Posted by: @pauli

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My big concern is the cosy heat pump is a 6kw unit, and from my testing a 1.5kw max input heat pump would allow me to heat up my house adequately without the current caveats (even in coldest of days). It would also mean I will need less expensive battery capacity to run it daily on cheaper cosy tarrif energy

Posted by: @pauli

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Buy a small (non hot tub) heat pump, which i can regulate down to a max 1.5kw, add some batteries to power it (with the batteries being charged via a time of day cheap overnight tarrif

Firstly very well done indeed for actually doing something leftfield.  If nothing else it proves that heat pumps arent so mysterious, tehy are just a source of hot water.  Your case is also an interesting one so, if you are willing to, please keep posting!

However you need to stop mixing up input and output.  The Cosy is a 6kW output heat pump (heat pumps are specified by their output not input), which will be max ~2kW input.  

The classic answer is get a proper heat loss survey done, get three quotes at least, don't accept any quote which has a buffer, low loss header or plate heat exchanger between heat pump and emitters, change several radiators and your DHW system so you can operate at a low flow  temperature.  Provided you haven't removed the gas boiler you should still be eligible for the BUS grant so it could be quite cheap.

However your 54sq m well insulated house is certainly well sub 6kW loss at design temperature, perhaps even sub 3kW.  In reality there arent many sub 5kW heat pumps out there (there are several with sticker capacities that are lower but they are generally just downrated larger ones). I know of only the trianco (also here at a much higher price) and Anzen neither of which anyone here has reported having (the Anzen btw is solid state, rather interesting but early days for the tech)!  Also you may want 5-6kW for your DHW depending on what reheat time is acceptable.  That may determine the heat pump size and if so it would be very tempting indeed to do a quick calc to see if you could reasonably leave the rads and change the minimum possible initially.

I think you need first to work out what you want to do about DHW.  Once that is decided you could either go the conventional route with a grant, with Octopus definitely being one to consider but Id also consider others including the 5kW Vaillant which is generally well reputed, or leftfield without a grant, going for one of the units I have indicated or a cheap self installed model from somewhere, probably for about the same cost!  

The 'safer' route is definitely the conventional one and I cant in all honesty, advise you to go any other way,  However, if I were in your shoes, I would definitely be sorely tempted by staying leftfield perhaps even considering the Anzen tech if its actually available and definitely considering the Trianco (subject to noise concerns as its inside, but then so was your boiler presumably, and also a house loss calculation to check if it will suffice) 

You do need a heat pump designed for domestic heating because you need a defrost function (basically the ability to get rid of ice collecting on the fins).  Im guessing your hot tub one doesnt have that, although given that hot tubs are popular in cold countries in winter, maybe it does!

@jamespa 

"The Cosy is a 6kW output heat pump (heat pumps are specified by their output not input), which will be max ~2kW input."

Wasn't aware of that, i wrongly assumed that that was the *max* energy it used, modulating/cycling up/down as/when needed, depending on flow temp and DT

That's info alone is pushing me towards octopus etc once i have three quotes

Although, it will be interesting to see how much they want over the bus grant, an important factor for me, as it will make the aternative option more or less desirable - I can easily re-hang the boiler if needed unconnected 

The current system is "tuned" so that the three way valve is sending more water to the radiators then the tank, and the water stays at just under 40 degrees all the time

Currently using a shower pump for showering directly out of the cylinder, for this I already have a smart plug controlling the immersion (via a contactor) - a 15 min session raises the temp above 40 enough to have a nice shower (no big deal waiting 15 mins before having a shower) - I do switch the pump off first as i dont want the immersion heating up my radiators 🙂

I went on heatpunk and did a detailed plan as possible, it gave a heat loss of just under 2.5KW , which make sense as I cant see that heat pump producing much over 2kw and its coping (with caveats)

I was expecting the hot tub to not last long using it this way, but pretty surprised how well it works, just seems to produce quite a lot off condensate (and it is under a verandha)
 
PS Above sound tedious, but its all automated using a HEMS called Home Assistant (also allows control of the heat pump using a thermostat in the coldest (aka highest heat loss) room in the house

Posted by: @pauli

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The current system is "tuned" so that the three way valve is sending more water to the radiators then the tank, and the water stays at just under 40 degrees all the time

You will struggle to persuade an installer to do this and nor would I necessarily advise it without careful thought (although I wouldn't rule it out either).  The invariable norm is a diverter valve so its DHW or space heating, in order that you can operate weather compensation for space heating and heat the tank to a higher temp, say 45.  I cant see a solid reason to depart from this TBH.  If you get weather comp right its more comfortable than no weather comp and also more efficient so cheaper to run.  

Posted by: @pauli

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I can easily re-hang the boiler if needed unconnected 

Not if you take the BUS grant, it has to be removed.

Posted by: @pauli

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I went on heatpunk and did a detailed plan as possible, it gave a heat loss of just under 2.5KW , which make sense as I cant see that heat pump producing much over 2kw and its coping (with caveats)

Sounds very plausible!  

@jamespa 

I do appreciate that for get the £7.5k I will **have** to...

