Reading the spec on passivhaus technical requirements, I see:

15kwh/sqm heating demand
10w/sqm heating load

I'm not sure how 10W/sqm times 24, times 365, divide 1000 comes to 87.6 kwh/sqm but the requirement for heat demand is 15kwh/sqm - why the disparity? What's the difference between heating demand and heating load?

The 10W/same in narrowly the maximum heating capacity you require when the outside temperature is -10C, so for a 100 sqm house you need a 1kW heater when the outside temperature is -10C.

The heating demand if 15 kWh/sqm is how much energy you need to keep the house at a constant 20C over the course if a year. This takes into account solar gains and other gains like electrical and humans etc.

The 2 figures are not directly related, Passivhaus with large windows areas and heat demand at 15 kWh/sqm are likely to require > 10W/sqm heating capacity because their heat demand benefits from solar gain but tends to lose out in very cold weather because even triple glazing has poor U values compared with (Passivhaus) walls.

Gyrogear: is all relative to floor area not surface area, and the floor area is not entirely straightforward to calculate as you need to exclude internal walls, and make specific adjustments for low height areas and boiler rooms.

I get it now, I think Nigel probably nailed it for me.. It's like saying "using a vehicle of maximum engine size X and a fuel tank of maximum size Y, you must achieve a distance of Z", the max engine size dictates the burn rate of fuel, (heat load) the tank dictates the total amount you can burn (demand) and the distance (the year) is the goal.

What I can't now understand is how, wih walls and roof of U value around 0.1 (not considering windows) how I've got a heat demand estimate (two now, actually) that are 25-28MWh, busting the passivhaus limit by about 7 times and putting my build on par with the actual gas bills of the draughty Victorian semi I live in right now. Where does the building go so wrong? But that's a question for the guy who's doing the reports and recommending a heating system..

Ventilation losses tend to be quite a large proportion of the losses in ph only because all the other losses are so low.

How about doing a thermal model?

Someone is doing calculations wrong and I reckon that at 0.1 you don't even need a heating system, I don't and my whole house heat loss was predicted to 100W for October, never used that much yet. Any boiler would turn the house into a pizza oven.

No heating system kind a guy but prefer cheese and tomato pizza

tony, tesco have a less than half price offer on Pizza Express 14 inch pizzas at the mo.. Cheese and tomato, and simple pepperoni are the two varieties stocked in my local Express, so you should be able to find some reasonably near ;)

What calculation method is being used? There is no right calculation method, because building physics is not completely understood. But some are better than others, and some are more appropriate to different situations, e.g. the sensitivity required for certain types of losses.

I'm interested to hear the results.

Why would that be the assumption? Other than the use of the term "passivhaus" in the OP... I don't necessarily think that means PHPP has definitely been used for cjard's design.

Exactly ;-)

Posted By: cjardWhat I can't now understand is how, wih walls and roof of U value around 0.1 (not considering windows) how I've got a heat demand estimate (two now, actually) that are 25-28MWh, busting the passivhaus limit by about 7 times and putting my build on par with the actual gas bills of the draughty Victorian semi I live in right now. Where does the building go so wrong? But that's a question for the guy who's doing the reports and recommending a heating system..

You haven't told us where this estimate comes from or who is doing it. Unless the estimate is produced using PHPP by a person trained and experienced in using it, then there's no point in comparing it against passivhaus standards.

As regards your actual house design and heating system requirement, it might be that if you share some information, people can help you understand it and perhaps help improve it.

Posted By: djhNo, you don't have to meet both parameters, you only have to satisfy one of them; your choice.

I've been educated:

http://www.passivhaus.org.uk/standard.jsp?id=18

In addition to the primary energy demand the standard permits that either the Specific Heating Demand or the Specific Heating Load must be met.

The peak heating load is based on using supply air to heat the building; this is quite feasible in Britain, but whether desirable or not is perhaps unclear. The annual demand is simply what the annual heating demand would be in Germany for a building that could be heated by the supply air. In any other context (North America, say) it's absurdly arbitrary.

Peak heating supply can't necessarily be easily met by supply air, largely because air doesn't have a significant heat capacity. The problem is you are limited to about 50C and the max flow of the MVHR, the 50C limit is the max temperature before which the air starts smelling (I think from burning dust). If you max out the air flow it is likely to be noisy, it is also advisable to insulate more of the duct work if using MVHR heating.

Although capital costs for the heating elements are low, heating using straight electricity without a heat pump has a high CO2 and is ~2.5 times more expensive than gas or a heat pump. The general advice is to provide either ASHP/GSHP or gas supplementary heating in the UK for Passivhaus rather than inline air heating.

Having said that you can get away with just air heating with careful design in the UK.

<blockquote><cite>Posted By: gyrogear</cite><blockquote><cite>Posted By: djh</cite>How would you reshape the criteria so as not to be arbitrary?</blockquote>

Well, one method might be to use the heating-degree-day method for a given temp (say 18 or 19°C etc.)

If one's HDD is 15% less than the "average German HDD", then de-weight the criteria accordingly; or in the opposite case, weight accordingly.

As a would-be PH chaser, it *does* seem quite odd to have to adhere to a German baseline, whereas one's local climate might be far different...

(After all, merchant vessels bear a Plimsoll line for just this reason...)

gg</blockquote>

You are right some of the Passivhaus requirements are very much focused around colder German winters and may be a little OTT for the UK. For example window U values of 0.8 are required to avoid convection currents setup because the inside surface temperature of the glass drops below 16C - but this is only when the temperature is -10C outside (or -5C????). If you look at temperatures in southern England I think you could get away with 1.1 U value double glazing 99% of the year without getting draughts because of internal surface temperatures dropping too low.

The best way to analyse your proposed building would be using a full building simulation using real local historic temperatures rather than degree days. The problem with parametric models like PHPP and SAP, or using degree days is that they don't really capture the dynamic balance of internal gains versus losses with real weather, and the building's thermal mass. Generally they breakdown compared with a full building simulation when you get a whole series of cold dull days with no solar gain as is more common in the UK than Germany; SAP and PHPP generally under-estimate the heating requirement...... and try to overcompensate with approximations.

Personally I feel that eventually SAP and PHPP's backends should move onto using full simulations rather than trying to fit parametric approximations. Particularly with PHPP, given the level of detail you enter in terms of building fabric you may as well use that to drive a building simulation engine, given computing power is so cheap these days and good quality local weather data is available (5 minute temps, solar gain, wind, rain etc.).