So, by means of a brief update, I've made some progress over the past week or so and the picture is becoming a little clearer.

Octopus have provided their calculations, floor plan and radiator schedule, which has given me a base of knowledge to work from.  I've also had productive conversations with EDF (who use an acquired company called CB Heating for their Heat Pump work, and who've come across as very knowledgeable so far), and had an initial quote conversation with E.ON.  In addition, I've spoken to several locally based independent installers, one of whom was able to carry out a house visit at short notice while I was at home yesterday (not a full survey, but it included an examination of the various planned installation locations, and a review of the pipework in various areas of the house).  I have two similar visits booked for next week.

It's becoming increasingly clear that the installation itself is a relatively straightforward proposition for which there are no overall concerns.  The common challenge for everyone is how to deal with the conservatory within the design, and this is leading to a range of options/solutions, so far.

The original Octopus survey treats the conservatory as a habitable space, with a 22 degree temperature requirement, based on the option I chose between doing that and treating it as non-habitable with a 16 degree requirement.  This is essentially why their 11kW Daikin solution potentially looks oversized.  As I've had more discussions with other installers, it's becoming clearer that not targeting normal room temperature for it might be the more logical option.  The EDF consultant probably summarised it in the simplest way, in effect, "if we include the conservatory as a habitable room, the remainder of the house will be heated less efficiently, as the system has to work harder for the conservatory heat loss.  The alternative option is to design a system for the rest of the house and accept that the conservatory will never get up to target temperature, but the design will be better suited for the main part of the house."

The independent installers are saying similar things.  For example, one has suggested looking at an electric air heater for the conservatory, when required, and I'm expecting to have a conversation with EDF's senior designer this week to discuss options (an unusual step in their normal quotation process, but a good indication that they're looking at our specific situation).  They've mentioned one option of perhaps using TRV's set low in the conservatory for the periods of non-use so that it effectively gets bypassed when not required, so that it is treated as an occasional room, which is more like how it gets used in reality.  From a heating perspective, we spend more of the year trying to keep it cool, rather than warm.

Logically, at some point down the line, the logical plan for the conservatory would be to replace the roof panels with insulated ones, at which point it becomes more like a 'normal' room in terms of temperature fluctuations, so the system design we choose needs to also consider this likelihood over the life of the heat pump.

If nothing else, getting the Octopus survey done at an early stage has at least enabled a better quality of initial conversation with the other providers, and is starting to result in a bit of variety in the solutions suggested.  There is undoubtedly going to be an element of compromise in whichever solution we go with, which also needs to take into account how we are likely to use the conservatory 'in real life', which puts it somewhere in between the standard options of habitable and non-habitable spaces.

In terms of practical next steps, I'll likely get a 2nd full survey done by E.ON, as this requires a refundable deposit of only £50, and by the end of next week I should also have had 3 60-90minute site visits from the independent installers, which should result in a reasonable range of quotes and calculations to make a better assessment from.  Boxt still remain the cheapest overall quote, although the gap has narrowed considerably to the next option, as more quotes are being added to the mix.  However, their requirement for full upfront payment to commence any form of site review is likely to rule them out of consideration, as there appears to be no sensible way to have the necessary conversations about tacking the conservatory without doing so.

One addition point worth noting is that I've been very upfront with every company I've spoken to about the steps I'm taking and the fact that I'm speaking to other companies.  The sales consultants at both Octopus and EDF, who've been the contacts for the initial quote generation, have both been very accommodating in terms of time spent on the phone and both have made comments to the effect that if I end up getting a heat pump, regardless of who I use, then they can take satisfaction from that.  It's only a 'failure' for them if I end up choosing a gas boiler after having researched heat pumps.  I have to admit, I found this attitude very refreshing to hear.

That all sounds very logical.

The only question I would ask is how well insulated is the conservatory from the rest of the house.  If its not well insulated then you wont succeed in maintaining much of a temperature difference and not heating it may cost more money than heating it.

It was added by the previous owners, but the conservatory is built on to what would have been the original back door for the property so has the original external wall separating it from the kitchen, accessed by a single door, and with two, previously external windows to either side of this.  In use, we can keep the doors and windows closed and it effectively feels like a different temperature zone in there in cold weather (it has a separate thermostat in there, so is heated separately to the remainder of the house currently, in terms of thermostat control).

