7:28
The air pulled from your loft is probably the best configuration...and here's why.
I'm Trying to think through this with critical thinking here.
The house has a heat loss coefficient and the heat lost through the loft will be a know number... X kWh.
That energy is a measure of how 'leaky' your loft insulation is.
If your losing energy through your loft (and we all will be!) then you want to recover as much of that as possible which drawing air from the loft space does. But doing so, as you rightly say, lowers the air temperature in the loft as air is drawn in from outside to replace the expelled air.
So it comes down to what is the efficiency of the energy extracted from the air from the loft verses the amount of energy lost through the loft. If the heat pump can extract more kWh from the air than the house loses through the loft you win.
If it can't then you lose...
That figure depends upon the size of your DHW tank and how much hot water you are using every day!
So their is no right or wrong answer to this question.
It depends upon your hot water use, the size of your house and DHW tank, the amount of cycling that that tank does and the heat loss from the loft which equates to the amount of insulation you have!
A well insulated roft space though will limit heat loss through the roft, will require a lower efficiency from the DHW to be more efficient than drawing colder air from outside. So I suspect that in most cases (especially new builds) the air extracted from the loft would be the better configuration.

Really informative video!

I just bought a house electric only in the village that is a total remodel - we would like to add solar at later date - live in north Oxfordshire - any referrals for designing a system for us and evaluating our insulation etc . Someone sharp

Hi Tim who is your installer ? I live in Oxfordshire as well .

We’ve had a Vaillant aroTHERM system installed for just over four years. Unfortunately, our experience has been disappointing: we’ve had multiple breakdowns, each resulting in long periods of downtime, sometimes weeks without heating or cooling.

What’s even more frustrating is that we have a valid maintenance contract, yet the support response has been extremely slow and inconsistent. In urgent cases, it has taken two to three weeks just to get a technician on-site — which is unacceptable for a premium system.

The system works when it works, but in our case, the reliability and service have not lived up to expectations.
If you’re considering Vaillant, I strongly recommend researching alternatives like Hitachi, Bosch or Nibe, which seem to have better performance and support, especially in Spain.

R290 is propane, R134A is tetrafluoroethane which I think is banned for new installations in the UK(?). Maybe Daikin and Mixergy using R32 now?

We are building a new house in Thailand and it's obviously very hot here year-round. We have a 36kWp solar array and batteries, and the inverters generate heat. Could we pull hot air in from outside to heat our water, and then vent the cold air into the solar inverter room? We could even place the heat pump in the same room.

The builder wants to go with a bunch of small on-demand, tankless hot water heaters in each of the bathrooms and kitchen sink.

What are your thoughts?

Thank you. 🙏

My son has just bought a top floor flat with, as it turns out, a broken Ideal Istor gas boiler which is obsolete. It has twin pipes for a flue, about 2 inch diameter, one input the other for exhaust gas. The boiler is in a cupboard with no outside wall. I'm wondering if I can replace the boiler with a heat pump cylinder and repurpose these two flue pipes for use with the heat pump. Alternatives such as a Tepeo and Sunamp Thermino are expensive and may be too heavy. Any gas boiler replacement would need a new flue which again would be expensive and require scaffolding.

We have just installed a dimplex heat pump cylinder on a school for an area with low demand. Works beautifully. We originally wanted the mixergy but were awful to deal with. Provided no support for technical design and suggested consultants were providing wild statements. This was even after seeing them in person at the installer show in Birmingham.  Mixergy seemingly didn’t even want to know.

Many thanks. Doing the thinking on whether to replace my costly and inefficient immersion with a hot water heat pump cylinder so this was quite handy, although it would be have been great to see some sort of comparison of how the apps work and also come approx costings too.

If you have a photovoltaic system on your roof (or you are forced by the law to install one, as it is the case for new constructions in many EU countries) it is better to produce hot water with a conventional, cheap, and fully static, maintenance free for decades, electric boiler. Infact you are probably selling for almost nothing (typically 4 €cents/kWh) most of your production to the grid. Just convert it in hot water and save money, complexity, and running costs (maintenance/replacement) of a heat pump heater.
If you have money to spare and space on the roof just install a few more cheap PV panels (they are really cheap nowadays) to better saturate the inverter power even in winter/mid season (i.e. 8kWp panels with a 4.5 or 6kW inverter).
If you have a well insulated house you can have winter space heating too with electrical resistance mostly for free (from PV) and save the cost and complexity (and maintenance) of a heat pump. Probably you will miss a few kWh in December and January, just buy them from the grid. You will not break the bank!
But running a house with fully static, maintenance free, and totally silent plants is priceless! You have only to switch on the air conditioner in summer.

Tim, I so wish this video had been around about 18 months ago when we decided on our heat pump cylinder, though at the time there were few choices available and the Dimplex EDel 270RF ended up being our choice. As mentioned in other comments, it looked to me that some of the then not yet available models such as the Valiant were pretty much identical anyway - it’s just the marketing information which is confusing and different.

