The house (and greentomatoenergy) has won a 2013 CIBSE Building Performance Award in the category of Refurbishment Project. The judges said:
“This Victorian Mid-terraced refurbishment to passivhaus standard was the standout entry in this category. The project shows what can be achieved in the refurbishment of homes, which will be increasingly important as we look to upgrade our housing stock.”
We couldn’t put it better ourselves!
Well done and thanks to all the team.
[Sorry so copy-heavy!]
We have just had our second summer The good news is that, with a little bit of occupant management, both summers have been a great success – the house has stayed cool and fresh, with minor instances of localised over-heating.
When we modelled the property in PHPP, the possibility of it overheating was highlighted by the software. The PHPP model of the building estimated that there will be overheating 2.5% of the time throughout the whole year, this is when the internal temperature is above 25°C, so we were interested to see how it would perform against the prediction.
Data from the MVHR control unit, which is located in the living room, the hottest room in the house show that the room has been over 25°C for 7.1% of the year so far, this shows higher overheating than PHPP suggested.
However, CIBSE defines over heating as 26°C and above in bedrooms and 28°C and above in living spaces, the criteria for an overheated building is when > 1% of annual occupied hours are at these temperatures. Due to enormous pile of data we would have to wade through to assess a whole year, we have looked at the data from May until the end of August 2012. It shows that the living room was 28°C and above for 4.3 hours during this period, all of these hours being on the same day when the maximum external temperature was 30°C. This gives the percentage of overheating defined by CIBSE, to be 0.11% for the year so far. This is nearly 10 times lower than what would be classified as an overheated building.
Occupants report that whilst the house can get hot on very hot days - like any other house - it is quick to cool down and the temperature can be quickly and effectively managed by simply opening windows.
It is worth explaining and exploring some features of the property pertinent to its summer heating profile:
South facing – the house faces more or less due south. There is a lot of glazing on the south façade, 53.7% to be precise. As a result, a great deal of direct solar radiation enters through the front of the building, causing rooms on the south side of the building to be noticeably warmer than those on the rear / north side.
Thermal mass – due to the wall build-ups and structural (read “budgetary”) constraints, it was not possible to include any thermal mass in the rooms on the south side, at any level of the 4-storey building. The kitchen at the rear of the building has a concrete floor, which appears to contribute to keeping that area cool, but does not provide any thermal mass to the hotter, south-facing rooms. We explored more affordable / lightweight options, but thermal mass plasterboard and clayboards were not easily available when we built the property.
Underfloor heat exchange – there is a ground-to-air heat exchange under the floor in the cellar. It is there to pre-heat incoming air before it reaches the MVHR in the winter; and pre-cool it in the summer. The air is taken in from the north side of the property, so that in its self means the air will be cooler in the summer than if taken from the south. Unfortunately, we do not have sophisticated enough monitoring in place to record its effectiveness from a cooling perspective. Similarly, because the windows are often open to provide passive ventilation (in addition to the MVHR), it is hard to know whether this element contributes anything meaningful to the internal temperature.
External shading - because the house is in a conservation area, it was not possible to add any external shading without planning permission. So before going down that route we decided to see how hot the house became and tackle this issue in future, if necessary. Whilst it’s true that external shading would increase comfort in the house, overheating is nothing like a big enough problem to warrant all the effort and cost involved in adding external shading. It’s always an option in future if the climate continues to get hotter…hmmm.
Passive ventilation – The main temperature control technique in the house is good old-fashioned passive ventilation; i.e. opening some windows. Because the house is tall and the ground floor generally stays so nice and cool in the summer (particularly with the north-facing kitchen door open), we generate an excellent stack effect bringing cool air up through the house and expelling hot air through the velux windows on the top floor. This generally means leaving one small bathroom window open (on tilt, so secure) most of the time when we’re in and one window at the top of the house open. If it’s very hot, we will open more windows to increase the flow of air. It is an exceptionally effective way of keeping the house cool and makes one wonder why anyone ever uses air-conditioning (in most scenarios).
For rapid cooling of particular areas that are “off the stack”, we also occasionally use cross-ventilation, but this is less desireable because of the presence of a reasonably busy road on one side of the house.
Some people ask why we don’t just turn the MVHR off in the summer seeing as we have windows open. The answer to this is twofold:
- We don’t open all the windows, particularly not at the front of the house which is on a relatively busy road. Therefore, some continued ventilation is required from the MVHR in these areas;
- Unless it’s extremely hot, we tend to close the windows at night and it would make sense to keep turning the MVHR on and off; particularly if doing so risked forgetting to turn it on again at night!