In summer, when it operates more like a greenhouse, we can equalise the temperature to some extent by leaving these internal doors and windows open.  The majority of the surface area wall which links it to the house will be the original cavity insulated brickwork, so the insulation between the two is adequate, from the perspective of keeping them as separate spaces.

Posted by: @sheriff-fatman

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It was added by the previous owners, but the conservatory is built on to what would have been the original back door for the property so has the original external wall separating it from the kitchen, accessed by a single door, and with two, previously external windows to either side of this.  In use, we can keep the doors and windows closed and it effectively feels like a different temperature zone in there in cold weather (it has a separate thermostat in there, so is heated separately to the remainder of the house currently, in terms of thermostat control).

In summer, when it operates more like a greenhouse, we can equalise the temperature to some extent by leaving these internal doors and windows open.  The majority of the surface area wall which links it to the house will be the original cavity insulated brickwork, so the insulation between the two is adequate, from the perspective of keeping them as separate spaces.

In that case treating it as a separate space seems eminently sensible to me!

I took a look back through the thread and I posted earlier sone sense checks with gas consumption you might like to do.  Im assuming you will now be looking at 5-7kW not 11kW.  Its best to avoid an oversized heat pump if you can.  That said if it does end up oversized you can heat the conservatory!

I had a telephone call with the senior designer at EDF today, but was somewhat underwhelmed to discover that they've moved away from the suggestion of using TRVs to selectively bypass the conservatory when not required to a plan to remove it completely from the wet heating system.  I accept that this might be most efficient in terms of heating the remainder of the house, but it ignores the existing infrastructure that we're working with, given that there are two sizeable radiators in there would could at least be utilised in some way, albeit not as a habitable room with the same target temperature as the remainder of the house.

It has got me thinking that an Air To Air solution would work best for the conservatory, particularly as it would allow cooling in the summer months, but that isn't the option I'd choose for the remainder of the house, so seems like something of a expensive add on alongside the main ASHP solution.

I much preferred their original suggestion, even accepting that there would be a trade off of efficiency in doing so.  I suspect the inefficiencies, in reality, would have to be sustained for quite a few years to equate to the additional costs of an entirely separate solution for the conservatory.

It will be interesting to see what solutions are proposed by the installers making visits over the next week or so.  It feels like I've covered the two extreme options so far - treat as a habitable room and end up with an oversized system or remove it completely from the wet system.  My gut feel right now is that there's something workable between these two that's a better option, albeit accepting that it will be a compromise solution.

I'm also tempted by the Havenwise optimisation system, after having seen a few videos regarding it.  It could potentially assist in getting whichever compromise solution we settled on run as efficiently as possible, taking regard of our electricity tariff.  I like the fact that it will sacrifice "SCoP chasing" in favour of options that minimise costs based on time of use tariffs, and the subscription cost seems reasonable.  Naturally, this potentially limits which manufacturers I could use, as its not universal across all brands, although in reality I think the smarter choice would be to disregard this from consideration completely and assume that it will eventually be integrated with more brands down the line.  It will probably be beneficial, if only in terms of understanding the system, to try to understand the system myself, in terms of looking for efficiencies, prior to passing the task onto an automated solution.

That's a consideration for another day, anyway.  I need to figure out a system choice first!

Posted by: @sheriff-fatman

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It has got me thinking that an Air To Air solution would work best for the conservatory, particularly as it would allow cooling in the summer months, but that isn't the option I'd choose for the remainder of the house, so seems like something of a expensive add on alongside the main ASHP solution.

I think you may very well be right and of course A2A can be used also for cooling (A2W can also in most cases but only if you have ufh or fancoils).

The permitted development regs are going to be changed soon and it seems almost certain they will allow 2 ASHPs, eg and A2A and an a2w.  This could well be a perfect match for you.

Today marked the 4th consecutive working day where I've had someone visit the house to do a heat pump related visit.  3 last week were potential installers coming to do a home survey, and today was a guy testing for Asbestos on behalf of Octopus due to us having Artex ceilings in some rooms.

