Whole‑House Renovation: Need Real‑World Advice on ASHP + Fan Coils + MVHR Integration

[NetTechie]

Hi all,

I’m in the middle of a full‑scale renovation of a very tired property — original 40‑year‑old fan boiler, vented cylinder, single‑glazed metal windows, the lot. The house is being stripped back to brick with new floors, ceilings, EWI, cavity insulation, new roof, the works.

I’ve been doing the rounds at the homebuilding/renovation shows, but I’m really struggling to find installers who actually understand integrated systems. I’m an engineer, so I like to get into the detail — but the moment I mention fan coil units, I get blank stares or “we’ll have to ask someone else”. Not reassuring.

My end goal for the refurbished house:

• ~16 kW reversible ASHP for heating and cooling - with UFH throughout + towel rads in bathrooms
• Fan coils for bedrooms, large open‑plan areas and south‑facing rooms
• MVHR with heating/cooling module (going with Zehnder Q600 + CW12, designed by Paul; armaflex radial ducting planned)
• DHW storage — Mixergy looks promising, but installers keep telling me I won’t see any benefit for the outlay
• ~7 kW solar + battery storage

Where I’m stuck is tying together the ASHP, DHW cylinder, fan coils, solar and batteries into a coherent system.

Heat pump thoughts

The LG Therma V R290 monobloc looks excellent on paper — strong COP, good flow rates — but I cannot find a single installer who actually works with LG. I keep getting pushed toward Samsung, Vaillant or Mitsubishi. Two installers have specified Joule cylinders. I’m personally not keen on Samsung due to reliability and integration quirks.

What I’d really appreciate advice on:

1. LG ASHP installers - Has anyone found a company that actually installs LG heat pumps in the UK?
2. Real‑world views on the LG Therma V R290 - Specs look great, but I’d love to hear from anyone who has one running.
3. Fan coil units - This is the big one for me. I don’t want split AC with another condenser outside and messy wall units — I want everything integrated off the ASHP with concealed units in the ceilings / walls. - Anyone used Megawave Energy or other brands? How well do fan coils actually cool during those 30–35°C heatwaves?
4. DHW cylinder choices - Is Mixergy X worth it in a heat‑pump‑led system, or should I be looking at something like Joule?
5. Solar + battery recommendations - I’m being quoted for panels I’ve never heard of at 5× their retail cost. Most installers push Fox batteries — any real‑world experiences?

 

If anyone has gone down a similar route — integrated ASHP + fan coils + MVHR + solar — I’d really value your insights. I’m trying to design a system that actually works as a whole rather than bolting together whatever each installer happens to sell. Oh and to throw so more complexity, control is important, I had my home automation company years ago, so can design this but the kit needs to integrate into HomeAssistant and Control4, Heatmiser is easy enough for the CH, DHW and FC's but more invested in the solar, battery / energy side of things. And how do I still get get hot water in summer when I'm running the ASHP in cooling mode?

Thanks in advance.

[JohnMo]

31 minutes ago, NetTechie said:

My end goal for the refurbished house:

• ~16 kW reversible ASHP

That is one huge heat pump - are sure you will ever need that big? Oversized heat pumps are not good. Correct sizing is important.

 

32 minutes ago, NetTechie said:

struggling to find installers who actually understand integrated systems

Integrated systems are almost never needed. Keep ventilation separate from heating.

 

33 minutes ago, NetTechie said:

MVHR with heating/cooling module

Don't waste your money, heating is only good for 10W per m² of floor area, cooling via MVHR is crap in the UK due to low flow rates and humidity.

 

35 minutes ago, NetTechie said:

DHW storage

Just use a normal heat pump cylinder (more later)

 

36 minutes ago, NetTechie said:

monobloc

Yes, but choose based on actual heat demand at -3 (location depending) and more importantly min modulation at 7 to 12 degs. Work out heat loss at these temperatures and choose a heat pump that will modulate down to that level.

 

Home automation - just isn't needed.

 

So system layout

PV and battery let them run on their own, supplying house and export excess. Keep it simple, no integrate quired it's there by default.

 

Heating, fan coils is fine for heat and cool. But again keep it simple.

 

Ventilation IF you are airtight better than 3 and are strongly making airtight decisions then ok. Just have it do ventilation nothing more nothing less, it runs in background 24-7.