All-in-all, whilst it is during the winter that the house really comes into its own, the good news is that it is a comfortable place to be even on those rare hot summer days.
Exciting news - the energy cost for the house has come in at less than £200 for the year 2011-2012; £189.40 to be precise.
The excellent people at Good Energy have provided me with a breakdown of all of our bills (gas and electricity) and credits (e.g. Feed-in Tariff and HotROCs) for the last year.
It looks like this for each element:
The gas is just there for cooking and more than half of it is for the standing charge. Every time I see this it makes me regret not installing an induction hob…
Avid readers of this blog (of which there are many millions) will know that we don’t yet have a year’s worth of data on the MVHR sub-meter. This means that we can’t split out (a) electricity used for ventilation, heating and hot water from (b) electricity used for lighting and appliances.
For those who are not familiar with HotROCs, they are a rather pleasing initiative offered by Good Energy for people with solar thermal systems who are still not receiving the RHI (which was “promised” by various governments, ministers etc, but is still not being paid and may well never be). Apparently Good Energy is reviewing this offer, so let’s hope they don’t decide to pull the plug.
Thanks for all the responses to our post about data - it’s great to know there’s so much interest and enthusiasm for passivhaus out there. As always, the blog was deliberately as non-technical as possible and it was clear that a variety of assumptions have been made, but some readers have asked me to elaborate a little on it, specifically about the provenance of the data and the assumptions made. So here goes…
The data is collected from the following meters:
- The electricity meter for electricity;
- The gas meter for gas;
- The PV generation meter for PV; and
- The control panel on the solar thermal system for solar thermal.
We also now have a sub-meter on the MVHR, installed in September 2011. On 27 Feb, the MVHR sub-meter showed 1,259kWh. Please note that the MVHR is a Genvex Combi 185 which does three things: provides mechanical ventilation, has a heat pump that produces hot water when the solar thermal is not doing so and provides any space heating required, also through the heat pump.
Turning to that MVHR sub-meter, we can address the assumption made about proportion of electricity used for ventilation, heating and hot water. In my post I assumed 50% consumption by the MVHR. This was based on two things:
- The sub-meter figure for the six month period, Sept 2011 to Feb 2012 inclusive, was 1,259kWh. 4 of these months sit within the year’s data period, but for the sake of painting the full picture, we extrapolated the 6 month figure to 12 months by doubling it and then applied it back to the whole of 2011 - i.e. 2,518kWh. Clearly, this is not perfect, but given that this 6 month period covers the coldest months and therefore the months when the MVHR would actually be consuming the most energy, I figured it would be fair; if anything it assumes too high a consumption by the MVHR.
- As a control on this number, I compared the balance - 2,549kWh - (i.e. energy used for lighting and aplliances) against average energy used in this way in the UK. There is a wide variation in estimates for this number, but somewhere around the 3,500-4,000kWh number is reasonable. 80% of our lighting is low energy and we are generally careful not to waste electricity, so it’s reasonable to work on the basis that our consumption will be lower than the average. That said, we still use an electric oven, microwave, fridge, kettle, TV, washing machine, dishwasher etc etc, so it’s likely that our consumption will not be cut by more than 20-30% - i.e. down to about 2,600kWh. And there is plenty of data to show that a young family, at home for most of the day has considerably higher consumption than average. So this seems to be a reasonable cross-check to support the assumption above about the MVHR.
From a passivhaus point of view, there is a bigger question here - what percentage of the energy is used to drive the fans and for heating water and what percentage is used for space heating? Unfortunately we don’t have metered data to show the relative percentages for each component of the MVHR and so have to make assumptions about how it breaks down. The assumptions are as follows:
- Fan energy consumption is 964kWh per annum (110w power consumption, running 24 hours a day, 365 days a year)
- Hot water delivered energy consumption is 531kWh (assumptions are COP of 2, 5% standing losses, 55% solar fraction)
- Leaving space heating as the balance - 2500kWh less (964kWh + 531kWh) = 1,005kWh for space heating. Multiply this by a 2.5 COP of the heat pump to give 2,513kWh for space heating. Divide this by the treated floor area of 195m2 to give 12.88kWh/m2/annum.
