ASHP vs Gas

[MrsDeS]

Hi,  this is my first post!  I've done a lot of reading here over the last few weeks and there's clearly some very knowledgeable people here, so I'd really appreciate a bit of advice.  

 

My husband and I have built our own house from SIPs.  We first-fixed a Vent Axia MVHR but haven't moved on for a while. Even without heating, the house is more temperate than the leaky 1930s box we live in now.   We're back, with funds to continue, but have procrastinated over the heating system. 

 

We were initially thinking of ASHP, then changed our minds to gas and didn't put UFH in with the slab.   Doing it now as a retrofit looks pricey so we're stuck with radiators definitely if we go with the ASHP, because of the initial outlay.  

 

The house is a 190sqm 1.5 storey chalet bungalow.  The 'as designed' SAP gives us a space heating energy requirement of 31KwH per square metre or 5766KwH, reduced to 3392kwH by an ASHP.     Putting the numbers in for RHI, it looks like we'd get £3900 back if the requirement was calculated as 5767.    

 

We have one quote at the moment of £10k (plus VAT to reclaim) for a 11.5kW Mitsubishi with a 250 litre Kingspan tank.   We will buy our own radiators.  

 

Connecting the gas is about £600 and a powerful combi boiler (to hopefully run the water for 2 showers at the same time) less than £1500 plus fitting costs, plus radiators.  I think then, that if the ASHP and tank is about £6,000 then the RHI will (eventually) give us back the difference in the inital outlay, minus the additional cost of larger radiators.   I've used an online calculator to calculate BTUs for the radiators if we used gas and they look happily small.  I don't know how to calculate those upwards for the low running temperatures associated with the ASHP.  

 

The questions are: 

 

Is it really that level a playing field IF we are prepared to wait for the RHI to pay back?   How much more expensive will an ASHP be in running costs and at what point in time will it actually start to pay back in real terms on heating bills?    

 

Part of me thinks that we've done the hard work in paying for the SIPs and the MVHR.  Should we just stick with what we know and go with gas?  

 

Oh, and if we go with ASHP, how do I calculate how much bigger my radiators need to be?

 

Sorry for the waffle.  It's about as clear as my head can think about this.  This bit is really not my bag! 

 

[scottishjohn]

If you have maisn gas,then that will always be cheaper using radiators 

had you used and insulated slaband UFH then it gets a lot closer -but if on mains gas --probably still cheaper 

[MrsDeS]

45 minutes ago, scottishjohn said:

If you have maisn gas,then that will always be cheaper using radiators 

had you used and insulated slaband UFH then it gets a lot closer -but if on mains gas --probably still cheaper 

Thanks for the reply!  I hadn't fathomed that it was less efficient that way as well.  I'm doing a bit more reading now, thank you.  

Edited May 14, 2020 by MrsDeS

[A_L]

1 hour ago, MrsDeS said:

Oh, and if we go with ASHP, how do I calculate how much bigger my radiators need to be?

 

Relative to 70°C outputs at 30°C/13.5%,  40°C/32.1%,  50°C/53.0%,  60°C/75.8% for rooms at 20°C and 'ordinary' radiators

[Gone West]

In your situation I would use gas with skirting radiators. UFH only works well with a very well insulated floor and even then is less efficient than radiators. There is also the potential problem of finding repair/maintenance companies for ASHP. ASHP supply and fit companies seem to spring up and then disappear but finding a service company for gas appliances is easy.

[ragg987]

Do you need gas in your house anyway e.g. for cooking. If not and you choose ASHP then you cut out connection and standing charges for gas.

 

If your primary driver is cost, gas if probably better as noted above.

 

If you want to be green then ASHP gives you the credentials as your are not (directly) adding harmful stuff into the air.

 

[DamonHD]

If you thought that you needed gas for cooking, from a green and a health point of view (gas cooking turns out to be horrible for indoor air quality), may I please nudge you towards induction + electric fan oven?  The combo has worked nicely for us.