 

DHW, you can have ASHP do hot water, if so just choose a cylinder with a 3m² coil. Or heat during off peak periods via immersion, with solar real cost difference is that big.

 

I have run every mode of heating control with a heat pump and weather compensation wins, on comfort, costs and ease of implementation. No third party control is needed and adds to running costs.

47 minutes ago, NetTechie said:

And how do I still get get hot water in summer when I'm running the ASHP in cooling mode?

It run priory hot water mode, it just switches from CH or cooling to DHW. No issues.

 

49 minutes ago, NetTechie said:

Oh and to throw so more complexity, control is important

No your just making it complex in your heat, each is a separate standalone solution that without the box without over engineering

 

Battery and PV work out the box

Ventilation set and forget (except filter changes)

 

ASHP set WC curve, get on with your life

 

DHW either set a stand alone immersion timer, or get the ASHP to it via a schedule.

[jfb]

Don’t much about fan coil units and ashp. 
my first thought is 16kw sounds way too big if you are already doing a full thermal/air tightness upgrade.

how large is the property?

[NetTechie]

The property will be around 385 sqm over 3 floors. One of the companies said a 12kW would be fine as they estimate the heatloss to be around 11.3kW on the coldest day at -2c, but having a 16kW would give more headroom and they also suggested additional electric backup, whatever that is meant to mean.

[Iceverge]

29 minutes ago, JohnMo said:

Keep ventilation separate from heating.

 

Correct. 

 

30 minutes ago, JohnMo said:

Home automation - just isn't needed.

 

Double correct. 

 

8 minutes ago, NetTechie said:

they also suggested additional electric backup

 

Plug in fan heater. 

 

 

 

[JohnMo]

24 minutes ago, NetTechie said:

estimate the heatloss

That isn't good enough for a heat pump, you need an accurate assessment. My house is 192m² all vaulted ceilings (225m² including plant room) my heat loss is 3.5kW at -9. Just about to remove a 6kW heat pump and install a 4kW one.

 

26 minutes ago, NetTechie said:

headroom

Not the correct way to go about heat pumps. Not really correct for has boilers either.

[SimonD]

+1 for the comments so far. Just a couple of questions for you:

 

- what is your rationale for using fan coils in the locations you're proposing?

- how much cooling are you expecting the ASHP system to deliver? A lot of people over estimate this.

[NetTechie]

9 minutes ago, JohnMo said:

That isn't good enough for a heat pump, you need an accurate assessment. My house is 192m² all vaulted ceilings (225m² including plant room) my heat loss is 3.5kW at -9. Just about to remove a 6kW heat pump and install a 4kW one.

 

Wow. 3.5kW at -9, could you share your build fabric details and what you did to achieve this, this sounds extremely high performance.

[NetTechie]

1 minute ago, SimonD said:

+1 for the comments so far. Just a couple of questions for you:

 

- what is your rationale for using fan coils in the locations you're proposing?

- how much cooling are you expecting the ASHP system to deliver? A lot of people over estimate this.

 

The South facing rooms are where we will spend most of our time, this is the one plan area, I work from home so the Study is also important to me and bedrooms for night comfort, which also includes 2 loft rooms.

 

I'm not expecting AC level cooling, but enough to keep the house comfortable in the peak summer months, so if it can deliver a delta of say 10c -15c I would be exceptionally happy. 

[Beelbeebub]

I would suggest that headroom is less important for such a large house.

 

Say we have a very cold snap and your 12kw heatpump isn't able to keep the entire house at 21C (or whatever the design conditions are) 

 

Just turn down a few of the unused rooms to 15C for that period. 

 

If you have a 2 bed flat and all rooms in use all the time you don't have that ability. 

 

But for such a big house the spare rooms can be hibernated for a bit. 

 

[SimonD]

10 minutes ago, Beelbeebub said:

Just turn down a few of the unused rooms to 15C for that period. 

 

I don't think that's a good suggestion as all that does is end up increasing the load on the other rooms that are heated (as internal heat losses increase and can do so significantly) and can result in higher flow temps from the heatpump as well as reduction of system volume which can also cause cycling and defrost problems. 

