Self Build Heating and Cooling Strategy

[Havkey100]

All,

 

PHPP calcs are in and we are firmly in between Passivhaus Plus and Passivhaus Premium by design, less than 0.6ACH, with overheating risk at 1%.  Amazing of course, but the one slight downside of all of this is the multitude of options for heating and ventilation.

 

The solar installation will be 9-13kw of solar (pending DNO approval), a 15kw battery, with no gas installed on site. May consider a V2X Sigenstor as my car has that feature and I have access to  free charging.

 

The basic issue is that we can almost heat the house with a toaster, therefore a heat pump system is for the most part going to provide HW in an unvented cylinder for a family of 5, with only periodic use via a wet U/F loop downstairs on the insulated raft.  The MHVR solution is likely to be a Zehnder Q450.

 

I am trying to weigh up all the permutations we can try in order to minimise capital cost and simplify controls.

 

So far my considerations are:

 

1. MHVR Heating/Partial Cooling with Direct Hot Water Cylinder - £16k

 

• Zehnder Q450 and Comfoclime add on HP, which gives about 2kw of heating and cooling capacity. It comes in at £13k and has the benefit that it likely has all the heating capacity we would ever need, can temper hot conditions (clearly not a full blown AC, but 2kw of cooling would help a lot).
• OSO 300L DHW cylinder. Can heat overnight on an agile tariff, economy 7, and recharge daytime from battery, V2X if needed. £2k installed.
• Contingency install U/F loop in case the above is less liveable than we hope. £1k

 

2. MHVR with 5KW HP HP Cylinder - £18k

 

• Zehnder Q450 £10k
• Vaillant Arotherm 5KW with 300l Vaillant HP Cylinder. £7k out of pocket.
• U/F loop in case the above is less liveable than we hope. £1k

 

3. MHVR with Direct Hot Water Cylinder and IR/Electric Heating - £14.5k

 

• Zehnder Q450 £10k
• OSO 300L DHW cylinder. £2k installed
• Strategic Electric Heating - IR or a few towel rails in the extract locations of the MHVR loop - £1.5k.
• Contingency install U/F loop in case the above is less liveable than we hope. £1k

 

Any other options I am missing here? I am inclined to go with option 1 as it is a bit cheaper, harmonises the controls, and in practice I can't imagine it will cost any more or less in practice to run, plus has some cooling capability. I was also considering the NIBE S735C, but am struggling to lock in pricing on that. I think it may come in at 20k and is an entirely integrated solution from which I could heat via air, u/f, ventilate, cool and manage HW with 300l of storage. But, the issue is the overlap of heating and cooling adds complexity to ownership of an installation between the MHVR designer and heating engineer.

 

Thanks for any input and experience!

 

 

 

[JohnMo]

Keep it all simple and functions separate

 

First ventilation is ventilation, it isn't heating/cooling.  Separate the ventilation keep it a stand alone item. Cooling via MVHR is an expensive folly and in the round doesn't work as flow rates are too low.

 

7 minutes ago, Havkey100 said:

Vaillant Arotherm 5KW

Why oversize the heat pump - bad for SCoP due to cycling. Plus Vaillant don't do cooling out the box and cost way too much

2 minutes ago, Havkey100 said:

almost heat the house with a toaster

So is that 3kW or less? if so heating bey direct electric isn't going to be cheap - 3 x 24 hrs is 72kWh.

 

3 minutes ago, Havkey100 said:

OSO 300L DHW cylinder. Can heat overnight on an agile tariff, economy 7, and recharge daytime from battery, V2X if needed. £2k installed.

Direct cylinder are fine, not sure I would pay the premium for an OSO A rated one.  Slightly oversize for house and heat to 50 degs. Low tariff charge not from battery, use battery for other stuff, not worth the round trip losses when you can charge direct, without going via battery.

 

I would do

Decent no frills MVHR, if you want a Passivhaus certified one, fine, but leave all the optional extras on the shelf

Stand alone small heat pump - A Haier 4kW is £2100 from Wolseley incl VAT (you can reclaim)

Telford Tempest 250L direct cylinder £610

UFH to ground floor, £1k - fully open system, no room stats or actuators, flow direct from ASHP circulation pump

Or direct cylinder and a good quality A2A heat pump for heating and cooling

 

Option 1 - no

Option 2 - no, almost but not quite - £10k for MVHR are you mad?

Options 3 - no

 

[Havkey100]

Thanks, that's why I posted.

 

Agree on HP controls. Have one at the minute and it's unzoned. If I can get that install at less than £5k I'm happy.

 

MHVR seems like it's 4k components, 2k ducting, 4k install, but am working on prices around that. Seems a big racket on it by installers at present.

[JohnMo]

29 minutes ago, Havkey100 said:

MHVR seems like it's 4k components, 2k ducting, 4k install

I installed two units in our house, self installed, well under £2k all in. Ebay can be your friend - if you accept warranty is unlikely.

 

But in the UK you don't need pre heat or enthalpy heat exchangers, you don't need CO2 sensors and really once setup, and running, you don't even need boost very often - we last had boost on (manually) over a year ago, auto boost just isn't activated. Manual boost outside bathrooms and in kitchen, or manual boost in kitchen and humidity activated elsewhere.