Someone pointed out that I shouldn’t have gone to a decimal place (i.e. 12.8kWh per m2) given the assumptions made. Fair enough, let’s call it 13kWh. This still shows PHPP in a pretty good light and is testament to the skill and attention of the builder in delivering the design.
Finally, why did I assume 50% of the PV was consumed onsite? Two reasons:
- That is the assumption for feed-in tariff export and has sort of crystallised in my mind.
- It struck me as reasonable based on my (frankly obsessive) observations of the system output and OWL monitor during the peak solar pv months last year (May, June, July, August, September).
But it’s true, it could be wrong…so let’s assume 100% of the electricity is used onsite (which is DEFINITELY wrong) and see what a difference that would make. It would take electricity consumption to 5,480kWh, but have no effect on the space heating kWh/m2/annum figure, which is taken from the space heating figure.
Of course, we would much prefer to have perfectly measured data for all of this and we will soon have better-but-not-perfect data, but in the meantime it would be a shame not to share and explain what we do have. In this case, we can’t do this without making some assumptions and are more than happy to explore these further with anyone interested.
Either I am really stupid, or tumblr really doesn’t show the date when a message was sent. So, if it has taken me ages to reply to your message, then apologies.
To answer your question, if you are doing a major renovation, you should add about 25% to the total cost for energy-related measures. If you are not doing work anyway, then this turns into 100% and then it depends on all sorts of things like size of house, finish/spec etc.
The key question is how deep a retrofit would you be interested in doing? You don’t need to retrofit to passivhaus to considerably improve the comfort in your home and reduce your bills.
If you would like to find out more about some of the less extreme projects we have worked on, feel free to drop us a line on firstname.lastname@example.org.
Sorry for the long, long gap since my last post - we’ve been busy applying what we learnt on this project to many others around the UK.
We finally have a year’s worth of data, represented (rather boringly, sorry) in the graph below:
A bit of background about the electricity consumption number:
- it includes all electricity used onsite - i.e. MVHR fans, air source heat pump for heating and hot water when needed and lighting and appliances etc.
- we now have a meter on the MVHR, so will be able to attribute proportion of consumption to that (i.e. ventilation, non-renewable heating and hot water) later this year once we have a full 12 months’ data.
- it assumes 50% of the PV system’s energy was consumed onsite.
The gas is for cooking only.
If we assume that half of the electricity consumed was for lighting and appliances, then the other half was for fans, heating and hot water via the MVHR. Over the 195m2 of (“treated”) floor area, this 2,500 kWh consumption equates to 12.8kWh per m2 per year - this is exactly the same number that we predicted in our PHPP model. Amazing!
Last Friday and Saturday we opened up the house for International PassivHaus open days. (Seems funny writing that - who woulda thought that’s something we would be end up being involved in.) Anyway, we are proud members of the passivhaus community so it was a pleasure to share the love.
I would write all about our wonderful tours, but luckily one of our visitors has kindly saved me the trouble -
Couldn’t have put it better myself!
So, we’re starting to get some data from the house. Admittedly, we don’t have the most advanced monitoring systems - i.e. me with a pencil (apart from the MVHR temperature data, which is quite snazzy, of which more to follow in another post) - but think we’re capturing the key data.
The electricity and gas readings are shown from 23 January 2011 - by which time the MVHR, solar thermal and solar pv had all been commissioned properly - up to today’s date. Both of the solar readings are totals generated since date of comissioning.
- Gas consumption - 5.8 HCF, or 180 kWh (@31 kWh per HCF) - note, this is just for cooking
- Electricity consumption - 1,837 kWh or 14.35 kWh per day (over 128 days)
- Solar PV output - 330kWh (since 9th January 2011)
- Solar thermal output - 1,198 kWh (since 16th November 2010) - total hours of operation (i.e. pump running) is 845 hours. Therefore, the 7m2 of solar thermal is putting out an average of 1.41 kW
The electricity consumption looks high, making me wonder about the extent of our parasitic load and actual power consumption of the MVHR. Theoretically, the MVHR should only use a maximum of 140w for its two fans and there will have been some consumption when the heat pump was heating the hot water, but we need to install a reader to confirm this.
It is instructive to look at the net daily electricity consumption (i.e. incorporating input from solar pv) by month, below:
- February - 20.46 kWh
- March - 14.51 kWh
- April - 10.1 kWh
- May - 8.96 kWh
We will put up some key data from the MVHR datalog soon. Most notable trend so far is impressive stability of internal air temperature.