted.

 

Simply what I am doing is trying to calculate accurately the differing walls that I have in proportion to their areas. Once I have a single wall u-value, I will obviously plug that into Jeremy's sheet. An example:

 

If you take this example of 3 different walls, it seems incorrect to just average out the u values since their areas differ. In this case, it's like bodgit builder wanting a low wall u-value so simply constructs a couple of walls that achieve that low figure (walls b and c) and then the remaining wall (wall a) achieves a really high u-value. If you simply average those out u get a not-so-bad figure:

 

image.png.38979013eebd10ccb37efb1de1437759.png

 

If however you taken into account their respective wall areas in relation to total wall area and then sum those areas you get a more representative (weighted) u-value of 0.290.

 

So surely where you have walls of differing u-values it's best to take that into account??

 

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8 hours ago, oranjeboom said:

Just noted that my entry included duplicated screenshots - now deleted.

 

Simply what I am doing is trying to calculate accurately the differing walls that I have in proportion to their areas. Once I have a single wall u-value, I will obviously plug that into Jeremy's sheet. An example:

 

If you take this example of 3 different walls, it seems incorrect to just average out the u values since their areas differ. In this case, it's like bodgit builder wanting a low wall u-value so simply constructs a couple of walls that achieve that low figure (walls b and c) and then the remaining wall (wall a) achieves a really high u-value. If you simply average those out u get a not-so-bad figure:

 

image.png.38979013eebd10ccb37efb1de1437759.png

 

If however you taken into account their respective wall areas in relation to total wall area and then sum those areas you get a more representative (weighted) u-value of 0.290.

 

So surely where you have walls of differing u-values it's best to take that into account??

 

 

I feel your pain! 

Calculating the losses for the bathroom as it's basically now an insulated box (and will be nominally airtight) is something I'd like to do later (along with an airtightness test), these things just out of interest. I've two different wall constructions. On one wall out of the 4 the insulation thickness tapers from 50mm one end to nom 100mm the other where I squared the room off. How I'd calc that I have no idea. 

 

Whatever, it's a damn sight better than before! ?

 

Could you not "work backwards" so to speak? As in if having fabric elements of varying construction just choose to ignore then, then measure overall heat loss for the whole house over a period of time and figure an average U value given to total wall/floor/ceiling areas? ...or something like that?

 

Edited by Onoff
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10 hours ago, oranjeboom said:

If however you taken into account their respective wall areas in relation to total wall area and then sum those areas you get a more representative (weighted) u-value of 0.290.

 

So surely where you have walls of differing u-values it's best to take that into account??

 

That is the way I would do it, whether that's strictly correct is another matter, but I can't see why it isn't.

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2 minutes ago, PeterStarck said:

That is the way I would do it, whether that's strictly correct is another matter, but I can't see why it isn't.

Thanks Peter! Seems a lot more accurate using a weighted average than just taking an average. This way you're looking at how much each wall's fabric build up (u-value) contributes to the whole house.

 

4 minutes ago, ADLIan said:

The area weighted average u-value is the way to go however if the main wall/Uval accounts for at least 90% of the total wall area just use that value

Yes, I think I took that approach first, looked at my the bigger walls, worked out the u-values and then averaged. But looking at each wall in greater detail (especially those walls where I have been able to squeeze in extra insulation) has altered my weighted figure in my favour.

 

3 hours ago, Onoff said:

 

On one wall out of the 4 the insulation thickness tapers from 50mm one end to nom 100mm the other where I squared the room off. How I'd calc that I have no idea. 

Again, if you want to be as accurate as you can, then I would use a weighted approach than just averaging that whole wall section. Did you insert various thicknesses of insulation against that wall? If you know their sizes still, then you can calculate using above method. If however you just used one sheet and rasped it down to get a level surface and average u-value would probable suffice. Just talking one wall here after all.

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Quote

If you take this example of 3 different walls, it seems incorrect to just average out the u values since their areas differ. 

 

@oranjeboom  If you know any German and one to go one step further than Jeremys sheet, without going as far as (or paying for) the latest version of PHPP, then you coud look at the freely available PHPP 2002 as another option.  https://passiv.de/en/05_service/02_tools/02_tools.htm

 

Edited by Dan Feist
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4 minutes ago, Dan Feist said:

 

@oranjeboom  If you know any German and one to go one step further than Jeremys sheet, without going as far as (or paying for) the latest version of PHPP, then you coud look at the freely available PHPP 2002 as another option.  https://passiv.de/en/05_service/02_tools/02_tools.htm

 

vielen dank!

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  • 1 year later...

Where do you live? The English average temperature for the winter months is closer to 4°C with the odds spell a few degrees below zero.  -20 °C would produce Texas style strain on the Grid; however assuming continuity of supply,  the heating could always be supplemented when the temperatures do plummet - for example I use an oil-filled electric radiator in the coldest month to add a little bit of extra heat for my top floors.

