Posted by: @l8again

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Is minimum disrupt just a marketing ploy to sell more heat pumps or is there science behind the change? 

My take on this isn’t a popular one, and I’ve actually been blocked by Heat Geek on several social media platforms as a result, which I personally find unprofessional. But I’m going to say it anyway. I don’t think zero or minimum disruption is the right direction for the industry. I know some mods and senior members here will disagree with me.

I saw a so-called Zero Disrupt install posted by an installer last week. The job took six days. Six. In that time you could strip out the old system, upgrade pipework, put in a new cylinder, resize emitters properly and commission a system that’s actually been designed for low-temperature operation. At that point, what exactly are we avoiding? Six days is not minimum disruption for a shaved down install. I don't get it.  

For me, this whole minimal disruption narrative feels like marketing first and engineering second. It sounds great to homeowners... "no mess, no changes, no upheaval, lower price", but heat pumps are fundamentally different to boilers. Pretending otherwise doesn’t make that difference go away. In many cases, disruption is necessary to do the job properly and to have long-term efficiencies. 

I also think it conveniently lowers the barrier to entry. It’s an easy sell for installers: get through the door by promising no changes, no radiator upgrades, no pipework alterations… and then hope the system performs well enough not to cause a complaint. Often it doesn’t. 

From what I’ve seen, especially with some of the Heat Geek-style installs, the end result is frequently a very expensive system that’s been engineered around avoiding disruption rather than optimising performance, resilience or long-term running costs. That might look good on Instagram, but it doesn’t always translate into warm homes and happy homeowners five years down the line.

And that's issue. In winter 2027, we'll start to see the cracks of the Zero Disrupt model. Remember, you heard it here first... just the same as Octopus Energy minimal upgrade/disrupt issues have started to manifest themselves this winter.

Heat pumps aren’t boilers. If the industry keeps pretending they are (just with better branding) we’re setting homeowners up for disappointment.

@editor1  Thanks for your view on this offer. On the face of it, it is very attractive. The initial estimate comes in low and then I guess the fun starts.

That said, I do think that the issue of whether to use a heat pump or an immersion heater to re-heat a HW cylinder is a real one that needs to be debated if, as we all hope, electricity prices fall in the longer term. However, as always, one has to consider the unintended consequences in terms of system design.

i will be interested to read what others have to say.

Posted by: @l8again

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@editor1  Thanks for your view on this offer. On the face of it, it is very attractive. The initial estimate comes in low and then I guess the fun starts.

That said, I do think that the issue of whether to use a heat pump or an immersion heater to re-heat a HW cylinder is a real one that needs to be debated if, as we all hope, electricity prices fall in the longer term. However, as always, one has to consider the unintended consequences in terms of system design.

i will be interested to read what others have to say.

I agree with you, the assumption that DHW cylinders must always be replaced needs to be questioned.  The mantra dates from the days of low temperature (R410 max 55C FT)) fixed output heat pumps.  We have moved on, we now have R290 heat pumps (max FT 75C),  that modulate!  Replacing the cylinder can be both expensive and disruptive so needs to be properly justified IMHO.

There are several considerations and options. 

With an R290 heat pump you can get flow temperatures up to 70C anyway, in in the same region as a boiler, and therefore very likely indeed be sufficient to heat a small coil cylinder to 48, the typical temperature.  Even with an R32 pump you have a good chance of heating to 48.  In both cases however it may not be able to do the (typically weekly) legionella cycle.  Many heat pumps do this on the immersion heater anyway, but some, eg Vaillant, by default do it 'natively'.  Of course there is nothing stopping you using the immersion for this, but some immersions are fitted near the top of the tank so it probably wont heat the bottom.  There again some consider that the leigionella risk is sufficiently small in a domestic environment that, unless there are vulnerable people in the house, it isn't necessary.

If you have a relatively new cylinder and want to increase the 'coil area' another option is to add a plate heat exchanger and circulator pump.  In fact some adopt this approach as standard.

If your existing DHW is fed from the loft not unvented then you might appreciate the upgrade to mains pressure DHW anyway, which might justify the replacement cost.

Suck it and see is another option, of it works keep as is if it doesn't replace or add a phe plus circulator pump.

Finally if you have a particularly high power (particularly a high power R32) heat pump (say 10kW+) then its possible that it will cycle when attempting to heat DHW with a small coil which would be a bit of a disaster for efficiency and reheat time.

Your actual DHW use is also worth throwing into the mix.  Think about how much you use daily and what this says about the need.

IMHO all of these need to be factored in and ultimately it is (or should be) the householders choice not the installers!  Of course some installers will do what's right for them not what's right for the customer!  The 'safe' option from the installers PoV is always to replace the cylinder, even if it adds £2500 to the price and involves ripping up floorboards.

For the avoidance of doubt Im not saying that your cylinder doesnt need replacing, I dont know enough about your system to make that statement, just that you should not assume it doesnt need replacing and should listen to the arguments. 

I went through this myself.  I had a 10 year old loft fed cylinder.  One installer was happy to fit a phe and recirculator pump instead of replacing it, which would have been cheaper.  In the end I opted for replacement because it enabled me to ditch two water pumps as a result of the upgrade to unvented.  Im happy I made the right choice, however had the cylinder already been unvented, I would definitely have gone the other way and probably would, just have had the installer plumb the R290 heat pump to the existing coil and suffered the performance hit from running the DHW at a higher FT.