 

http://www.earth.org.uk/note-on-IKEA-SMAKLIG-built-in-induction-hob.html

 

And yes, ASHP is locking in green gains from our electricity getting greener, whereas gas locks you into burning stuff, already less green than electricity, for many many years.

 

Rgds

 

Damon

[Bozza]

Go for ASHP if a long term investment.  You’ll save money in the long term and it’s better for our environment,  in a few years time younger buyers won’t want fossil fuel houses as we wouldn’t want to buy a house now heated by coal.  Won’t be long before house builders won’t be allowed to install mains gas. It’s old technology.  I’m not an eco warrior BTW just my mindset has changed since looking into the options for my own build.  

 

 

[Mr Punter]

You could DIY an overlay UFH system on ground floor only.  If the house is well insulated you will not need anything upstairs but you could get a portable fan heater and use it for 10 minutes for the exceptional days.  Heated electric towel rads on timers in the bathrooms.

 

The gas combi seems more attractive though.

[MrsDeS]

I did a bit of maths.  Assuming the big number on the SAP is what I'd need to heat the house without the ASHP cutting the energy required, based on just heating and my current energy tariffs.  

 

Electricity £453 heating, £256 service.  

 

Gas £117 heating, £91 standing charge, £50 service.  
 

SAP says 'other fuel'.  That looks like £241 in electric vs £37 (or maybe £63 if the ASHP is 170% as efficient) in gas in their estimates, not accounting for teenage addictions to endless showers.  Is hot water really that cheap?  
 

Cooking will be induction/fan oven, so no difference.  
 

I'm adding this up as the ASHP costing me £659 a year more.  Plus the additional cost of buying and fitting it. 
 

 With that fact that most electricity is still produced with fossil fuels, I'm struggling to justify the additional cost.    The payback on PV is over 20 years for us too.  
 

We've paid a lot on SIPs, on the MVHR and this looks like saving us nothing in bills and we've already spent a fortune insulating it and making it airtight.  

 

What would happen if I oversized the radiators now to accommodate a future ASHP when the boiler dies?  
 

Or maybe I do spend the extra now on UFH (we have 150mm of insulation under there) so that when it genuinely makes sense to go to ASHP, we are ready... 
 

 

[SteamyTea]

20 hours ago, MrsDeS said:

The house is a 190sqm 1.5 storey chalet bungalow.  The 'as designed' SAP gives us a space heating energy requirement of 31KwH per square metre or 5766KwH, reduced to 3392kwH by an ASHP.

I don't understand the differences.

Generally as ASHP will run with a CoP of 2.8.

So 31 kWh/m².year X 190m² = 5893 kWh/year

 

5893 [kWh/year] / 2.8 [CoP] = 2104 kWh/year (this is the electricity consumption for the same thermal output)

 

It is kWh, not KwH, or KWH, or kwh, or kw/h

 

Edited May 15, 2020 by SteamyTea

[ragg987]

1 hour ago, SteamyTea said:

I don't understand the differences.

Generally as ASHP will run with a CoP of 2.8.

So 31 kWh/m².year X 190m² = 5893 kWh/year

 

5893 [kWh/year] / 2.8 [CoP] = 2104 kWh/year (this is the electricity consumption for the same thermal output)

 

It is kWh, not KwH, or KWH, or kwh, or kw/h

 

Or the easier way would be divide cost of electricity by CoP to get effective price per kWh of heat dumped to the house. In our case CoP is usually 3.5 to 4, so effective cost is equivalent to that of gas.

 

If you factor in solar PV or E7, the cost of heating with ASHP can be significantly lower. Neither offer a benefit with gas.

[SteamyTea]

28 minutes ago, ragg987 said:

In our case CoP is usually 3.5 to 4

That is quiet an impressive CoP.

How much oversized is your ASHP?