[Iceverge]

Single A2A and direct 300l UVC here and pull cord heaters in the bathrooms.

 

Pros. 

 

Very simple to control. Daikin app for a2A and time clock for UVC. 

Spacing heat is cheap.

Cheap to install.

Heating the UVC to 70/75 on immersion allows it all to be done on TOU tarriff. 

Simple to repair. 

 

Cons. 

 

A2A sounds like a fan. 

I can't fix or install the A2A, needs an f-gas man..

DHW uses 10kWh/day. Vulnerable to elec tarriffs.

 

It works just fine, add a few PV panels and I reckon it's the cheapest thing you could have over a lifetime. 

 

Next time I think a monoblock A2W, cheaply bought. A 300l UVC. Simple UFH.

 

Under tile electric mats in the bathrooms.

 

It'd be as cheap to run but be more luxurious than the blown air option I have. 

 

Allow a 70mm blanked duct through the wall and a fuses spur for an A2A for cooling if ever needed. 

 

 

Ps. I'll take suggestions of "simple" UFH. Could a single 100m loop be enough. I don't know enough about these things 

 

 

 

 

Edited Saturday at 17:33 by Iceverge

[SteamyTea]

Re your overheating 1%, is that really an issue, 4 days a year.

Also by how much will it overheat, there is a difference between 24.5°C and 30°C.

Did they take into account the reduced solar gain in the roof area because of the PV, it will take out the worse 20% of the energy at the sunniest times.

Finally, where in the country are you, there is a difference in climate between coastal, rural and urban.

[Havkey100]

On 01/02/2026 at 20:12, SteamyTea said:

Re your overheating 1%, is that really an issue, 4 days a year.

Also by how much will it overheat, there is a difference between 24.5°C and 30°C.

Did they take into account the reduced solar gain in the roof area because of the PV, it will take out the worse 20% of the energy at the sunniest times.

Finally, where in the country are you, there is a difference in climate between coastal, rural and urban.

 

Yes, you're right on the overheating. I am inclined to not commit to anything on that front as I would rather run the system for a year and see how we manage. If we find we have an issue then we would maybe consider split air in one of the main rooms.

 

1% over 25 degrees, I believe peak was 26 degrees.

 

Not sure on the solar gain impact of the PV, I assume it is embedded in the PHPP methodology as the calculation takes input on the array size.

 

Location is Bristol, non coastal.

 

 

 

[Havkey100]

On 31/01/2026 at 17:32, Iceverge said:

Single A2A and direct 300l UVC here and pull cord heaters in the bathrooms.

 

Pros. 

 

Very simple to control. Daikin app for a2A and time clock for UVC. 

Spacing heat is cheap.

Cheap to install.

Heating the UVC to 70/75 on immersion allows it all to be done on TOU tarriff. 

Simple to repair. 

 

Cons. 

 

A2A sounds like a fan. 

I can't fix or install the A2A, needs an f-gas man..

DHW uses 10kWh/day. Vulnerable to elec tarriffs.

 

It works just fine, add a few PV panels and I reckon it's the cheapest thing you could have over a lifetime. 

 

Next time I think a monoblock A2W, cheaply bought. A 300l UVC. Simple UFH.

 

Under tile electric mats in the bathrooms.

 

It'd be as cheap to run but be more luxurious than the blown air option I have. 

 

Allow a 70mm blanked duct through the wall and a fuses spur for an A2A for cooling if ever needed. 

 

 

Ps. I'll take suggestions of "simple" UFH. Could a single 100m loop be enough. I don't know enough about these things 

 

 

 

 

Yes, have been speaking with the heating engineer. Looking at the options on HPs at present. @JohnMo thanks for the input. Direction is low power HP, reasonable cylinder size of 300l or so, wet U/F and as per @Iceverge electric u/f in bathrooms and electric towel rails.

 

On the MHVR I am investigating options as I am very tempted to DIY as I believe I can probably do the lot for 4-5k in a week myself with a spare pair of hands from my wife.

[Nickfromwales]

43 minutes ago, Havkey100 said:

I believe I can probably do the lot for 4-5k

Don't cheap out on the choice of unit  

 

Get everything else you need from begging / stealing / borrowing, but the beating heart should not be bean-counted.

 

Remember this....you will switch the MVHR on, and then NEVER turn it back off again for the rest of your time in the dwelling. If you choose poorly here, it'll not pay dividends from day 1, plus it'll come back and bite you on the ass when it fails early.

 

Also make sure you pay homage to box attenuators before the distribution manifolds, as noise needs addressing. If you buy a compact (cheaper) unit, it'll have inherent problems as 'size matters' here I'm afraid.

 

Look for units which have pre-heaters as an integral feature, so the unit doesn't shut down when incoming air temp comes close to freezing the unit.

 

Brink for me, great price for an excellent unit. Watch for grey imports as they are 'return to source' warranty only, and do not come with on board controls; you need to buy these as a bolt-on, and then all of a sudden it wasn't so cheap after all.