 

When you're designing a system for a deep retrofit like this one, there's no excuse not to design and install a system that can supply 100% of heat requirements, which is also required by MCS.

 

[SimonD]

25 minutes ago, NetTechie said:

so if it can deliver a delta of say 10c -15c I would be exceptionally happy.

 

That may be too much expectation. Usually assume a few degrees - but usually enough to take the edge off excess heat to provide more comfort.

 

 

[SimonD]

1 hour ago, NetTechie said:

The property will be around 385 sqm over 3 floors. One of the companies said a 12kW would be fine as they estimate the heatloss to be around 11.3kW on the coldest day at -2c,

 

For a little comparison, my place is deep retrofit with EWI and new timberframe 1st floor with 3g glazing. We had a heat loss of 3.8kW at -6 with a floor area of 176sqm. We have natural ventilation.

 

If yours is really going to be 12kW, you could also consider 2 smaller units in cascade to provide the modulation you need for the shoulder months, in particular when you have a great deal of solar gain.

[JohnMo]

Fan coils work well for cooling and heating, but as with radiators they need to be sized correctly for the heat loss first and then for the cooling load. So you need proper room by room heat loss calculation. You basically set the cooling flow temp to match output needed. That flow temp for cooling can be anywhere from 6 degs to 20. The fan coil fan speed modulates to match output needed in a micro scale. Running cooling below about 14 degs required condensate drains at the fan coils.

[NetTechie]

Thanks for all the responses thus far. So it seems my heat loss calculations seem pretty bad at 11.3kW, these were done by an MCS company and it did seem a little high. 

 

Our new extensions fabric will be double block wall with 120mm cavity fill with PIR boards, 40mm EWI, existing walls are double brick with 70mm cavity, which will be blown with EPS beads and then 60mm EWI applied. 

All windows, skylights will be triple glazed. Ground floor will be screed throughout with 150mm PIR insulation below and 150mm PIR insulation in the roof envelope.

 

I'm not sure what else I could do to lower the heat loss, or have the company simply calculated this wrong or provided enough margin on their design?

[crispy_wafer]

run your own calcs to sanity check the MCS company, you'll learn (from others on this site), if you haven't already, to double check what you are being told by 'experts'.  

[JohnMo]

15 minutes ago, NetTechie said:

Our new extensions fabric will be double block wall with 120mm cavity fill with PIR boards, 40mm EWI, existing walls are double brick with 70mm cavity, which will be blown with EPS beads and then 60mm EWI applied. 

All windows, skylights will be triple glazed. Ground floor will be screed throughout with 150mm PIR insulation below and 150mm PIR insulation in the roof envelope.

 

I'm not sure what else I could do to lower the heat loss, or have the company simply calculated this wrong or provided enough margin on their design?

Trouble with a lot of MCS installers they simply ignore all improvement made to building fabric. They then apply a huge ventilation heat loss if you're not careful, then add a bit just in case.

 

As said by @crispy_wafer do your own calculations make sense of everything in your own head. I recently asked about a heat pump quote via an umbrella scheme, they had my EPC, my house drawings, quoted a 12kW heat pump. I have just bought a 4kW, without grants. 

[SimonD]

17 minutes ago, NetTechie said:

I'm not sure what else I could do to lower the heat loss, or have the company simply calculated this wrong or provided enough margin on their design?

 

Did the company have specified u-values to work with for all building components, including internal walls? And did they have a floor plan to use? Do you know what heat loss tool they used?

 

The most common factors I find regarding heat loss calcs are:

 

1. air change rates being over-estimated (this is expecially risky when you have a deep retrofit because the designers might put in build date and use default figures for those, rather than use the new figures based on the retrofit upgrades)

2. using default u-value figures for fabric elements instead of clarifying - even on solid walls the default values are know to be pessimistic. Default window u-values are just so far off it's not even a joke

3. don't correctly adapt the floor u-values. In the CIBSE book you need to adjust the floor u-values according to depth and how many exposed edges the floor has and some floors can be calculated using an exposure factor. I've seen a constant u-value inserted for this.

4. don't corretly look at building construction details to adjust the psi values for thermal bridging. In a well detailed, good quality and highly insulated deep retrofit, I'd expect to be able to used SAP reduced thermal bridging values instead of default

5. then there are other minor details like the geographical position used to determine outdoor air temperature, building exposure (which is used to calculate infiltration ventilation losses) and is subjective. etc. 

