Energy use in a Structurally Insulated House

Structural Insulated Panel houses have greater thermal efficiency due to good insulation properties (0.2 W/m2 K or better) and also air tightness. Below I have tried to work out how much energy was used in one year by my SIP house.

The summary is that I used about 11,000 KWh of imported energy for the year. A typical UK house uses 22,000 KWh (I expect a detached house of this size would normally use much more than this).

Imported energy splits as:

Electric = 6500 KWh (approx half not used for space heating)
Wood Burner = 4500 KWh (all for space heating - see below for calculation)

Home made energy contributions include:

Solar thermal/hydro = 1500 KWh (minimal space heating)
Passive Solar = 2700 KWh (I'm only including the special sky light window)

The split of energy is thus:



My conclusion is, while this SIP house has made a very large impact on my energy use, to go energy neutral would require a LOT more than a SIP house. I have no idea how Government targets to make energy neutral houses will be achieved unless they use some strange criteria. I would need a very large and expensive PV system to cover the gap. I've worked quite hard to get my energy use down,  to do better every single detail of a house would need very careful consideration if you want to carry on living your life in a business as usual way. Maybe that is the truth of things - to have a house which is energy neutral requires us to use and consume far less.

Data for comparison and assumptions:

House area: 196 m2 detached
SIP Panels: 150mm Urethane and OSB
Location - South West UK
Occupants = 3 on average

Wood Energy
Dried (20%) wood = 11MJ per Kg. (note 1KWh = 3.6MJ) - oak/beech about 15MJ per Kg
1 m3 of 20% dried hardwood = 650Kg approx
1 year use = 3 m3 approx
Wood burner efficiency = 75% approx
1 year energy = (11MJ x 650Kg x 3 x 0.75%)/3.6MJ = approx 4500 KWh pa.

Passive Window
Based on a east/west contribution of 410 MJ m2 over a 33 week heating season the large 8x3 m2 window contributes (410 MJ x 8m x 3m)/3.6 MJ = 2700 KWh. (note window is actually inclined south facing in one direction and inclined east facing on another plane - so this is very approximate)

Solar Thermal
20 x 58mm evacuated tubes (approx 2.454m2 aperture)

Much of the data I use is from "Environmental Science in Building" by Randall McMullan. It's really useful for calculating heat losses, thermal storage, heat gain from windows, light, sound, ventilation etc. Simple breakdown of calculations and lots of worked examples at the end of each chapter. Not a cheap book, but good quality.

LED lights

My very good friend Patrick hand made one of his beautiful LED lights for me. I really like the idea of hand made electronics when normally they are all mass produced.

I always pictured the jungle plants lit with green light and then another strong color in the background, and years later it's here.

I need the plants to grow a bit more, and get some of Patrick's new wall lights. It is very hard to do justice to quality and brightness of the light using a digital camera, but this was my best effort (much brighter in real life). You can select from hundreds of different colors for the front and back of the light, quickly changing to whatever you want. Some calm, some outrageously strong and pure.

Building can be fun, but I guess it is what you are left with that counts. If you build a house I think you need to list at least 5 really strong ideas before you start, and I think the indoor Jungle is one. Things have changed a lot since this post.

To see more of Patrick's lights and contact him visit www.smoo.co.uk

Are Heat Recovery Ventilation HRV worth it ? (Vent Axia HR250)

Back working on the house again. This time fitting the Heat Recovery Ventilation system (HRV). It's a Vent Axia HR250 from ebay; £250 brand new with speed controller.

All houses need to be ventilated. Highly sealed SIP houses like this one need mechanical ventilation (passive is hard to design and control and cannot recover heat) otherwise you get condensation and poor air quality problems.

An HRV exchanges the heat energy from stale exhaust air to the incoming fresh air. This creates a rats nest of pipes at the HRV unit. Fresh air in, stale out, two feeds of warmed air to rooms, extracted air from bath room, small toilet room and, experimentally, a filtered cooker hood.

I've made my own control unit. Essentially it gets three boost signals to raise the unit from L1 (default trickle level) to L2 for small toilet (90m3/h @ 36w), L3 bathroom (112 m3/h @ 51w), Cooker hood (220 m3/h @190W).

