I'd wait for the heat loss to come in and see what the report says. In the meantime, do you know the size of your main fuse? You should be able to get up to 100amp (24kW) for free from your grid operator. 

Do you know your current peak power use?

I'd also recommend getting some more quotes for the system. I've also posted somewhere a heat loss calculator you can use to go room by room. Have a look at some of my posts to find it and just do your own. You don't need to be perfect, just know the dimensions and roughly the insulation. It'll give you a solid ball park to go from

@batalto thank you.  I'm going to ring our grid operator tomorrow. I have tried to get more quotes but everyone I have rung is too busy, don't cover this area yet or haven't got back to me.  We have a "quick" quote from the solar guys but they haven't done a survey and are pricing for a 14kW unit. I will push to see the survey report and try and do my own calculations as you suggest. Thanks again 

@lizzylou 

Hi Lizzylou,

Welcome to the forum.

18kW heat demand does seem on the high side and will cost you a kings ransom to supply using an ASHP. Do you know the floor area and the type of insulation (double glazing, loft insulation, cavity wall insulation etc.)?

What is your present boiler and how much energy does it use? If you can provide details of its manufacturer and model it should be fairly easy to calculate the approximate heat demand.

The better you can insulate your property, the lower the heat loss and hence the reduced heat demand, which in turn would require a smaller ASHP, that would be cheaper to run.

You will probably find that most of the good ASHP installers are very busy, because the RHI payment is being reduced next year, so everyone who wants an ASHP is jumping on the bandwagon. There is a school of thought that when the RHI is reduced, the cost of installing an ASHP will also fall, since who is going to pay an arm and a leg to have a system installed, that probably costs approximately the same to run as their present system.

@lizzylou, welcome to the forums. As Derek says, 18kW sounds like a lot. What’s the approximate square footage of your property that you’ll be heating?

@lizzylou Which Essex company, I have had some "experiences"

@witchcraft. Thank you for replying. Use the Sun in Basildon have completed the heat loss survey and are recommending the 2x12Kw LG cascade system.  Eco Greenenergy Solutions Ltd installed the solar panels and battery but their after sales service is definitely lacking and we have pretty much been left to work everything out ourselves despite constant calls and emails which doesn't instil us with confidence about also using them for the ASHP installation.  I only managed to get him back out to answer queries, give us the handover pack (which is incorrect and doesn't really explain anything anyway) and install the iBoost which they had forgotten by dangling the carrot of an ASHP installation.  He was still unable to properly explain the Solax app.  He seems to be a great engineer though and I think our solar installation is sound, I will just have to research and learn how to make the most of the solar system myself. Anyway, he was here within days with a glossy brochure and a quote which calculated without conducting a heat loss survey though so it's difficult to do a comparison between the two.  

@lizzylou Different company to the shower who installed my ASHP then, their site surveyor and lead installer (same chap) in particular turned out to be hopeless!

That said the company have "got-rid" of a whole team of people including him from the mob who installed my kit and I'm prepared to give the remainder and their new management a fair crack of the whip as they seem to be a lot, lot more switched on now.  

Only an MMSP fit and a new Vaillant back panel to go now and we're done (after five months)...

I might even end up as a satisfied customer.

@derek-m thank you. We have double glazing throughout and although we have 2 very large roof lanterns (1.7m x 4.2m) on the flat roof they are also double glazed with 4/16/4 toughened glass. Whilst the original outside walls are brick built there aren't many of those left following our 2 extensions and all the new external walls are fully cavity wall insulated. We converted the loft space and that was then fully insulated during that build too. I think our floor space across the 3 floors is approx 190/200sqm (105+53+35).  Our ground floor ceiling is 265cm high, 1st floor 240cm and loft 210cm.  Our boiler is a Potterton Suprima 70 which we put with the 1st extension in 2001 (loft 35sqm 2006 and 2nd extension 53sqm in 2011). It is working hard! Our hot water system is not working properly at the moment and I think we need a new tank.  Basically I think we need bring the heating and hot water systems up to date and to better match the size of the house which we changed from 3 bedrooms to 5 bedrooms over the last 20 years. We have 4 bathrooms - 3 with power showers and 1 with an electric shower.  We are a family of 6 - 2 adults and 4 teenagers!  I naively thought this would be relatively straight forward and would kill 2 birds with one stone (replace the boiler and become greener). I am feeling quite anxious and apprehensive about it now but am trying to hang in here and not just ring our gas engineer/plumber and just get a new boiler and tank.  Any help gratefully received. Thank you  

@lizzylou given you have about 200m2 of floor area and a well insulated house (I will assume its pretty well insulated given the age) 18kW seems excessive. I have a 260m2 house and my ASHP is 12kW

@lizzylou 

Hi Lizzy,

According to the data for your Potterton boiler it is rated at 20.5kW output. So as you say, if your boiler is working hard, lets assume that it is close to 20kW. A correctly installed and optimised ASHP, should be possible of achieving a Coefficient of Performance of approximately 3, down to an ambient air temperature of 0C, so in theory a 20/3 = 6.67kW ASHP should be adequate. But we live in the real world not the ideal one, so I often suggest that the size of an ASHP should be 1.5 to 1.75 times the required heat demand. In your case, because you have 4 teenagers and probably have a high demand for hot water, then a 12kW to 14kW may possibly be the better option.

To check your overall heat demand, look at your old gas bills and also record your present daily gas and electricity usage along with the daily weather conditions, to see how your energy consumption varies.

You could also download the heat loss spreadsheet and carry out your own heat loss assessment. A good job for your teenagers. 🙄 

The major problem with ASHP's is that their primary fuel source, the ambient air, keeps varying, and has the lowest calorific value at precisely the time when you would prefer it to be the highest. The efficiency of ASHP's therefore reduces as the ambient air temperature falls.

A further factor that affects the efficiency of an ASHP, and hence the size required is type and size of the heat emitters (radiators and/or UFH). The larger the heating area of the heat emitters, the lower the required water flow temperature needs to be, an hence the more efficient the ASHP will operate.

I am in the process of finalising an Excel spreadsheet, which you may find useful in performing some 'what if' scenarios on any proposed system.

@lizzylou 

Hi Lizzy,

Whilst I would always try to convince anyone and everyone to use less energy, I also feel it is important to also let people know of the financial considerations. At the present tariffs for gas and electricity, it would probably be more expensive to heat your home using an ASHP, as against a gas boiler. As I'm certain you are aware, electricity is approximately 5 times the price of gas. So if your old gas boiler has an efficiency of say 80%, then the true cost per kWH would be 1.25 times the tariff rate, so for an ASHP to be operating at comparable cost, it would require a Seasonal Coefficient of Performance (SCOP) of 4 or greater.

There has been discussion of moving some of the costs of renewable incentives from electricity to gas, but I have not yet seen any proposals. The problem that the government and Ofgem face is that approximately 85% of UK homes are heated using mains gas, so any changes may not be popular to the voting public.