 

All these and it adds up to a lot. On one recent design, the default values applied by the design tool I was using suggested a heat loss of over 11kW on the property and after I adjusted the relevant values it was 6.9kW. On my own house, Heat Geek's AI tool tells me I need a 12kW heat pump, which is obviously total nonsense.

[jack]

18 minutes ago, JohnMo said:

Home automation - just isn't needed.

  

100% agree, but if the OP is installing HA anyway (or already has it installed), it doesn't hurt to use it.

 

I've intentionally kept my MVHR and heating systems completely separate. MVHR is just left to its own devices - I may eventually integrate it into my (Loxone) HA system for more flexibility around boosting times and power, but for the moment it works fine without.

 

My heating system is kept relatively simple, in the sense that weather compensation, flow temps, etc., are all left to the ASHP, which is programmed via the ASHP controller. Calls for heat are made via the usual live switching approach. However, instead of using a thermostat to call for heat, I have my HA system control a relay. The HA system decides whether to call for heat based on the temperature upstairs, the temperature of the slab, whether the Octopus cheap rate applies, and a couple of other factors. 

 

Throughout December and early January, we had anything from 2 to 9 people staying in the house, and daytime temperatures of everything from low single digits to low teens, including long periods of sunny days and cloudy days. The slab temperature never dropped below 20, never rose above 22, and didn't change by more than about 0.8° (usually a lot less) in any 24 hour period.

[NetTechie]

34 minutes ago, SimonD said:

Did the company have specified u-values to work with for all building components, including internal walls? And did they have a floor plan to use? Do you know what heat loss tool they used?

 

Thanks @SimonD. The only information they asked for was current building elements, external walls, floors, windows and ceilings, no specific u-values were requested or given. They were provided detailed floor plans of the build and I have no idea what software they used. One thing I did notice and I called them out on it was even though I specified no radiators, the specified radiators to all the first and second floors. The ground floor, they broke down into 3 sections, front, rear and side, not sure why room based calculations weren't done for the GF, but they were for the FF and SF.

Edited 5 hours ago by jack
Fixed quote text

[SimonD]

1 hour ago, NetTechie said:

 

Thanks @SimonD. The only information they asked for was current building elements, external walls, floors, windows and ceilings, no specific u-values were requested or given. They were provided detailed floor plans of the build and I have no idea what software they used. One thing I did notice and I called them out on it was even though I specified no radiators, the specified radiators to all the first and second floors. The ground floor, they broke down into 3 sections, front, rear and side, not sure why room based calculations weren't done for the GF, but they were for the FF and SF.

 

Ah, okay. Yeah, that all means your heat loss 'estimate' is worth less than a pinch of salt. Unfortunately.

[Dillsue]

If you use Excel it's straight forward to do room by room heat loss calcs. Work out the area of each surface x U value x temp diff and that's your heat loss. If a surface has an element with a differing U value ie a window or door, deduct that elements area from the main surface and work out that elements loss separately, if it has a differing U value. Add all the losses for each surface of the room, 4x walls+floor+ceiling, and youve got the room loss. Repeat for each room and you've got the whole house loss. Simples:)

 

After that you need to account for ventilation losses and I can't remember how I calculated that without opening up my laptop which I haven't got access to!! Someone else will likely guide you on that??

[Iceverge]

6 hours ago, NetTechie said:

Our new extensions fabric will be double block wall with 120mm cavity fill with PIR boards, 40mm EWI, existing walls are double brick with 70mm cavity, which will be blown with EPS beads and then 60mm EWI applied. 

All windows, skylights will be triple glazed. Ground floor will be screed throughout with 150mm PIR insulation below and 150mm PIR insulation in the roof envelope.

 

I would get this right before going anywhere near a heat pump. 

 

I would rather chew my leg off than put PIR in a cavity wall or between roof timbers. 

 

Why the combo of EWI and cavity in the new build? I would do one or the other. 

 

Why so little EWI?  The insuation is a small cost vs the install and render. 

 

Wait.........do hear a greasy haired white toothed kingspan salesman boarding a plane..........I think he's off to Marbella for a jaunt on the kooltherm sales he pawned off to you via your architect......