Looks like this is a fundamental difference in how to design a house. I was thinking of the discussion in the 'Breathing walls' thread.
If the house has relatively little mass heat storage, a lot of the heat energy is in the air. So it is particularly inappropriate to use that hot air to drive a passive ventilation system - it must be replaced immediately by some high carbon source of heat energy.
If the house has a lot of heat stored in the walls and floor, it may be more appropriate to use hot air to drive a ventilation system, the lost heat can be restored from the mass heat store.
If ALL the house's heat comes from the sun heating thermal mass, such a house could use the sun to (indirectly) drive it's ventilation system.
Again, my question is how realistic is this? How possible is it build a house that is heated adequately only by solar insolation stored in thermal mass, in the UK and elsewhere?
Comments would be much appreciated(hides under the desk in anticipation)
Peter
Peter Clark
posted on 06-06-06
Answering myself, Robert Vale clearly thinks it is possible at Hockerton.
But I note:-
1. The houses have mechanical ventilation and heat recovery.
2. The interior temperature can be as low as 18 Celsius.
3. They are very weird looking houses - earth sheltered.
Paul in Montreal
posted on 06-06-06
Peter,
interesting stuff on the Hockerton houses. But 18C interior temperature in winter (it was actually as low as 17.6 according to the link you provided). That is just too cold for anything but sleeping through! No wonder the UK has a reputation for cold houses if people think that is an acceptable temperature! Of course, my situation is special as I work from home. Before our GSHP was commisioned last October, there were a couple of days where the interior temperature had gone down to around 18C - it was impossible to work at that temperature no matter how many warm clothes I put on. My fingers and feet were just cold - since we were without any source of heat at that point I had to resort to putting a 500W halogen construction lamp on under my desk.
As to the thermal mass issue, air has a very low heat capacity so even in a "low thermal mass" house, most of the energy in the house is stored in the fabric - walls, floors, furniture etc. and not in the air. Thermal mass is useful if you can store solar energy - but the challange is to get that energy in since you have to effectively reduce insulation levels to allow it to pass - meaning that it can also escape. Windows with external thermal shutters can work though.
In my climate, we have the opposite challenge. In the summer, there is too much solar gain and so it affects cooling loads. Interesting, though, that the Hockerton houses had MERV equipment - as we've discussed to death on this forum "build tight, insulate right" and then you need a MERV. Solar energy through PVs could supply all the electricity to run a MERV - though the array size is such that the payback time could be a long one. My calculations for London and current start of the art mono-Si PV arrays show that an array size of around 10m^2 is required to run the MERV for a year. That's a rather expensive array! Perhaps a small wind turbine would be more cost effective.
Paul.
Tony
posted on 06-06-06
Try reading about "passivhaus". They have been at building houses with no heating systems for quite some time and using high thermal mass to both moderate temperature fluctuations and to store solar gain. The theory is to reduce heat losses so much that just living in the house pluss solar gain will heat it on all but the very coldest days. Paul they are mostly built in areas of Europe that have similar winter climate to yours!
Paul in Montreal
posted on 06-06-06
I'd like to see the consumption per heating degree day for the passivhaus examples. Despite being "similar", our climate is significantly colder in winter than most of Europe except the far north of Scandanavia and much hotter in summer. This is the challenge: a solution for both extremes. This past winter I kept detailed records and have the consumption per heating degree day (HDD = 18C - average temperature over 24 hours). For the larger house (436 m^2) we consumed 18300 kwh but there were 4285 HDDs in that period. Factoring in the area, this works out as 0.009783 kwh per m^2 per HDD. This was total consumption including all heating, hot water, lights, appliances, MERV etc. Using the numbers enables me to predict what future consumption will be. By the way, this was with the thermostat set on 20C for 5/7 of the time (weekdays) and 21C for the weekends. Thermal mass of the house is a mix of low (timber frame construction) and high (1 3/4" lightweight concrete floor on the ground level with radiant heating embedded). There's lots of windows, all the lowest air leakage rating on the market and the highest U-rating. Low-E coatings prevent (some) heat gain as well as heat loss so we were optimizing to reduce heat gains in summer, not increase them in winter. Air leakage assumed to be around 1ACH@50 due to the construction techniques, but it wasn't measured. Empirically, that number seems about right. Another thing about the energy m^2 ratings is that they don't take into account the house volume. I personally like high ceilings and so that's what we built - so total volume is significant IMHO.
Paul.
Paul Teather
posted on 07-06-06
Just a quick one: Degree days in the UK.
the degree day is the accumulated temperature difference required to heat a building to the target temperature. In the Uk this is typicaly around 18-19 oC (as used in building regulations).
This is not the total temperature difference as you need to take account of passive gains (solar/occupants/cooking/lighting etc).
Green Building Bible ((both volumes) fourth edition) These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.
1 year Green Building magazine subscription Green Building magazine is the UK's only eco-building magazine. It always features a wide range of eco-building projects from all around the UK.
Ecohouse 3 Sue Roaf never fails to impress with her inexaustible energy, forthright opinion and attention to detail. Ecohouse has become a classic in the green building genre.
