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Author Topic: ASHP - monovalent, bivalent or hybrid?  (Read 5410 times)
Antrobus
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« on: February 24, 2013, 08:58:31 AM »

Hi

We're currently building a house; a new build rather than self build though we retain quite a bit of control over the design. The builders have told us we need to have a heat pump so that the house meets Scottish Building Standards emissions targets. Can't do PVs as the south-facing roof elevation has a number of dormer windows which would be difficult to fit around. Can't do a GSHP as the garden has a number of big trees, and can't really afford a bore-hole. This really just leaves an ASHP. We're a bit worried as the neighbours have one and they're finding it very expensive to run despite extensive trouble shooting with the fitters. We've persuaded the builders to fit UFH which I hope will help.

We live in rural Scotland (and can get pretty cold here) and have a fairly large house (260m2), and the street has mains gas. I understand ASHPs are reasonably new technology and, having never heard of them up until a couple of months ago, I've been trying to research them but finding it difficult to find unbiased advice (i.e. companies promise a CoP of up to 4.5, govt data suggests an average CoP of around 2.5?)
Could we reasonably expect a heat pump to provide all our heating and DHW needs, without breaking the bank in electricity bills? Or, seeing as the street has mains gas, would we be better to get a gas back-up system?

Thanks in advance. Very interesting forum by the way.
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Bodidly
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« Reply #1 on: February 24, 2013, 09:27:39 AM »

Welcome to the forum  Smiley

I am a fan of Ground source heat pumps and ASHP are getting better all the time but if you can have mains gas I would take advantage of it. Most who opt for ASHP or GSHP do not have mains gas as an option and are choosing between wood,oil or bottled gas. I think you are very unlikely to save any money using a ASHP over mains gas in the near future. Just re read your post and you say you are required to have a heat pump?

Beau
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sam123
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« Reply #2 on: February 24, 2013, 09:32:19 AM »

You have money to build 260m2 house, but not money for borehole?

Have ever thought about heating cost after you move in?

When I start my project 10 years ago, I used excel and calculate how much my energy bill will be with different systems.

I got conclusion that as long as my house loan interest rate keeps below 6% it will be cheaper to buy GSHP with 210m borehole.

From that days electricity price has been more than doubled and interest rate is below 1%  hysteria

We use about 13000-14000kWh electricity per year (about 8000kWh to GSHP=> 20000 free kWh from ground) . 100-120m3 domestic water. 270m2 middle of Finland. We have every winter months with average temperature less than -10 degree.

So calculate, calculate and calculate. That is my advice.

cheers, Sami



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pdf27
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« Reply #3 on: February 24, 2013, 10:51:44 AM »

Gas is ~5p/kWh, assume 90% efficient so ~5.5p/kWh of heat delivered.
Electricity is ~17p/kWh. To break even with gas, you need a COP of 17/5.5 = 3.1 or greater

SAP 2009 assumes 0.198 kg/kWh CO2 for mains gas (realistically, ~0.22 kg/kWh allowing for efficiency).
It also assumes 0.517 kg/kWh CO2 for mains electricity (current values are ~0.55 - see http://www.ecotricity.co.uk/our-green-energy/energy-independence/uk-grid-live )

On that basis you need a COP of 0.517/0.22 = 2.35 to show any environmental benefit, although as the grid is decarbonised this will go down.

Personally in your situation I'd go for the underfloor heating no matter what (it will always improve the efficiency of any heating system to provide heat at a lower temperature), then go for a mains gas boiler and spend the money you're saving over an ASHP on lots of insulation. That will give you lower fuel bills in the long run, ensure you meet building regs now, and if you've gone for underfloor it won't be very hard to replace the gas boiler with a heat pump when the grid decarbonises enough to make it worthwhile. Heat pump performance is also improving over time, so it may actually make sense to fit a cheap gas boiler now and replace it in ~5 years or so...
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kennyt
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« Reply #4 on: February 24, 2013, 12:07:16 PM »

It would be easier to answer your question if you could provide the heat loss of your house, and is the mains supply single or 3-phase.

The CoP for most ASHP is about 4 at external temperature of +7c and flow of 35c, but deceases to 2.5-3 with flow of 55c. So for heating using UFH with a flow of 35-40c will be more efficient.
The CoP for most ASHP is about 2.5-3 at external temperature of -7c and flow of 35c, but deceases to less than 2 with flow of 55c.

You could ask your neighbour if their heating system has built in or external backup immersion heater which has been set to come on, their installer would have programmed the external temperature that the backup heater switches on, but this could be set too high and on more than it should.
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Antrobus
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« Reply #5 on: February 24, 2013, 12:11:14 PM »

Thanks for your answers - your advice is helpful.

