@lucia ah sorry I wasn't clear, I meant with BGs price being higher than Octopus how much lower was that from say the price that upgrades.heatgeek.com spits out

@lucia Sorry, Ive lost track of this thread as I have a lot on today. Obviously the output of the heat pump must meet or exceed you estimated heat loss in worse case conditions (unless you have secondary heating to pick up extreme temperatures.)

@bontwoody Theirs (from your link) is even lower than the MCS calcs.  HG = 3kw

          BG = 3.5kw

          Oct =4.? 

These are all estimated on 'cavity wall insulation'  

If done (via MCS calcs) with 'no cavity wall insulation' BG = 5.6 

But allegedly a 4.5kw Daikin can output more than 4.5kw - which is a bit confusing. But even on a 50º design temp it's not enough. See image from Daikin EU installer's portal

I might add I still don't think it is enough for cold weather because you can only run the Daikin graphs for under floor heating or fan coils - it must be an EU thing. Also it doesn't take into consideration the UK's humidity and the cold weather defrost cycles.

@lucia

With regard to the bucket analogy, think of your home as a bucket with lots of pin holes covering the whole bucket, with the bucket placed inside a larger vessel containing a certain level of water that can be raised or lowered (OAT).

If the water level inside the bucket is increased (higher IAT) then water will leak through more holes (greater heat loss), if the water level inside the larger vessel is increased then less holes will be uncovered, so less water will leak (lower heat loss). 

Having cavity wall insulation is like having less holes or smaller holes in that section of bucket, while having no cavity wall insulation is like having more holes or larger holes in that particular section. This is similar to having different windows with single, double or triple glazing.

If possible assess the area of wall that has cavity wall insulation and the area that does not. It should then be possible to approximate the heat loss in the cavity wall insulated section and the section without cavity wall insulation.

Other factors that may have an affect is if the missing cavity wall insulation is on a North, South, East or West facing wall, since solar gain may then become important.

I can explain in more detail if you wish.

Posted by: @toodles

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@lucia Rogue tradesmen have a lot to answer for! 😳😩 (Bring back Hanging I say!😉) Toodles.

I'm from the old school, I much prefer hanging, drawing and quartering. 😋 

@derek-m I 2nd that and keep it slow and agonising for cavity wall insulation sharks. 🤬

@derek-m Thank you - that's great imagery - I have that kind of a visual brain so it appeals.

I will play with it this weekend and have a go at trying to plot a course between the two extremes of insulation/lack but if I get stuck I'd love to take you up on it. I think I have to make a strong case to persuade Octopus (and work out what's the wisest for me) because I can't afford the BGas deal. 

This forum is great and I really appreciate the knowledge, experience and help. 

Posted by: @lucia

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@bontwoody yes, I understand about larger rads and lower flow temperatures what I don't grasp (and that page doesn't explain because I don't see any mention of heat pump size) is how much a possibly undersized heat pump can make up the heat temps.

If my heat loss is 5.61 kw how much do I need to improve the emitters with a 4Kw heat pump to get it to supply sufficient heat to be warm? (I'm an absolute wuss about the cold 😁). 

Will exchanging my 11s for K2s make a 4kw heat pump meet or exceed that heat loss for example? Can a 4kw be coaxed to do that? I'm not considering water heating here either or defrost cycles. 

Or should I do both.... lobby for the 6kw and bigger emitters? 

Unlike a number of heat pump manufacturers, Daikin appear reluctant to publish heat pump data tables, or at least I have not yet found any.

The primary energy source for a gas or oil boiler is the supplied fuel, but with a heat pump the primary energy source is thermal energy absorbed from the outside air. This of course varies with the OAT, so that as the OAT falls, less thermal energy can be 'harvested'. To make up for the reduction in absorbed thermal energy, the compressor has to work harder and therefore consumes more electrical energy.

The other important factor is that the heating demand is greater at lower OAT, so the heat pump suffers a double whammy. Less absorbed energy, more consumed electrical energy, higher heating demand, which equals much lower efficiency.

Some heat pump manufacturers rate their heat pumps for high demand conditions, so one rated at say 8kW is capable of producing 8kW of thermal energy at an OAT of -3C and a LWT of 55C. Other manufacturers are not so generous, so their 8kW model may only be capable of supplying 7kW under the same operating conditions.

I would suggest that you ask the supplying companies for a copy of the data tables for the various models they are intending to supply.

@gunboatdiplomat I was being a muppet, sorry. 🤦🏻‍♀️

The Heat Geek guesstimate is double the Octopus quote for a proposal of a 3.5kw Vaillant. It would definitely go upwards if I got them in to assess. 

British Gas only proposed a Daikin, sadly we agreed a Vaillant was out of my reach. Although he did say it would be approx £2000 on top of whatever they offered the Daikin for. In my dreams, I'd like a nice little 5kw Vaillant - for the capability and the after service. 

@derek-m

Daiken capacity charts 

@lucia In my view, the Daiken is a better unit as it controls the Delta T of the heating water (The Vaillant does not ), and the controller is very much simpler that the Vaillant. This will lead to a very much more stable installation and will more than likely, lower running costs, as long as a buffer tank is not installed.

@derek-m Thank you. If you get time please either have a look at the images I put in a post roughly a page back. Or see below: 

I got a link from Open Energy to the  Daikin EU installer's tech hub portal . I ran it with both a 4kw and a 6kw with a 180l boiler on the Applications page EDLA04EV3.  Octopus/BG is EV2 which is an older model (2022 I think). It only lets you set it as either 'fan coils' or UFH I chose the latter.  

This then takes you to a page where you can play with sliders for different flow temps and it shows you 2curves for power used and output at different temperatures.

I guess it's only approximate but it does show a 4kw Daikin (I guess because of the software that is limiting what is really an 8kw) going to about 5kw @ 0º . I didn't trust it too much because of the lack of variation and it not considering de-frost cycles or radiators.. (Or maybe the defrost is built in).