SAP calcs are (finally) in! Now searching for ASHP & MVHR in earnest - any recommendations / ones to avoid?

[Great_scot_selfbuild]

Well it's been a long journey getting here!

 

We've finally got the SAP calculations done in detail and although we had the assessor use an assumed 6kW MCS listed ASHP, we haven't committed to any brand and are now looking at them all. Some time back we visited a local self-build with a similar performance and sqm specification and have used that as a handrail for what size HP we should expect to need (they had a 5.5kW).

 

Additionally we need to choose the MVHR system, and confirm what overall system / combination will be right for keeping the house warm in the right places. We were advised that UFH downstairs and MVHR around the rest of the house (for a house with passive-levels of insulation) would suffice. Now we're getting close to finalising this though, we're questioning the detail - MVHR seems to extract heat (& humidity) from bathrooms, but if we only have a towel radiator in there will it be enough?

 

Timber frame build summary:

UFH downstairs

MVHR to move heat around the rest of the house

Electric towel rails in bathrooms (1 x family, 1 x en-suite upstairs)

Walls and roof U=0.11

Floor U = 0.14

Target ACH = 0.6

Downstairs ceiling height = 2.9m

Upstairs is into the roof eaves, with a flattened top at c.4m

Area = 260 sqm total

 

As you'll notice form the summary, the design has a large volume, hence my focus on understanding how an MVHR and heating system is sized. Welcome your thoughts & advice. Very early on we got some quotes from companies specialising in renewables and 'whole system' design - and we quickly scared by the VERY large quotes. Felt like they just saw us as a self-builder money pot, which is a long way from the reality!

[ProDave]

You are clearly building a well insulated house.  Have you tried using Jeremy's heat loss spreadsheet to work out your heating demand?  Many of us have found it clearer and more accurate than SAP calculations.

 

Regarding no heating upstairs, I am one of several here that took that bold leap and am happy with the result.  Heat transfer throughout the house is not really via the mvhr but more just convection through the house etc.  It works for us and many others provided you are happy with a bedroom temperature a little lower than downstairs in winter.

[JohnMo]

MVHR is ventilation, it is rubbish at moving heat around. It is needed in all rooms. Read what is said above. Ventilation is basically designed around building regs. But pretty to design and install yourself if needed.

 

UFH downstairs is fine downstairs, but loops need to be design to provide full house heat output. But again not difficult to design and specify if needed.  

 

ASHP when you get a grant can come with a big billy bonus to the installer, which they rip you off for. As many on hee I just didn't bother with the grant and just bought a heat pump from eBay for many thousands off list price.

[Great_scot_selfbuild]

1 hour ago, ProDave said:

You are clearly building a well insulated house.  Have you tried using Jeremy's heat loss spreadsheet to work out your heating demand?  Many of us have found it clearer and more accurate than SAP calculations.

 

Regarding no heating upstairs, I am one of several here that took that bold leap and am happy with the result.  Heat transfer throughout the house is not really via the mvhr but more just convection through the house etc.  It works for us and many others provided you are happy with a bedroom temperature a little lower than downstairs in winter.

@ProDave thanks - good to know it's a valid approach (we've seen one, but encouraging to hear directly from others. Not seen Jeremey's heat loss spreadsheet - will do a search for it; the SAP output is certainly not signed up to the 'plain English campaign'!

[Great_scot_selfbuild]

1 hour ago, JohnMo said:

MVHR is ventilation, it is rubbish at moving heat around. It is needed in all rooms. Read what is said above. Ventilation is basically designed around building regs. But pretty to design and install yourself if needed.

 

UFH downstairs is fine downstairs, but loops need to be design to provide full house heat output. But again not difficult to design and specify if needed.  

 

ASHP when you get a grant can come with a big billy bonus to the installer, which they rip you off for. As many on hee I just didn't bother with the grant and just bought a heat pump from eBay for many thousands off list price.

Useful nudge RE: alternative approach to the ASHP grant. It certainly comes across as less of a grant, more of a 'we'll charge you £7500 more and try to convince you you're getting a saving'.

[ProDave]

8 hours ago, Great_scot_selfbuild said:

@ProDave thanks - good to know it's a valid approach (we've seen one, but encouraging to hear directly from others. Not seen Jeremey's heat loss spreadsheet - will do a search for it; the SAP output is certainly not signed up to the 'plain English campaign'!

Here is a link to the heat loss spreadsheet

 

 

[JamesPa]

The important thing with ashp system design is KISS.  You just need a correctly sized (not grossly oversized) ashp connected directly to the emitters with a diverter valve to the UVC, maybe a volumiser, (almost) never a buffer.  No external controls, not even a thermostat.  No glycol, use anti freeze valves. Use the ashp controller to set the weather comp curve as low as possible and leave ticking over 24*7 (unless you plan to batch charge your slab to use it like a storage heater which is a whole different design). 

 

Choice of ashp itself not so important, but some have better controls, quieter, less ugly etc than others.  Many of the ones with poor (personally I would say inadequate) controlled can use homely, an external physical controller, or havenwise, a virtual controller service) as an overlay.  Under no circumstances fit any smart thermostats etc other than these (or adia) which are designed specifically for heat pumps.

 

Reject any installer who is going to install a buffer, phe or llh between ashp and emitters, or external controls other than those mentioned above.  Be suspicious of any installer who specified a 'pre plumbed cylinder'.  This is often an indication that they are employing rookie plumbers and some pre plumbed cylinders come with built in llh.

 

Post back for explanations of the above if needed.

Edited Thursday at 08:09 by JamesPa

[JohnMo]

1 minute ago, JamesPa said:

maybe a volunteer,

Or even a volumiser

[Great_scot_selfbuild]

10 minutes ago, JamesPa said:

The important thing with ashp system design is KISS.  You just need a correctly sized (not grossly oversized) ashp connected directly to the emitters with a diverter valve to the UVC, maybe a volumiser, (almost) never a buffer.  No external controls, not even a thermostat.  Use the ashp controller to set the weather comp curve as low as possible and leave ticking over 24*7 (unless you plan to batch charge your slab to use it like a storage heater which is a whole different design). 

 

Choice of ashp itself not so important, but some have better controls, quieter, less ugly etc than others.  Many of the ones with poor (personally I would say inadequate) controlled can use homely, an external physical controller, or havenwise, a virtual controller service) as an overlay.  Under no circumstances fit any smart thermostats etc other than these (or adia) which are designed specifically for heat pumps.

 

Reject any installer who is going to install a buffer, phe or llh between ashp and emitters, or external controls other than those mentioned above.  Be suspicious of any installer who specified a 'pre plumbed cylinder'.  This is often an indication that they are employing rookie plumbers and some pre plumbed cylinders come with built in llh.

@JamesPa very useful advice - TVM 😁