The fans in the toilet and bathroom are icon 30 and icon 60 fans which automatically open when triggered by humidistats, IR sensors and lights. They each have in built timers. I disabled the icon fans and just use the boosted HRV to pull the air through the units. This makes them practically silent in operation now. Boosting is achieved with three 240v coil relays on IDC carriers in a small consumer unit, connected to 3 of the six 6 transformer taps in the speed controller (white box above). The relays are daisy chained to pass the highest boost level currently on demand to the HR250. Later I might 'break' the default L1 trickle ventilation rate if the outside air temp is very low or PIR sensor detects the house is empty.

It all works really well, is quiet and makes the air much fresher, especially in the mezzanine areas of two bedrooms which were poorly ventilated and uncomfortable before. As the fan is balanced (pulls air in while pushing the same amount out) it runs MUCH quieter than the noisy cooker and bath room fans it made redundant and more efficiently. The bathroom humidistat runs for a noticeably shorter time. The HR250 has two exhaust ducts. I'm using one on the cooker hood via a filter and I hope that stops it from clogging the heat recovery unit. I'm sure it will be OK as the cooker hood has run for year, and the pipes are clear.

Now how much energy do these things save over just exhaust fans? Well it's not easy to calculate, but here goes (making lots of assumptions):

Taking the volumetric specific heat capacity of air as 1300 J/m3K and the average outdoor temp here as 12degC, indoor temp 21degC, the designed ventilation rate for this house 30 l/s or 108 m3/h according to building regs, HR250 has a 70% efficiency.

• The temperature of the incoming air is raised: 21-12 x 70% = 5.6 degC
• The energy reclaimed = (1300 x 108 x 5.6)/3600 = 218W/h
• Now the fan will use 51W to achieve this ventilation rate, so energy saved = 218 - 51 = 167W/h
• Assuming continuous operation that's = (167x24x365)/1000 = 1,462 kWh
• At 0.12p per kWh, that's £175 per year or about 750Kg of carbon saved.

Now that sounds good, but I don't run it at 30l/s, I run at less than half that rate in the summer due to high air infiltration from open windows and doors, and I use boosts when required, so perhaps the savings are going to be more? When running at lower speeds the system seems more efficient. I have measured > 80% efficiency at L1. Later, when the house is empty I want the system to shutdown, so not ventilating at all. I don't know how to calculate all these variables and you start to see why it's hard to work out the saving. I might know next year when I get my full year energy bill.

The system cost £800 when you include the HR250, ducts, vents and controls. As you have to ventilated the house to reduce condensation, smells and pollution it's definitely better than just simply ventilating with no heat recovery.

Tips:

• Put a fresh air inlet close to the entrance of the kitchen to make cooking smell extraction more efficient and encourage air to going into the kitchen.
  
• To reduce noise don't put a T duct junction closer than 1.5m from HRV unit, and use a soft Y junction.
  
• Use acoustic ducting between rooms if there is 'cross talk' via the ducting, or for short outlet ducting. It is expensive if you get the proper stuff - it HAS to be acoustic ducting, don't accept insulated ducting with acoustic properties. This can cost 3x more than insulated ducting.
  
• If you connect the cooker extract, disable the cooker hood fan which locally accelerates grease around the ducts and use the fan signal to demand maximum HRV boost. Use an additional in line-filter if there is any doubt about grease as this is not recommended operation and regularly check and clean the exchanger. Note as the system is balanced you will not need to run it at such a violent rate as before to get the same effect. I find the default trickle vent enough for cooking with lids on pots in summer.
  
• Build time about 3-4 days for this job.
• I choose the Vent Axia HR250, because it older technology, cheap, robust, easy to speed control, has multiple extract vents, and the company have a good and long reputation. I'm not sure the newer versions with pollen control and summer by pass filters, electronic control systems really justify an extra £1K.
• In winter when the UNICO forced air heating system is on and the windows are normally closed, the HRV uses the house as a giant air mixing box. The UNICO system pushes the air to all corners of the house. I have seen designs with the HRV air inlet feeds only into the return of the forced air system to save using two sets of ducting. This really only works for forced air systems with cooling and heating ie. 24x7x365 operation. I do not use the UNICO system in the summer (The UNICO fan is 200W and I have no cooling option - not very eco!), so I need two sets of ducting. Fortunately my HRV ducting is very limited and 80% is in the loft room.