Hi Jules. I am just swapping out my oil boiler for an ASHP. I won’t worry that your developer would rather have put in a gas boiler - the building industry is ultra conservative when it comes to “new technology” despite the fact ASHPs have been around for 40+ years. 
If you have a new build, it will have been built to the latest standards, meaning well insulated. So with UFH, your ASHP should work very efficiently. 
Your installer should explain how the system works. The controls and functions. He should be MCS accredited and should support you for a few years. Before you take out a support contract (like you would with a gas or oil boiler) to get it serviced annually.  
I hope this helps. But you’ve done the right thing to reduce your CO2 footprint. Electric car next. 
I would also put solar PV on the roof

Thanks for the post Jules, and welcome to the forums.

From our experience, we had two very heavy heating months this year (Dec-Feb) with a few cold days now in April, and they were just marginally more expensive than running oil. Over the course of 12 months, the ASHP is much cheaper than oil (in our case).

The biggest benefit of running an ASHP is that it will keep your house warm 24/7 over the cold months, as opposed to an oil or gas boilers that runs on schedule. There is also a big benefit of using underfloor heating and ASHPs.

At the end of the day, the running cost and efficiency of the ASHP will come down to how well the property is insulated. If it's well insulated you should have little to worry about.

I would, however, recommend that you examine the heat loss calculations for your property and ensure that the right-size pump has been installed. If it's too small, it may struggle to heat the property which is what has happened to some users on this forum. 

Valid points @julianc

I wouldn’t worry too much about the developer stating he would have preferred a gas boiler if possible. It’s the easy low cost option. I’ve not met many developers that are willing to go beyond the cheapest/easiest option...

As Mars has stated, as long as the ASHP is correctly spec’d to the property you have nothing to worry about. 

Quite a few of us are off grid in regards to not having access to gas and have minimal issues running ASHPs.

You will just need to invest some time into understanding how the system works best with your property and how to get the best out of it in terms of performance and efficiency. 

Most of us here have already been through this and are happy to help if needed. 

Spot on @george.

@jules, the key is understanding how these systems work, as each one works differently in every house, and it takes time to figure out what the best and most efficient way to run it will be. It'll take trail and error, and patience.

Thank you to everyone for your replies but I am still concerned that this form of heating, even if it works satisfactorily, will cost more to run than other systems.  I also think that it may be prudent to have some form of back up heating eg a woodburner, in case the level of heating is not adequate during colder months or something goes wrong with the system.  Also, does this system provide enough hot water to satisfy several bathrooms?  Does the air source heat pump have to remain on 24/7 and thus using constant electricity.  What happens in Summer?  Does the pump have to remain on 24/7 to provide hot water.  As you can see I am quite ignorant about this system and at the same time quite worried. I get the overall impression that people think yes there may be problems but as long as the property is well insulated and you have the correct size of pump for the property, you can overcome those hurdles but no one seems to be saying this is the best form of heating and hotwater and is economical to run.

@jules, these are all valid and relevant questions, and there is a lot to get your head around. Ask as many questions as you need.

Yes, the ASHP should remain on throughout the winter, consuming electricity all day long, which from our experience is much more efficient and it keeps the house warmer. Remember, heat pumps are slow burners that take time to heat areas up. That is why it's essential you get a really good, low tariff from your electricity provider.

What target temperatures do you like in your house, because that will also address your consumption? We like it warm (21-22C) so we consume a lot more power.

In terms of a backup, I would definitely recommend a wood burner. We have two, and they are great at spiking room temperatures and taking a load off heating zones so that heat can get distributed elsewhere. If you can install a wood burner, from our personal experience, we would recommend it.

The pump will also turn on in the summer when needed to heat hot water, but the pump is super efficient in the summer because there is a lot of ambient heat in the air.

Over the course of 12 months, we've found our ASHP to be much more economical than our old oil boiler. We have also got solar PV, so that takes a load off, and it heats our water too. You can read about our 2020 performance here: https://myhomefarm.co.uk/air-source-heat-pump-performance-for-2020

What size heat pump do you have and what is the approximate area you're going to be heating? Also, what's the insulation like and is there double glazing?

@Jules, just to add to what Mars said, an ASHP is not going to be as cheap as gas at current prices, nor will it be as flexible. Electricity is 4-5x the price of gas per kWh and even allowing for gas boiler inefficiency and ASHP COP, gas is still cheaper.  You can also switch a gas system on and off and on it will heat your house up quickly, which an ASHP won't.  ASHPs are less efficient when it's cold whereas gas (I think) is unaffected. If you can have gas and you don't care about burning fossil fuels then gas wins hands down.  Most comparisons say an ASHP is comparable to or cheaper than oil. 

But... if you can't have gas and/or don't want to burn fossil fuels, then an ASHP is a good choice, especially in a new build.   In really cold weather the ASHP might either struggle to heat the house or, more likely in your new build, just cost a bit more to run because it’s less efficient.  A wood burner would certainly help (and is nice to have as well). You'll also have to be a bit more mindful of HW use; an ASHP is going to have to work very hard to heat several bathfulls of water a day if it's cold out. 

Unlike Mars, I'm speaking from experience of just one week's use of our ASHP and although it’s been quite a chilly week, ours (retro fitted to a 1990 house) worked just fine.  The whole house is nice and warm and we used about 34kWh per day for heating and HW (but bear in mind our HW usage is low as there are just two of us). 

I'm happy to compare specs with you - heat loss, house size, radiators, etc. if that would help). 

Posted by: @kev-m

Unlike Mars, I'm speaking from experience of just one week's use of our ASHP and although it’s been quite a chilly week, ours (retro fitted to a 1990 house) worked just fine.  The whole house is nice and warm and we used about 34kWh per day for heating and HW (but bear in mind our HW usage is low as there are just two of us). 

34kWh per day is great. 

We've used considerably more.

Hi Mars

Thanks for your information.  

I understand from the Builder that there will be adequate insulation and all windows will be double glazed.  The property will be approximately 2800 sq. ft.  I have sent an email to the company supplying and fitting the ASHP to ask the size and make of the pump and asked them to confirm that they are MCS accredited and I will let you know.

Meanwhile, looking at your beautiful property, I cannot see where  the solar pv is.  I would have thought they would be panels on your roof.

Thanks again to yourself and everyone on this site for all your help and valuable information.

Jules

@jules, thanks for the update and we'll wait to hear what they say.

Our panels are in the back garden on our studio roof and not visible from the front of the house - it's the perfect location for them because we hardly see them and they're on a south facing roof: https://myhomefarm.co.uk/one-year-solar-pv-array-anniversary