 

IMO, you need to do the sums (perhaps assuming that you can do most heating overnight on E7 rates) and work out what your annualised eating building will be at a CoP of 1. An ASHP will give a CoP between 3 and 4, so you can calculate the annual saving if you add an ASHP, and this will give you the investment case to see whether it's work while installing one.

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@TerryE Sorry, that was a bad question - was meaning more, need to provide 2643W of heat to maintain 20 deg inside, if -20 outside (the default value)?

Yes, appreciate that it also has the months/temps further down. Just wanted to understand what it's telling me.

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I believe so!

 

The spreadsheet is very useful, but I need to do some tweaks to the monthly table of my version (which would drive cost values), to take into account some 'gain'.

Values in July and August are around 400 but obviously you don't normally have heating on.

 

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  • 9 months later...

Hi all

Thanks @Jeremy Harris for your useful calculator.

 

I'm just trying to get my head around some of the numbers so we can assess different insulation levels, double vs trouble glazing etc.  I just wanted to make sure I am reading the calculator correctly!

 

I've plumbed in the following values:

 

Air changes per hour = 4
MVHR efficiency = 85%
Wall U value = 0.19
Floor U value = 0.1
Roof U value = 0.16
Average window U value = 0.8
Average door U value = 1.2
Average roof light U value = 1.3

 

Along with the following dimensions of our property

 

Internal wall area = 272.00 m2
Internal roof area = 125.00 m2
Internal floor area = 128.00 m2
House total internal volume = 700

 

Its a 2.5 storey house, 240 total square meters and I've used an online calculator to ascertain the internal volume using our working drawings.

 

It appears to be generating a space heating requirement of 23,933kWh per annum for average OAT (the sum of row 84).  This is giving an average cost per square meter to heat of about £100.  This seems really high, but perhaps I am misreading or misunderstanding?

 

I am effectively trying to work back from this to understand the electricity costs for an ASHP to heat this space. It seems that the "Total daily heat loss power for average OAT (W)" is coming out at 4109W (4.1kW), but then it seems that this value is multiplied by 24, so I'm assuming this isn't the total Daily heat loss power, but the total Hourly heat loss power?

 

Thanks in advance.

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16 minutes ago, SBMS said:

I am effectively trying to work back from this to understand the electricity costs for an ASHP to heat this space. It seems that the "Total daily heat loss power for average OAT (W)" is coming out at 4109W (4.1kW), but then it seems that this value is multiplied by 24, so I'm assuming this isn't the total Daily heat loss power, but the total Hourly heat loss power?

 

Thanks in advance.

4.1kW is the heat input, i.e. run two 2kW electric heaters continually and it will maintain the desired temperature. so that tells you the size of the heat source you require.

 

So 4.1kW over a 24 hour period is 98.4kWh  Note the h for hour, Killo Watt Hours.  That is the amount you will be metered and charged in a 24 hour period.

 

In rough figures a heat pump consumes about 1/3 the electricity than it outputs in heat, so a typical heat pump delivering 98.4kWh would consume about 32.8kWh of electricity in a day.

 

You don't say under what conditions that is taken, i.e. is that the coldest day?

 

Our house (half the size of yours) needs about 2.5kW to heat it on the coldest day, +20 inside, -10 outside.

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@ProDaveThanks, that makes sense.  The coldest day value (+20, -10) is coming out at around 7776 (7.7kW), average is around 4.1kW.

 

A worst case scenario for an ASHP generating 184kWh of heat in a 24hr period, would be 61kWh of electricity (or £12). Thats a whopping bill of £380! But I guess this is extremely unlikely.

 

I also don't think Jeremy's spreadsheet makes a huge amount of sense for summer values? i.e. we never have any central heating on from around May to September, so in theory there's a 0kWh requirement in these months, but this isn't what the spreadsheet is showing.  Is that typical with an ASHP too?

 

Also - am I right in thinking that a worst case of 7.7kW would require an 8kW ASHP?

Edited by SBMS
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1 hour ago, SBMS said:

I also don't think Jeremy's spreadsheet makes a huge amount of sense for summer values? i.e. we never have any central heating on from around May to September, so in theory there's a 0kWh requirement in these months, but this isn't what the spreadsheet is showing.  Is that typical with an ASHP too?

The spread sheet makes no allowance for incidental heat, e.g. from cooking, showering and just people being in the house, and solar gain through windows.  So when you get to the months that fon't show as needing much heat, the reality is none at all.  Typically I find once the outside temperature gets to about 12 degrees we need no heating at all.

1 hour ago, SBMS said:

 

Also - am I right in thinking that a worst case of 7.7kW would require an 8kW ASHP?

An 8kW ASHP would struggle, it would have to be running 24/7 doing heating.  The reality is it has to stop heating the  house to do DHW heating from time to time so it would not manage.   12kW might be closer to the mark.

 

I have a 5kW ASHP and it runs 6AM to 10PM and meets our max 2.5kW demand.

 

Now is the time to experiment and see what you can do to improve some of the U values to reduce the heating requirement.

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