How to calculate heat load…

[MikeSharp01]

The heat load is based on the worst day of the average year not the worst monthly average. So bunging the average in won't get you the passive house outcome - you need the worst day, the maximum energy needed to maintain the temperature difference. See here: https://passipedia.org/basics/the_passive_house_-_definition 

 

[SteamyTea]

I seem to remember that the Passivhaus standard for energy usage was based on primary energy i.e. how much coal burnt in the power station and not the delivered energy i.e. what your meter reads.

Am I correct here?

[JohnMo]

4 minutes ago, SteamyTea said:

I seem to remember that the Passivhaus standard for energy usage was based on primary energy i.e. how much coal burnt in the power station and not the delivered energy i.e. what your meter reads.

Am I correct here?

Think that's where it all started from, but believe it's been distilled down to energy meter - but not really true for any more for heat pumps - as even a leaky shed could be Passivhaus with a well designed ASHP system. So think it's really delivered heat now.

[SteamyTea]

5 minutes ago, JohnMo said:

Think that's where it all started from, but believe it's been distilled down to energy meter

Makes a lot more sense as we clean up our generation, but there should be a penalty for gas boilers.

 

The main thing is to get the overall W.m^-2.K^-1 number as low as possible.

More correctly would be to use W.m^-2.ΔK^-1 as that takes the users preferences and the house location into account.

[IanR]

3 hours ago, SteamyTea said:

I seem to remember that the Passivhaus standard for energy usage was based on primary energy i.e. how much coal burnt in the power station and not the delivered energy i.e. what your meter reads.

Am I correct here?

 

The PH targets are independent of space heating method, they're a measure of fabric energy loss.

 

The targets are set so that you could avoid conventional heating and heat the house via the MVHR, with a resistive in duct heater (not a wet duct heater via ASHP).

[G and J]

5 hours ago, JohnMo said:

even a leaky shed could be Passivhaus

So I am building a Passivehouse - brilliant!

[MikeSharp01]

1 hour ago, IanR said:

 

The targets are set so that you could avoid conventional heating and heat the house via the MVHR, with a resistive in duct heater (not a wet duct heater via ASHP).

But the PHPP has a cooling via MVHR option that cannot be done feasibly with direct electric - unless you can afford a big enough thermoelectric cooling plant, so ASHP will be a good option for that.

[Alan Ambrose]

So, to summarise, I think there are 2 situations and temperatures to answer these 2 questions:

1. How much energy will I use to heat my house over the year 'i.e. Heating Demand'? Result in energy per annum per m^2. Uses average tempertures per month etc. Multiply by your m^2 and you get forecast annual heat demand.
2. What power do I need to heat my house on the 'coldest' days 'i.e. Heating Load'? Result in power per m^2. Multiply by your m^2 and you get the minimum heating power required to maintain your place at the set temperature on the coldest day. Uses 'coldest' temperatures. However 'coldest' is a bit of a tricky statistical value as it's looking towards the end of a tail on a distribution (see @SteamyTea's graph above) - so it's a percentile number taken from an averaged distribution.

In PHPP, both calcs take into account ventilation heat losses, solar gain, internal gains etc. Doesn't take into account how that heat is produced - that's a later step.

 

For East Anglia, PHPP uses -0.4C for coldest temperature.

[G and J]

4 hours ago, Alan Ambrose said:

For East Anglia, PHPP uses -0.4C for coldest temperature.

Fab. I’ll plug that into Jeremy’s spreadsheet.  

[IanR]

20 hours ago, MikeSharp01 said:

But the PHPP has a cooling via MVHR option that cannot be done feasibly with direct electric 

 

I believe active cooling was a later add to PHPP to support warmer humid climates get under the 10% Max over-heating target and still achieve the 15 kWh/(m²yr) heat demand and the 10W/m² heating load targets (including active cooling).

It shouldn't be needed in the UK climate to achieve the 10% Max over-heating (>25°C), for the UK that target seems to me to be quite excessive and should easily be achieved via passive means, but I can understand how including active cooling within the PHPP model can help understand ways to really get the over-heating risk down to a low, comfortable level, say <3%.

 

My comment regarding the resistive in-duct heater for the MVHR is just acknowledging the reason for the 10W/m² heating load target. The PH heat load target is set around one person needing about 30m³ of fresh air per hour, and that volume of air heated to 50°C (below the dust burning temp) via an electric resistance heater in the MVHR supply can carry 300W of energy into the property, so assuming a minimum 30 m² of living space per person the maximum heating load at a given point of time should not exceed 10W/m² of living space.

Edited August 26 by IanR