JohnMo

Not seen any cylinders with moving parts? Copper and stainless cylinders lasts for decades. There is no reason copper should not last a lifetime. The real advantage of sunamp are size compared to normal cylinder, and better heat loss profile. But any cylinder in a cupboard surrounded with accoustic insulation will loose the best part of nothing, especially at heat pump temperatures.

I don't see the issue with them not working, the refrigerant is going in to the evaporator at minus lots, well below the coldest day. So not different to ASHP. The issue come when you try to run them as in warm climates, with just one panel on a 2kW unit. You need to use 2.

You just take the whole extract nozzle out the ceiling, the filter cone goes over clips that are there already and slots back in place. No need to move any adjustment parts.

Basically no fans, instead of a radiator and fans it uses a couple of 2m X 1m black panels on roof or wall.

There about £3 each, not sure I would bother washing. https://www.epicair.co.uk/products/extract-air-valve-filter-for-8960-125mm-valve-10-pieces

The extract points should be installed with a cone foam filter, that keeps the ducts clean

I tried to extract from the bedrooms as that made sense to me as well, but the plans were rejected, so reverted to the normal supply. With a Sip build you will be quite airtight without trying, so will have no option than go for MVHR. The building regs do state balanced ventilation, so the option would need PIV and MEV or dMEV. But that makes no sense, when you can recover the heat. You could move air inside the house with dMEV fans within the internal walls.

Before you run weather compensation, you will need to balance the system. Then run a single high limit stat, and use and tweak the system flow temps, so you get the temp you want without the thermostat kicking in at anytime. It is normal to run weather compensation 24/7 with only a 2/3 degree setback at night if you want.

Not using mixergy or Vaillant, but same thermodynamics apply. You will have a max temperature settings within the control scheme, the system is slowly working upwards that limit, hole trying to maintain the correct deltaT.

What I forgot to mention, is the temperature ramps up, because the control system is trying to maintain a delta T between supply and return, not enough heat is being extracted, so it sees higher than expected return temperatures so ramps up supply temperature.

I have a gas boiler and I am using the indirect coil at very low temperature 30 degs. I see exactly the same as you. My flow temp increases well past the set point and eventually trips at 55 degC. So what is happening. In my case and most possibly yours, is the coil surface area is not large enough to transfer the heat to the cylinder contents. The lower the heating medium temperature the larger the coil required, to transfer the heat to the cylinder. Two ways to fix, can you heat the cylinder directly instead of indirectly (this will increase volume of glycol required). If not, a pumped circuit could be used through a large plate exchanger. So HP is used on hot side of the plate exchanger, the cold side connected to you cylinder. In my case my boiler should be generating 7kW, with boiler output of 30 degC, and return of 25degC. Have sized for a 2 deg approach temp, so that 28 degC is coming out on the cold side to the cylinder. PHE is a 40 plate and sized at 430x81mm. So quite big. Handy calc tool https://www.heat-exchangers.uk/online-calculator/

Two things worth looking into. Separate ring is normal for storage heating as they are supplied via the E7 part of the electrical system. The rest of the may not be. 1. PV produces the best part of nothing during the heating season, so would not offset the heating costs, even slightly. What is produced in winter will just be self consumed. 2. In winter they will be charging up the storage heaters and charging the battery via E7. Quick calc they are spending over £800 on heating. I would be tempted to give the storage heaters a bit of an overhaul and good clean out. Review, improve the insulation situation and fix the drafts.

Confused why you are even bothering fixing stuff in a rental, that's what the owner is responsible for. Call home owner or their agent, tell them to get it fixed. If it rented via an agency your contract will be with them so call them, they will get the plumber out and back charge the owner.

No Cost is simply how much something costs. The cost benefit, is what it save you by installing product A compared to B. A normal fan, on demand high flow rate when it's on, no flow when off. dMEV more expensive than a normal fan, but can lead to better air quality as its on all the time, uses low energy motors low flow rate all the time. Single room, MVHR, same flow rates as dMEV, but two fans to drive. The saving come from heat recovery. So if comparing dMEV with MVHR, the cost to install each will be £xxx or £xxxx, then you need to calculate the ventilation heat loss for the room and see if the payback time is worth the expense taking into account the heat recovery efficiency.

Why not a water cylinder instead of a sunamp? Cylinder, couple of immersions, 180 litre cylinder heated to 70 and drawn down to 25 is 9.5kWh. A2A heat pump? As you have all block work walls, what about in wall heating, same as UFH but in the walls, the blocks acting as the energy store from a Willis heater? What are you doing for DHW?

Servicing - just a matter of replacing filters in the main unit and at extract nozzles. While you are changing filters have a look round hoover out excess debris and give s general clean. They generally save about 3 to 4 times the running cost in recouping lost ventilation heat. Expense compared to not installing, if your air tight better than 3 you have to install to comply with building regs. Flows are not sufficient to move much heat around, also not noticeable sir movement in the house. We are in a bungalow bedrooms are cool, rest of house is not. A thought about install should not be noisy event in boost it will only just be heard if you are looking for the noise. Deathly silent compared to normal extract fan.

As said the more things you have that make the system smaller such as trvs, thermostat etc, the more likely you are to get short cycling

CT1?

I also bought from insulation hub.

On the boiler questions, if it's heating the house now, it will heat with an alternative heating system, as the energy lost from the house will not change. Unless of cause you do UFH without suitable insulation below the pipes, then loads of energy will be pumped into the ground under the house, as that would be the easiest path for the heat to travel.

I would be careful with thermostats everywhere, that the boiler can turn down enough to cope, otherwise the boiler will just short cycle and use loads more gas, or if heat pump lots more electric. Remember the house heat demand is for the coldest day, anything warmer it requires much less energy, every zone or rad that is off is even less energy being consumed, the boiler cannot turn down enough to dissipate the heat generated, so shuts down because the return temp is too high, waits for 5mins starts again and repeats. Going Smart everywhere is the the smart thing to do.

Not normal but could work, if the loft space is outside the heated environment, your heat loss could be large, even when very well insulated. Don't you have a cupboard or wardrobe you put the manifold in?

Design it yourself? Keep flow velocity low, semi flex duct and plenum/manifold. Silencer between MVHR and plenum. Keep MVHR unit central so duct runs are as equal/short as possible. Specify MVHR unit one size up from your target boost flow rate. Extract air out of wet rooms, supply it to dry rooms. Supply and extract flows match overall.

If they are installing and supplying they should not be charging you vat at all on anything.

What ever you do smart, will be old hat in 10 years time and not supported. Keep it simple, and replaceable, but install cat 5/6 data cables to TV points.