The good news is that this gives me a much more solid foundation of information to work from, but with the downside of there being so much variation in the output.

Just to give an indication of this, here are the list of Heat Pump solutions that have been proposed so far, before any consideration of price, etc comes into play:
11kW Daikin - full site survey with detailed heat loss calc provided
7kW Vaillant - online quote only (no survey yet)
8kW Ideal - remote survey via photos/floor plans provided by me (but this design specifically excludes the conservatory from the wet heating system)
10kW Ideal - remote survey - site visit done last week so am awaiting a confirmed design to update this now
8.5kW Mitsubishi - home visit
11.2kW Mitsubishi - home visit
9kW Grant - full site survey with detailed heat loss calc provided

I'm also awaiting another remote design from a Heat Geek certified provider for whom I've provided floor plans, photos, etc which I expect to receive this week.  Another one provided me with indicative costs of various solutions, including Stiebel Eltron and Panasonic pumps, but the overall cost of any would be way beyond the others, so not one worth progressing any further as I couldn't justify the cost, relative to the other quotes available.

One thing I do have available now is two room by room heat loss calculations, which allows me to do best case / worst case scenarios based on their figures.  One calculates a total heat loss of 9.2kW, the other says 8.9kW, so on first sight they look closely aligned.  However the individual room calcs vary quite a bit in percentage terms, including almost a 1kW difference in calculation on the conservatory alone.  Given that this room is the key part of the design challenge, that's quite a difference to overcome.

One of the surveys is very useful in that it breaks down every element of the room calculations - ventilation loss, wall loss, window loss and floor loss - and I can see the U-Values and room temperatures used in the calculation, as well as all the relevant measurements of doors and windows.  This gives me a chance to play around with the calculations directly to try to factor things such as changing the roof in the conservatory at a later date, etc.  I'd be inclined to go with the provider who produced this calculation, purely on the care taken to do it, but that quote is a complete outlier in terms of radiator replacement.  The consensus view from just about everyone is that 9 or 10 replacements are required, but that quote has recommended just 1 radiator change, with the others at additional cost.

Ideally, I was hoping that I would be able to select an installer by the end of this week, but I think awaiting the two outstanding designs is going to be necessary to see how well they align with the others.  The one being generated from the visit on Friday could potentially take two weeks to come through, so I might ultimately have to make a call without this if it reaches the end of that timeframe.  I'm hoping to get an install arranged for late June, after my daughter's GCSEs have concluded, so don't want to delay the decision process indefinitely.

I've also done a lot of background research into BOXT and their installations.  The little that is available out there is generally positive with regard to design, installation and follow up.  The downside is that to progress anything with them I have to pay the full quoted cost (on a refundable basis) to get them to just carry out the survey.  As much as I'm reluctant to do this, I'm interested in seeing a Vaillant system design being carried out within the quotes to choose from (their 7kW solution is currently from an online generated quote using EPC data, so I'm expecting a survey to re-size this upwards, based on the other quotes received).  They remain the cheapest overall quote at present, although the gap has narrowed significantly to a few others, so I may ultimately have to bite the bullet to get the survey carried out by them prior to making a final choice.

I think, having done a bit of research on the heat pumps, I'd be comfortable with a Vaillant or Mitsubishi heat pump.  I also don't mind the Ideal or Grant solutions, but I know less about them currently, particularly in terms of post-installation product support, and they're not yet compatible with Havenwise, which is something that I'm also considering trialling at some point to optimise the efficiency of the system once up and running.  The Daikin pump is the only one I don't really want to proceed with, mainly due to the less than stellar feedback regarding their customer support offering.

So, in summary, a lot of progress made but still relatively little clarity developing from the muddy waters so far.

Aren't you chasing too many quotes?

Have you attempted your own heat loss calculation, to double check your quotes?

EPC data can be sh!te in sh!te out, so be careful.

System design are super important, design temp flow temperature, can add a huge margin to running cost. Lower flow temp is king. Zones (loads of thermostats) mean you end up with buffer - which will increase running costs.

So a good design is a simple system.

Big radiators or UFH, design flow temperature no more than 45 degs, low is better. One single zone open system, maybe TRVs on bedroom rads only. Running weather compensation. Your cylinder should have minimum of 3m² heating coil.