1) remove my simple elegant DHW system to be replaced with the "standard" system

2) boiler will be removed (only hanging it up since i assume installer will need some photo proof of existing boiler when they claim the grant)

Its just a shame, as the hot water system works really well and cheaply, and I have lived by the principle "if it isn't broke dont fix it", an enviromental waste 🙁

@pauli Have you considered an Air to Air HP, looks like they are part of the BUS grant after the budget. Albeit, the grant is less than £7500.  But, the costs should be lower too.

Posted by: @pauli

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@jamespa 

I do appreciate that for get the £7.5k I will **have** to...

1) remove my simple elegant DHW system to be replaced with the "standard" system

2) boiler will be removed (only hanging it up since i assume installer will need some photo proof of existing boiler when they claim the grant)

Its just a shame, as the hot water system works really well and cheaply, and I have lived by the principle "if it isn't broke dont fix it", an enviromental waste 🙁

Re (2) I misinterpreted your earlier post.

The actual requirement is here

"(ii)the system providing space heating or water heating, or both, of the property prior to the installation and commissioning of the eligible plant (the “original heating system”) is—

(aa)fuelled by fossil fuel(12),"

and

(c)[the heat pump]replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i)supplementary electric heater, including any immersion heater,

(ii)circulation pump, or

(iii)solar thermal collector.

Obviously there is some 'interpretation' in your case and installers may be picky.  If you have already removed the gas boiler  then possibly you need to reinstate it, although this is something you might discuss (off the record!) with your installer given that you havent really yet substituted it for anything else other than as a temporary measure.  To be absolutely safe though you should reinstate it before anyone comes round!

Re (1) whether you can do it or not is a matter for discussion with your installer.  If your installer will agree to your approach (frankly I doubt it) you will be forgoing weather compensation, which will both compromise comfort and increase running cost, by perhaps 20%.  Also I think you said your existing tank is gravity fed.  There are installers who will retain existing DHW tanks, although if its gravity fed less likely because of the legionella risk.  I do accept and agree with your principle here, but you are fighting an industry that likes to replace perfectly good components at the slightest provocation, but also a very good reason to replace at least some of what you have.

Personally I would consider retaining the tank but using the standard diverter valve approach in order that you can operate weather compensation.  I would discard the idea of retaining your half open valve because it denies WC.  That said, if you go with Octopus, their approach to WC is a bit half-hearted, so you wont be getting the full benefit anyway!

Posted by: @chandykris

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@pauli Have you considered an Air to Air HP, looks like they are part of the BUS grant after the budget. Albeit, the grant is less than £7500.  But, the costs should be lower too.

 

It seems a good idea, but feels a bit low when you need say atleast 3 or more units to cover your house over two plus floors, at which point if you already have a central heating system installed.......

Seems like that level of grant is more suited too flats or peoples needing air conditioing in summer months as well

Be interesting to see what people are quoted when it gets going

@pauli 

For the conventional approach, have you considered the 4kW Grant R290 heat pump? The specs say that it can modulate down to 1.25kW output (see image from tech data below), this seems a better fit for your house's 2.5kW heat loss than most 5-6kW heat pumps that can only modulate down to around 2kW.

GRANTUK.COM "https://www.grantuk.com/media/awohwdor/grant-aerona-290-uk-doc-0204-rev-16-february-2026.pdf"

@robs 

Interesting, thanks, I'll now be asking my other two "quoting companies" can they supply this unit, I'd love to buy it myself and carry on using my unconvential system, but at over £2.5k i'd be a fool to not pay upto that above the £7.5k bus grant and get my whole system upgraded 🙁 

I've asked octopus to send me details of what the cosy 6 modulates down too, also after seeing the latest rhh video i noticed their cop values are specified ONLY at 7 degrees, naughty!

hope its comparable to the grant heat pump, because I do want to go with cosy because.....

1) They are built/manufactured in the UK

2) The more that get installed the more the data octopus will get and the more the models will develop/get optimised iteratively over time

3) This is a biggie ....... I really want to install some batteries and get the cosy tariff - currrently three 7p dips and make sure my batteries have enough charge built up at 7p to cope with the 40p plus expensive evening hours (and hot water at 7p)

4) On the geeky side, most of the real time cosy data is available via whats called an API, and there are industrious people out there who have freely developed addons for Home Assistant allowing you to view all the data and make automations based on the data. For example I can say send me a message on my phone if the live cop value goes below a certain figure (even an automation saying switch the damn thing off if it goes below a certian figure!!)

Posted by: @pauli

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4) On the geeky side, most of the real time cosy data is available via whats called an API, and there are industrious people out there who have freely developed addons for Home Assistant allowing you to view all the data and make automations based on the data. For example I can say send me a message on my phone if the live cop value goes below a certain figure (even an automation saying switch the damn thing off if it goes below a certian figure!!)

We have quite an extensive amount of posting on the forum about exactly this. Home Assistant is a regularly discussed topic, so a quick bit of searching may well answer many of the questions you might encounter.

Cosy tariff is more like 14p, at least in the South East region. Intelligent Octopus Go is around 7p, please do check.

Posted by: @pauli

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3) This is a biggie ....... I really want to install some batteries and get the cosy tariff - currrently three 7p dips and make sure my batteries have enough charge built up at 7p to cope with the 40p plus expensive evening hours (and hot water at 7p)