Just to clarify - this is a new-build, not a self-build house, and the builders have an ASHP included in the package. Anything we want over and above this (such as a bore-hole) we have to cough up the cost for and this can't be added onto the mortgage (as this affects the already tight loan-to-value ratio). We are led to believe that we can't have mains gas as the sole source of heating as SBS emissions regs won't allow it, and the builders plan to put in an ASHP at no extra cost to us. My understanding is that ASHP are more efficient in spring and autumn, and when the CoP falls below about 2.3 (below 1-3 degrees or so?) gas is cheaper. Therefore a hybrid system might be cheaper during the winter months. However all of the hybrid packages I've seen seem to suggest they should be used for older houses which already have a gas boiler, and new-build houses should use a monovalent system. I'm sure the builders will want to put in a system that will be cheaper for them, rather than cheaper for us so I don't really want to ask their opinion just yet.

Like I said I'm a bit of a beginner at all this, and just trying to find some unbiased advice.

Cheers
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Antrobus
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« Reply #6 on: February 24, 2013, 12:17:53 PM »

kennyt: haven't got the heat loss figures yet. As you said I suspect the neighbours haven't got their programming right, plus they have oversized rads (we're getting UFH), and they've got the thermostat set at 23 deg (we prefer 18 room temp and plan a 35 degree flow temp) so I imagine our bills will be less than theirs. Also their after-sales (Worcester Bosch) advice has been pretty poor - little advice re programming or usage. They're currently paying about 300 per month in electricity.
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pdf27
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« Reply #7 on: February 24, 2013, 01:12:43 PM »

We are led to believe that we can't have mains gas as the sole source of heating as SBS emissions regs won't allow it, and the builders plan to put in an ASHP at no extra cost to us.
That sets off all sorts of "scam" alarm bells for me - I've never heard of any rules like that and can't find any references to them with google. And saying they'll put in an ASHP "at no extra cost" makes it sound like they've just jacked up the price of the build by 5k or so and are using the ASHP to justify it. I'm not at all familiar with Scottish building regs, so they may be telling the truth, but if I were you I'd insist on them giving you the reference to the section which says that and go look it up yourself - or even phone the council planning/building control department up for advice.

They're currently paying about 300 per month in electricity.
Assume 50/month is for general gadgets, cooking and stuff. 250/month at 17p/kWh is 17,600 kWh/year for heating. Assuming they get a COP of about 2 (doesn't sound a very well set up system) and their house is the same size as yours, that's 136 kWh/m2/year.

Putting the 17,600 kWh/year into SAP 2009 money (to get an idea what the rating is) is ~2,600/year. That gives me an ECF of 3.97 and a SAP 2009 rating of 46 (Band E!). That's absolutely horrendous for a new-build property - from the sound of it probably down to bad heat pump installation/commissioning and user misuse. It would be enough to make me want to take a proper look at the heat loss calculation though to figure out what the actual heat load should be...
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dhaslam
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« Reply #8 on: February 24, 2013, 01:32:46 PM »

There is a secondary issue with air source  heatpumps in cold damp conditions.  If the unit is spending a good part of the winter covered in ice which is constantly  being  thawed out by reversing the flow  the electrical components are  much more likely to have problems from condensation and  dripping water.    A ground source heatpump should last a lot longer. 

Some heatpumps have two  running modes, one for heating and a separate one for DHW.   This allows heating to be  achieved at a near optimum COP with  just DHW heated to higher temperature.    This option might get the air source heatpump into  a more competitive  position. 
   
A well insulated house  would need roughly the same  energy  for water heating as space heating and in summer the ASHP usually has a  higher source temperature. 

It is unfortunate that your house design didn't take solar panels into consideration.         
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
titan
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« Reply #9 on: February 24, 2013, 03:02:06 PM »

Gas is ~5p/kWh, assume 90% efficient so ~5.5p/kWh of heat delivered.
Electricity is ~17p/kWh. To break even with gas, you need a COP of 17/5.5 = 3.1 or greater



NEP publish monthly fuel comparison costs, a COP of 2.7 with an ASHP is the same as mains gas.


http://www.nottenergy.com/energy_cost_comparison/
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pdf27
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« Reply #10 on: February 24, 2013, 04:57:16 PM »

Only if you can buy your electricity at 14.4p/kWh :p
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Ev
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« Reply #11 on: February 24, 2013, 05:09:55 PM »

Quote
Can't do PVs as the south-facing roof elevation has a number of dormer windows which would be difficult to fit around.

Will you have a garage that any solar panels could be put on instead of the main house roof?

If your house is to be built by an NHBC builder, it might be worth an email to their building control enquiries department at buildingcontroladmin@nhbc.co.uk (their technical enquiries address for guidance on building regulations), but I'd start with your local council building control guys.  Here in Fife they have always been quite helpful (especially when compared with the planners!) when I've had any queries.
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