@johnmo I'm not chasing quotes, primarily.  I'm chasing designs - or at least a vague consensus of opinion for our particular situation with the conservatory.

The home surveys are to ensure that I'm not relying on EPC driven data or quotations, and I've had 4 done in total, which doesn't feel excessive to me.

Ultimately, the system we use is going to be a single zone system, operated via weather compensation and not reliant on thermostats or TRVs.  The fundamental question I'm trying to answer is which sizing of heat pump (not just the quoted number of the unit, but it's ability to ramp down efficiently without excessive cycling) is going to deliver the best solution.  There's very little consensus on that so far.

Posted by: @sheriff-fatman

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ramp down efficiently without excessive cycling

That is really down to system volume. Plus the ability to get the heat out of the system. So radiator surface area.

Low design flow temperature drives bigger radiators, which drives system volume up, so is better able to move low flow temperature (heating on and 12 degrees.outside) out of water into the air. Which in turn means less cycles. Big volumes are forgiving for over sized heat pumps or heat pumps that don't modulate as low as others.

Bit like a kettle being nearly empty, it boils very quickly, but also cools quickly - fill the same kettle fully, it takes a long time heat up and cool down. The kettle has the same 3kW element but cycle time is longer the fuller it is.

You mentioned 7 different heat pump? So assume 7 different companies, that a lot of quotes.or information to trudge through.

Posted by: @sheriff-fatman

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@johnmo I'm not chasing quotes, primarily.  I'm chasing designs - or at least a vague consensus of opinion for our particular situation with the conservatory.

I'm not sure expecting consensus is realistic as the solution, given the conservatory situation, depends as much on your preferences and use patterns as it does on technical issues and, to make things even more complicated, bumps up against BUS and PD rules (BUS says the installation must heat the whole house, PD says that it must be used only for heating.

My understanding is that you have two thermally separate areas.  The house which you intend to heat constantly and the conservatory which is used very intermittently (and presumably more outside the peak heating seson than during the peak) has a significant loss of its own, and really needs cooling also.

That's rather different to the normal zoning question, so many people won't have thought about it.

I think I would approach this systematically as follows:

• first make sure you understand your requirements ignoring the bus and pd rules.
• then I would think about whether compromises are possible to meet the bus/ps rules and or whether it could be split
• I would get the best estimate you possibly can if the house loss (excluding the conservatory), taking into account and ideally rationalising surveys and measured data.
• Then make a choice, if necessary going out to the market again.

For example, if my interpretation above of what you want is correct, then a heat pump sized for the house alone, with fancoils in the conservatory, might be a technically good solution.

This avoids oversizing to cope with a rare requirement (heating the conservatory in the peak of the season) and would mean that, so long as you get a heat pump capable of cooling (bizarrely not all are), you could cool the conservatory in summer.  Fancoils heat (or cool) the air quickly, so are good for intermittent use and for the vast majority of the heating season a pump sized for the house will have sufficient spare capacity for the conservatory if you want it.

This solution bends the rules, to put it mildly, but a little judicious interpretation of the house loss (and possibly making the conservatory fancoil a separate job that just happens to be done at the same time) would probably resolve this.

This isn't a recommendation as such because I don't know the fine detail of your requirement, it's your an example of how this might be approached.

Hope that helps

It is not clear to me that you have a clear idea of what you want the final solution to achieve in terms of the conservatory? Perhaps you do in your head, but it doesn't come across in your posts.

I am also not clear on how much you use the conservatory through the year, how important is it to get it right.

My late mum had a lovely large conservatory on the back of her bungalow. She used it almost as much in winter as in summer I suspect, with a large double radiator in it. It had sofas and a dining table in it.

Ironically perhaps is was the most important and well used part of her home.

I can't tell how important the conservatory is for you, albeit you mention it many times.

As another example, the in-laws a have smaller conservatory that they have just had an insulated roof put on to replace a glass roof. It is now much cooler in summer, but to be honest they still hardly use it and it has a couple of cheap wall mounted convention heaters.

I wonder if you were clearer about what you want to achieve with the conservatory and how you use the home overall then it may be easier to compare solutions albeit there may still be a lot of variety across installers?