[JamesP]

Early days as we only moved in March but as of today we have a CoP of 6. Delivered energy 4070 kWh, Consumed energy 677 kWh. Ecodan W85. Well insulated, minimal heating and existing PV array with a diverter all helps in reducing demand. No gas here.

Edited May 15, 2020 by JamesP
Fair

[MrsDeS]

2 hours ago, SteamyTea said:

I don't understand the differences.

Generally as ASHP will run with a CoP of 2.8.

So 31 kWh/m².year X 190m² = 5893 kWh/year

 

5893 [kWh/year] / 2.8 [CoP] = 2104 kWh/year (this is the electricity consumption for the same thermal output)

 

It is kWh, not KwH, or KWH, or kwh, or kw/h

 

The SAP has given it 1.7.  
 

I'll have a look again, but I'm minded now to go with gas and UFH with the view to changing over to ASHP when the boiler dies.  ?
 

 

[SteamyTea]

That is a ridiculously low CoP.

 

[scottishjohn]

3 hours ago, SteamyTea said:

That is a ridiculously low CoP

its not a real fogure cos he has not included the cost of the pv system -so he is saying its free electic -but its not really 

will take 8 years + just to get  repayment of cost probably-if a professionally fitted system 

[Miek]

8 hours ago, MrsDeS said:

What would happen if I oversized the radiators now to accommodate a future ASHP when the boiler dies?  

 

 

Not a lot, however it's slightly more efficient to run a gas boiler central heating circuit at a lower temperature so bigger radiators will allow you to do that . If the return is below 50 degrees (ish) you get the boiler running in condensing mode.

[ragg987]

8 hours ago, SteamyTea said:

That is quiet an impressive CoP.

How much oversized is your ASHP?

330m2 house with theoretical demand of 3kW for space heating as calculated using PhPP. Using 7kW nominal split HP for DHW and space.

[SteamyTea]

6 hours ago, ragg987 said:

330m2 house with theoretical demand of 3kW for space heating as calculated using PhPP. Using 7kW nominal split HP for DHW and space.

Just goes to show that oversizing really does help to keep the CoP high.

330 m² is larger than the 6 houses that make up my street.  You must have one of the posh houses in you village.

 

15 hours ago, JamesP said:

Early days as we only moved in March but as of today we have a CoP of 6

Had the weather for getting a decent CoP, been dry and not silly cold.

[JamesP]

@SteamyTea Sunniest April on record according to the Met office. 

[SteamyTea]

2 minutes ago, JamesP said:

Sunniest April on record according to the Met office. 

Yes, apparently so, not looked at the figures for temperatures and humidity, which are the things that affect ASHPs the most.

[MrsDeS]

On 14/05/2020 at 18:59, A_L said:

 

Relative to 70°C outputs at 30°C/13.5%,  40°C/32.1%,  50°C/53.0%,  60°C/75.8% for rooms at 20°C and 'ordinary' radiators

Could you help me with the maths, please? 
 

Would I run the radiators at the same temp as the UFH?  What would that be, about 40 degrees? 
 

If I needed a 'normal' radiator of 1500 BTUs, for example.

[DamonHD]

FWIW, I pretend that my house is on ASHP, though it is still a non-condensing gas combi, and I run the flow at about 45C.  Because our insulation is (now) good, our newer rads are double-skinned but not huge, and they do fine.

 

Look here at the new small rad tucked away behind the sofa here, replacing two long ones at either end:

 

http://www.earth.org.uk/superinsulating-our-living-room.html#Radiators

 

Rgds

 

Damon

[A_L]

1 hour ago, MrsDeS said:

If I needed a 'normal' radiator of 1500 BTUs, for example.

 

At 40°C you would need a radiator with a 70°C output of (100/32.1)*1500 or 4673 Btu.

 

That is to get the same output. In practice ASHP houses are usually heated for longer each day so the radiator can be smaller, say 2/3 value calculated.

Edited May 16, 2020 by A_L
add second line