Apologies this is a long comment, but thought I’d share some useful things we learnt in the process:

270L is way bigger than we need for just the 2 of us, but it was virtually the same price as their others and so we figured we can use the extra capacity as a ‘thermal store’ instead of an extra battery, to take surplus solar PV generation. Note also that the Dimplex can be hooked up to your PV inverter so you can programme excess solar generation to ‘boost’ DHW production - Not sure if this is linked purely to the immersion element, nor have I worked out yet if or how I can link this with my SunSynk Hybrid inverter. I may be more efficient to use smart home automation to simply run the Dimplex’s heat pump when solar PV export occurs at set criteria.

As explained below, the Dimplex works exactly the same as the Mixergy in that it also heats from the top down, so you can still use what available water is up to temperature before the whole cylinder has reached target. Stratification is not some new technology unique to Mixergy - in fact lots of really old DHW cylinders work on the same principle.

Re what you said at 3:30; If you have an ASHP for heating, there are a number of good reasons to have a separate heat pump cylinder for DHW. We went for Mitsubishi Ecodan (5kW) for heating, but this exclusively runs the heating and the Dimplex runs separately for our DHW.

Efficiency - There was no Mitsubishi Ecodan data published (when we bought ours) on the CoP performance for its hot water mode, but I contacted them and was told the efficiency is only around 1.7 during hot water production, as their design prioritises speed for DHW production over efficiency. The Dimplex CoP of 3.3 isn’t quoted as a SCoP, but they confirmed to me it was a average figure (based on ambient external temperatures) and I seem to recall it may have even factored in using the boost function from the immersion heater inside - which isn’t optimal. So, separating heating and DWH this way gives ultimate efficiency, as the ASHP can stay in weather compensated mode for heating 100% of the time, while the heat pump cylinder just does hot water.

Cost - was very surprised to find that the heat pump cylinder was actually significantly cheaper than than buying the standard Mitsubishi pre-plumbed cylinder unit that would normally go with our Ecodan outside unit.

Location - the Dimplex is in our plant room, with intake and extract vents running out of adjacent external wall (minimal ducting/bends). Noise from the unit here isn’t a problem, as this is on the lower floor and opposite side of the house to our bedrooms, but in use the unit is not very noisy anyway (pleasant surprise).

We also have an MVHR system (Zender Comfo Air Q) which has an extract vent located in the plant room ceiling, meaning 93% of all the heat generated in the room (IT equipment and the Dimplex cylinder etc) is used to preheat the fresh air being brought into ventilate the house. The MVHR reduces the heat losses of the whole building by over 30%, so provided you put anything that generates heat within the insulated envelope of the building, none of that goes to waste (technically 7% of it does, but overall still way less with an MVHR system). For us, this also negates any concerns about standing heat losses which you may encounter if located in say a garage.

Stratification of hot water - AFAIK this is not a ‘design feature’ unique to the Mixergy cylinder as our Dimplex Edel 270 RF operates the same way (heating from the top down); on the phone app for controlling it shows you how many litres of hot water (at the set temperature) are available at any time, with an example of how many showers or baths that equates to.

Again, apologies for such a long comment but hope this is of use for others considering these issues.

The Dimplex and Vaillant 200 & 270 are exactly the same unit under the skin, they are made for these manufacturers by a third party.

Thank you for distilling all of that - saved me days!

Extracting heat from the attic to input into your hot water heater makes sense if you have a well insulated ceiling. In a ventilated attic the air temp in winter should be close to outdoor temp. During the summer months the extra heat will make the hot water heater more efficient. Instead of a vertical pipe into the attic I would use an elbow or tee stove pipe at least a foot above the insulation with wire mesh cover to keep out dust and rodents.

Regarding legionella and the mixergy system. The bacteria could easily survive in the lower cooler part of the tank so I guess the routine cycle is more important in that case. My wife is a microbiologist and she insists we take this seriously.

Thanks for this. I'm actually most likely to choose a air to water system and had been thinking of the Mixergy. I must admit that I was a bit worried as I misread that heat loss figure and thought it said around 1.01-1.75 kWh per hour not per 24 hours and was really worried that I'd be losing 24-42 kWh per day and figured that I'd be using huge amounts of electricity just keeping the water hot. When I read it again and realised it was that much in 24 hours I was a little less worried. 🙂

Hi Tim & Kat, I have a zero-emission (and very effiicient) Bed and Breakfast down here in the lovely Ancient town of Rye, in East Sussex. I'd very much like to invite you (and family) down for a night for you to review, comment, critique and discuss going from a regular domestic user to "light commercial", with a 34 panel solar array, a 12kw ASHP, 10x Air to Air heat pumps, underfloor and heated skirting boards....oh, and  4x 7kw EV chargers to boot :-)  I hope you catch this comment :-) All the best, Javed Khan, Magnolia House Rye.

I've had one for the last 10 years works perfectly no issues the heat pump pulls 660 watts I have also have also a heat pump for house heating, the logic of this is I prefer to to run 660 watts for hot water opposed to run the larger 2000 watt house heating compressor though both are run on off peak electric

If you have 10Kw of battery and 5Kw inverter in the loft creating heat, would that make it a better option for the inlet coming from the loft? Possible switchable vent so you can switch to drawing for outside during the winter to not over-cool the loft?