J1mbo

Ok, so the curve and the outside temperature dictate the flow temperature and, if you change room temperature from thermostat to “temp mod”, the room temperature observed by the VRC700 itself will also modify it. The set point and programmes set in the VRC700 will be overridden by the Internet gateway (assuming you mean Ambisense). The room temperatures observed by Ambisense room temperature sensors is not used in the flow temperature calculation. The min is simply the floor value. If the calculation yields say 30*, the heat pump will aim for 40* anyway as you have it. Likewise the max is a cap. Really the system needs to be designed so a curve in the region of 1.1 is used which will yield 50* at freezing outside when room temperature is set to 20*.

It will cost you a lot to run the heat pump with a design max of 65*. What is the heat curve configured as? And what was the design flow temperature on the MCS certificate? The min and max are applied to the curve; these settings might not do what you’re expecting. Also, do you have any controllers other than the VRC700? The larger buffer is not a problem at all and actually is required if the system has only low water content radiators. Is it plumbed 4-pipe like the Vaillant diagrams? Hopefully the external temperature sensor has been fitted?

Vaillant controls have load compensation. It works by shifting the weather compensation curve at 45* in either direction in their case. It means that the flow temperature is based on three temperatures - outside, inside, and set point, which makes the system quite responsive.

Can you expand on the configuration issue you’re seeing? A larger buffer will generally reduce short cycling and improve overall COP. The VF1 sensor should be connected to the heat pump interface at the appropriate terminal.

Perhaps I'm missing something but this question seems to be, "shall I get a brand new heat pump and free money"

Does the house have a damp proof course?

It absolutely does not. Electricity at my location has doubled in two years. It is widely expected to be getting another 50% hike in April. Quite why the press are so blinkered on electricity is beyond me.

The electric UFH mat is literally just wire in a loop on a mesh former that is connected directly to the electric supply. Power input = power output. It's advantage is simplicity and easy of installation - it can be set in tile adhesive for example with almost no build-up of floor height. However it's very expensive to run as electricity is expensive (currently 21p per kWh, compared to maybe 7p per kWh for gas). A wet system uses plastic pipe looped across the area and typically set in concrete (although other systems are available) and basically behaves like a huge radiator. The heat is applied by anything that can heat the water flowing through it - direct resistance electric heating, ASHP, GSHP, solid fuel boiler or just a plain old gas boiler. The big advantage with wet systems for heat pumps is that the operating temperature is low because the surface area is so large, which typically means the heat pump can work with a higher COP than it would be able to achieve using radiators (since radiators will usually have less surface area overall and therefore the required water temperature would be higher to achieve the same output), and this reduces the electricity demand. However, at least when part of the concrete slab, wet systems provide steady output and this in turn means that they can't offer the quick response heating that a gas boiler with radiators can. Instead the house will be heated 24x7 - which again suits the lower power output of the ASHP (when compared to a gas boiler).

Electric UFL is a matt consisting of resistant wire. An ASHP would be connected to wet UFH just the same as would be used with a gas boiler.

Yes it needs DNO approval prior to installation if the post installation maximum demand exceeds 60A (as does a car charger), otherwise notified within 28 days. Maybe request a 60A supply? And then ask for a free upgrade to 100A later...

or write to your MP...

What size is the prospective ASHP? And, what size electricity supply (main fuse) do you have currently?

Agree. The aluminium Tropical95's (and their numerous copies) simply have oodles of surface area. The datasheet gives the output at all flow temperatures from dT=20 to dT=79: https://www.warmrooms.co.uk/Controls/List/Download.ashx?ListItemId=22198&File=1

RHI payments are simply based on the EPC annual demand and the ASHP approved SCOP for the quoted design flow temperature (each product has a table). Aim for the lowest possible design flow temperature. Also bear in mind that in use, the number of daily compressor run hours will also affect the flow temperatures needed. If you only heat the house for 3 hours in the evening due to work patterns etc, go really big on radiator sizing. If you have it running all day or 24x7, the radiators can be much smaller relatively. Especially with the cost of electricity now and it’s likely to rise significantly further, my advice FWIW would be to design for the lowest practical temperatures as it will reduce your running costs and increase your RHI payments at the same time. So just replace the radiators.

@MattB67 aluminium radiators might be worth a look, I’ve had great success with Faral Tropical 95s. They are though very expensive,

Output is not proportional (in a linear sense) to flow temperature btw.

A small four pipe buffer cylinder is just a large LLH really. So maybe that’s the happy medium.

See the explanatory note on page 26: Draft: Boiler Upgrade Scheme (England and Wales) Regulations 2022 (publishing.service.gov.uk) Broadly like RHI - MCS installer, EPC with no recommendations for cavity wall and loft insulation unless listed, MCS approved published SCOP of 2.8 or more if ASHP. But it does cover new-builds.

It's a obviously a headline, but the principle is pretty straight-forward - make more of something and the unit cost is reduced. They appear to be partnering with Vaillant based on the videos they have published of their research and training centre.

Octopus Energy have said that they are planning to offer ASHP at £5.5k fitted within 18 mo. As the sector is currently so small, the prices will fall dramatically as it grows.

Picking up on this. I presume the 38p is exc.VAT @ 5% and the boiler efficiency will be about 90% in the end. With 7.08kWh per litre in the fuel, this works out at 6.26p per kWh at point of use. With electric currently at 20p on the cap then SCOP of 3.2 breaks even. If it's under-floor heating throughout then SCOP of 4+ is achievable. It seems likely that both of those prices will rise dramatically in the next six months. CCL will also likely be moved away from purely electric. You know that the property will end up with ASHP at some point; it would make sense to just do it now in my opinion to avoid system design changes (and costs) later. I'm not sure if an underground tank was envisaged but the quotes I had suggested that the installation costs of such can be more than the cost of an ASHP itself.

16°C is probably warmer than the incoming cold water supply. Then again that doesn't run 24 hours I suppose.

ASHP doesn't run out of gas.

Sometimes the manufacturer will specify the minimum buffer size based on defrost requirements.

15% is a good ratio. Though the COP would likely be a lot higher mid season meaning the output is likely more like 25% or more?