2xASHPs and WBS

[rondurrans]

Andy with your 8 kWp PV system will you effectively be carbon neutral?

[andyrob]

With 8 kWp PV system will you effectively be carbon neutral?

If I were more cynical I'd smell a trap here! I've calculated that the schools that we have built in recent years, ones that have had shedloads of extra money to have buildings that were intended to be carbon neutral, haven't a cat's chance of being that. The so-called "Carbon Calculator" used in their design never worked properly  - hardly surprising as it was never intended for use in the way that the old 'Building Schools for the Future' scheme used it.

You can use carbon calc's in two ways - to compare similar projects using different LZC (low/zero carbon) methods, or to see the effect of changing aspects of a design within a single project. If we tried to calculate whether, or not, a scheme was actually, or going to be sometime in the future, carbon neutral I could be shot down in flames - many times over.

But, to answer your question - I hope so but doubt it. But if I could get PP for a wind turbine ........

Andy

As before, I think you shouild do what you can; try to be carbon negative - some projects are. But trying to make my project as good as possible is in the context of trying to help my kids make their lifestyles (houses, transport, lifestyles etc) lZC as well. I suppose it's a  team thing - locally, nationally, internationally.

[rondurrans]

My name does not begin with ‘M’! 

I am genuinely curious……you have a blank canvas more or less, will you be designing to a ‘passive’ standard (I know passive standard means many different things to different people). When I talk of it I think of approximately 15 kWh/m2/yr for space and water heating; however to achieve that it seems you need very low u-values; less than 2.5 air changes an hour at 50pa; MVHR and good solar gain at the right time of year (isolated or direct). 

Regards,
Ron 

[andyrob]

Sorry Ron

I've had to battle my way out of too many corners in arguments with my pals on a Friday night pub session to be unaware of the occasional trick question.

I think my issue with the carbon neutral building is that it is two-sided - demand and supply, input-output etc. I would argue that we start on the demand side - reduce usage (dual flush toilets, aerated/percussion taps (in the public arena anyway), etc), reduce need (insulation, woolly jumpers,  RWH etc), recycle (heat recovery, etc) and re-use (eg burning cardboard packaging on a WBS). These can be cheap and easy measures but require a commitment to specify/install/use.

So, if we cut the need and demand for fuel and power, especially of the grid-supplied and carbon-based variety, we can then pay attention to the generation of that power.  I would have more time for a house that was less carbon neutral than another if it had tried really hard to get there in difficult circumstances; more so than the house that's of new construction, orientated south with solar panels, on a district biomass heating scheme, and/or whatever.

The theoretical carbon-neutral school building is not so because the growth in the use of electricity has been exponential (PC's etc). Its natural ventilation strategy therefore doesn't work as designed without support from mechanical ventilation and air-conditioning. Overheating is a bigger problem and has a higher cost than heating; solar gain is underestimated because the theoretical models used are inappropriate. Excessive light caused by south facing windows sized to meet the regulation natural light levels result in blinds being drawn in 75% of all classrooms at any one time - this to be able to see screens and whiteboards. This then reduces the effectiveness of natural ventilation (windows have to be shut to pull down the blinds and stop them billowing and rattling), and ironically requires lights to be on, etc.

When I've done the best I can with my building, modified my household's behaviour and installed all the renewables I can afford, then if I'm not carbon neutral I'll be ..... disappointed, but happy.

Andy

PS Can you tell from my postings that I write more than I work?  It's because I'm due for an arthroscopy on my knee on Monday - at the moment I can't even get up a ladder onto my scaffolding!

[rt29781]

We have 3 ashp pumps.  Two are Mitsubushi units, air to air and they have been working for 7 years.  A few issues with one of them now (think it is a duff temperature sensor).  We also have a 12kW air to water unit from Trianco that is plumbed into our underfloor heating system along with our solar thermal panels.  Our experience is that once the outdoor temperature goes below about 5C none of the heat pumps stay on for long as they ice up.  We have a woodburner here when it gets too cold.  The solar thermal still works though if it is sunny :-).

[andyrob]

Our experience is that once the outdoor temperature goes below about 5C none of the heat pumps stay on for long as they ice up. 

I don't know if any, all, or only some older ASHPs have a defrost cycle (almost the same as a fridge defrost I guess) but my Calorex units have a so-called "Reverse cycle defrost" of 4 minutes per hour. This de-ices the unit if necessary.

Basic physics say it will be if air of any given humidity cools and then the condensing water vapour (mist/fog whatever) comes into contact with parts that are below freezing. Or if the air is around 4 or 5 degrees, because the heat pump extracts around 3-4 degrees of heat the equivalent of snow or a hoar frost is formed.

I believe the defrost cycle can be quite demanding in electricity terms so minimising the activation of the defrost cycle would be worthwhile.


Andy