Posted by: @l8again

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HP installers like Heat Geek and Octopus are now offering minimum disrupt heat pump installations utilising the existing HW cylinder and radiators. My 210L Worcester Bosch Greenstore has a 0.845 m2 heat exchanger surface area whereas a Valliant Unistor has 2.5 m2 heat exchanger area. Clearly, the time to reheat my existing cylinder will be considerably longer than with a Unistor.

That said, Heat Geek suggests that a Unistor would add £1850 to its initial estimate. This cost is not insignificant. I can reheat my existing cylinder on Intelligent Octopus Go for less than £100 per year. If I took a HW reheating COP of 4, then I would save £75 per year on cylinder reheating which would give me a new cylinder RoI of £1850/75 or 24 years!

One local heat pump installer that I have just spoken to said that he would insist on replacing the HW cylinder to avoid future complaints about poor performance. I am not sure who to believe? Is minimum disrupt just a marketing ploy to sell more heat pumps or is there science behind the change? 

Hi, my take on this is there are 3 things you need to be aware of so you can make an informed choice:

1) the installation is a 20+ year investment, taking a bet on the future of electrical costs is a bet and inflation is against you as well as the Government and mechanisms in place currently, chances are it stays the same cost or gets more expensive. You will be running the heat pump harder to reach temperatures, also there is no research on the life expectancy of running the heat pump harder with the wrong heat exchanger in the cylinder, for all we know is it takes a year off the life of the unit, manufacturers have tested their systems with their cylinders or equivalent cylinders past the life expectancy or warranty of these units, they spend a lot of money on R&D and I mean a lot. If you purchase their kit you get their research and warranty

2) It will take longer to heat the cylinder 

3) it will be noisy due to the velocity of the water through the coil 

As long as you are happy with the short comings then the rest of the cylinder is a cylinder (unless your old one is poorly insulated then its a heat dump that will cost you money)

@ashp-bobba Thanks. I appreciate your helpful comments.

In our case we deliberately chose to change the cylinder because 

1 the cylinder is in our bedroom and whistling pipes if we re-heated overnight would be awful 

2 we are in a moderate limescale area and the cylinder will have scale deposited on the coil so it would be slower (a bit!)

3 if we reheated in the heating period it would take longer and I’m sensitive to a colder house 

4 the pipe work up to the 3 way valve was in the airing cupboard and needed to be increased in size anyway

5 the pipe work for the volumiser and the store itself is also in the airing cupboard and in front of the cylinder. So changing the cylinder afterwards would be a nightmare 

6 the cylinder was 15years old and while for me this is new (compared with 50year old copper one it replaced) I was separately told that this is old for a stainless steel ready insulated one

So even though I hate spending money unnecessarily the conclusion was change it!

As far as i'm aware Heatgeek guarantee a SCOP, so unable to see what the issue is?

You've got the figures in front of you to make the decision based on costs.

@deltona I just wonder how many HG surveys end up with a discussion about the need for a new HW cylinder? My HG estimate for a minimal disruption installation comes in at £4850 for a HP and one radiator. A new cylinder would cost £1850. HG is suggesting that I need a 12.43kW HP in a 172 M2 property with an EPC 87 rating. (It is likely to be EPC A as we installed 6.35kWp of PV solar a few years ago). I also have a R290 HP siting issue which could well stop me having a high temp HP. PS: I have no idea how the calculate the need for a single radiator upgrade.

Posted by: @l8again

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I have no idea how the calculate the need for a single radiator upgrade.

This is done through the survey.  Basically they measure each room, check the construction, and from that you can calculate the loss.  You then check existing rad sizes and model their output at the design flow temperature, compare it with the room loss and bingo you know which ones need upgrading. 

The problem is that the survey/calculations rely on a whole load of assumptions which may, or may not, be true, particularly for older houses which have been 'messed around with' a lot.  This can lead to spectacular oversizing or in rarer cases undersizing, either of which may have significant consequences. 

This is, however, a whole other discussion, but is why I personally advise doing a 'sense check' by comparing the surveyed loss with annual gas/oil consumption.  In fairness to Heat Geek they appear to be better at interpreting survey results than many, judging by what we hear on this forum, but 12kW sounds high unless your house is very poorly insulated, very large, say 300sq m or more, or very long and thin.  

If you have the info (and assuming the house is heated) take your annual gas consumption in kWh and divide it by 2000h.  Separately divide it by 3000h.  The loss is very likely somewhere in between.  So for example my gas consumption was 18,000-20,000kWh, divide by 2000h and you get 9kW, divide by 3000h and you get 6kW.  The actual measured loss is 7kW.  For oil just assume 10kWh/l and do the same calculation.  If the survey result isnt in this bracket, it should be interrogated.

@jamespa As far as my property is concerned, the HG calculator is broken. I recall when I last looked at it 2 or so so years ago, my property defaulted to pre-1900s build even though the build was completed in 2018. Previously, it was possible to change the input data but this is no longer possible.

An independent installation installer has estimated a 5.2kW Heat Pump plus cylinder with a 45C flow temp and SCOP of 3.70 for £6350 (after the BUS Grant).