Where are you based @glacken? I'd definitely get more quotes than just Octopus.

Posted by: @glacken

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Octopus have surveyed and said the heat loss is 11 kW

That's a big or rather lossy house.  How certain are you that this is correct.  Do you have any existing consumption figures to act as a sanity check, what's the floor area and construction/level of insulation?

I agree with @editor, you should get some more quotes to be comfortable.

Nice guy from Octopus insisted I had 8.8 kW heat loss and wanted to install a 9 kW Dakin,

Local guy that I have a lot of confidence in (appears on this site with great answers) gave a figure of 4.4 kW which I still think is slightly high but my installation is booked for two weeks time. (Vaillant 5kW)

Please get other quotes, or calculate yourself using Heat Punk (takes a while to learn but gives a great understanding of what is involved)

If you are in Kent I am happy to share name of my installer.

@glacken

With 3 years on this forum, I am immediately suspicious whenever I read a surveyed heat loss greater than about 9kW, unless its for a mansion of a very poorly insulated (usually listed) building,.  Of course it does happen but its rare, and more likely that the loss has been heavily overestimated.

Here is a scattergram from heatpump monitor of measured demand at design temperature vs floor area, taken from about 250 systems.  I think this illustrates that I am right to be suspicious!  Each spot represents a specific system, except in the dense area where multiple spots may overlap.

Conclude from this what you will.

Octopus had two goes. The first, (I am reading from his performance estimate) Floor area 183.9 sqm, heat loss 12083 watts and average watts per sqm 65.7 The flow rate of 51* with 6 radiator upgrade.  Double glazed windows, 250 m loft insulation. 

The second Octopus man got in down to 11.1 heat loss. target temp of 18 and 21.

BG had a target of 21* in every room. The empty ones, the bedrooms and utility all 21*

His was a Vaillant 10Kw but need 13 of 16 radiators upgraded.

The big advantage of Octopus going forward is their Guarantee, service costs and size. 

My neighbour used a small local firm who installed a Grant and when it developed a fault after 18 months they had gone out of business. His cost to get somebody else was high.

I have contacted an installer recommended by the Renewable Hub and await their response.

They have installed a heat pump near me here in West Yorkshire.

Posted by: @glacken

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Octopus had two goes. The first, (I am reading from his performance estimate) Floor area 183.9 sqm, heat loss 12083 watts and average watts per sqm 65.7 The flow rate of 51* with 6 radiator upgrade.  Double glazed windows, 250 m loft insulation. 

The second Octopus man got in down to 11.1 heat loss. target temp of 18 and 21.

BG had a target of 21* in every room. The empty ones, the bedrooms and utility all 21*

His was a Vaillant 10Kw but need 13 of 16 radiators upgraded.

The big advantage of Octopus going forward is their Guarantee, service costs and size. 

My neighbour used a small local firm who installed a Grant and when it developed a fault after 18 months they had gone out of business. His cost to get somebody else was high.

I have contacted an installer recommended by the Renewable Hub and await their response.

They have installed a heat pump near me here in West Yorkshire.

There is onl,y one system currently on heatpumpmonitor that is both under 200 sq m and have a measured heat demand of >9kW at the design temperature.  So if your house is really 11kW its an outlier by some margin; the highest measured loss recorded on heatpumpmonitor for a house your size or less is 8.3kW.   An oversized heat pump is not necessarily bad, although it can be, but if the options to which you are being constrained are causing you a problem, then you may want to do some more due diligence.  

By way of another sanity check if you wish, do you have previous annual gas/oil consumption figures and, if you do, what pattern of heating did you have/what temperature do you heat to.  No need to answer if the current choices you have don't bother you.  I am offering the information so you can make a choice informed by as much information as possible.  Both Octopus and Vaillant have a good reputation and appear to be good at coping if they are oversized and Octopus may well solve the noise problems with the Cosy 12.

We have solar and 9.6kw of batteries.

We pay Octopus £120 per month and go from £470 credit in August to £50 debit in February before the sun shines.

We heat to 19.5degs. twice a day in winter and switch the gas boiler off in early May until early October. House is usually warm. In summer we us the emersion heater for our usually one bath and one shower per day.

Being told that we have to have and pay for a large cylinder just in case the house is sold to a large family or group of persons in the future does not sit well.

A replacement Vaillant gas boiler is looking better by the day.

Posted by: @glacken

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Being told that we have to have and pay for a large cylinder just in case the house is sold to a large family or group of persons in the future does not sit well.

What cylinder do you currently have?  There are very likely installers who wont insist on replacing it (eg Heat Geek 'Zero disrupt')

BTW I used to turn off my boiler in the summer just like you.  I don't any more, instead I heat my water with the heat pump on a nighttime rate at ~3p/kWh delivered, and export instead at 12p/kWh.  This trade off is a no brainer!

If you are seriously considering a heat pump then I strongly recommend you look at other installers.  If you aren't seriously considering one then I strongly recommend you don't as it can waste a lot of time!

Posted by: @glacken

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The second Octopus man got in down to 11.1 heat loss. target temp of 18 and 21.

Calculating the heat loss isn’t a competition whereby the lowest value wins.  You want to see more than one quote reporting pretty much the same estimate.  Even then it isn’t a guarantee.

Heat Geek and Octopus both calculated a heat loss within 200W of each other which was encouraging.  I did go with Octopus and due to overheating problems, Octopus redid the survey and calculated 1000W lower.  I’m still not convinced it’s not nearly 50% of the calculated value.  The other problem with Octopus, in my opinion, is  they design to different temps in different rooms with a differential of 4c across the house.  This makes it much harder to balance out the system to a consistent temperature but does make it more imperative the room-by-room calculation is correct to give you a chance. 

I’m not saying don’t go with Octopus but affirming what others have said: you must get more than one surveyed quote, preferably 3, even though it’s quite expensive.  

Better still learn how to use Heat Punk (free) and do your own calculations.  It does not have to be exact (none of them are, plus or minus 20% would be good) and you will learn a lot in the process.