MVHR and cooling

[JohnMo]

5 minutes ago, James Allen said:

We've decided to go fully electric, dimplex for hot water and a few electric radiators

That's different.

[James Allen]

7 minutes ago, Nickfromwales said:

Thanks Nickfromwales what's going on here?? Aircon attached to MVHR?

[Nickfromwales]

33 minutes ago, James Allen said:

Thanks Nickfromwales what's going on here?? Aircon attached to MVHR?

Yup, kinda. Refrigerant gas duct cooler, with external HP, which can heat and cool.

 

If you want cool, it's gotta come from somewhere, so I am more intensively exploring options for these situations atm.

 

There is also the Brink Air Comfort (AHU) but if you don't have a heat pump you're options are very much limited, hence what I showed you in that image.

 

FWIW, I think if you want all electric space heating and cooling, with MVHR, then you'd be far better off going multiple A2A AC throughout.

 

Space heating via electric radiators is pretty poor tbh, unless it's a PH+ spec build, and I think you'll come to regret that imho.

 

I assume there won't be UFH then, so nowhere to implement slab cooling? This is the better option and far less aggressive means of heating and cooling, and imo the most economical operating costs/least impact the occupants.

[JohnMo]

4 minutes ago, Nickfromwales said:

UFH then, so nowhere to implement slab cooling? This is the better option and far less aggressive means of heating and cooling

We had 30 degs yesterday - cool by England standards, but we still pulled 50+kWh of heat out the house by UFH run in cooling mode. Including heating DHW, used all of 11kWh of electric, all provided by the sun FoC via solar PV.

[JohnMo]

1 hour ago, James Allen said:

We've decided to go fully electric, dimplex for hot water and a few electric radiators

What's you heating requirements in kW at -3?

 

If no heating system why not modern storage heaters, this would allow you to run on off peak electricity, to massively reduce running costs. Also if you PV, charge the storage heater with excess PV to make running the heating in shoulder months free or almost.

[Benpointer]

I looked at adding the ComfoPost to our Zehnder MVHR but in the end it seemed like an unnecessary complication and expense since we are going to have UFH anyway and we can run in cooling mode from our Panasonic ASHP.   

 

The need for insulated ducting on the supply side is a big negative imo.

 

It remains to be seen how effectively UFH in cooling mode works for us but it's got to be better than nothing.

[Nickfromwales]

3 minutes ago, Benpointer said:

The need for insulated ducting on the supply side is a big negative imo

The radial stuff comes pre-insulated if you want, so no other issue than the slight uplift in cost.

[SteamyTea]

5 hours ago, JohnMo said:

I would need to increase my MVHR airflow rate by 4 to get 3kW cooling

Which is only doubling the ducting diameter (assuming it is round).

[JohnMo]

1 hour ago, SteamyTea said:

Which is only doubling the ducting diameter (assuming it is round).

And change terminals and possibly the manifolds, and the MVHR unit. So that's easy. All to get 3kW cooling.

 

Then in winter the unit will not turn down enough, so you spend all winter over ventilating.  

 

May as well just do A2A

 

[Marvin]

We find when its really hot the sun is out so we use a mobile air conditioner which has a heat pump. It doesn't do the whole building but then we tend not to be in most of the rooms.  

[Nickfromwales]

1 hour ago, Marvin said:

We find when its really hot the sun is out so we use a mobile air conditioner which has a heat pump. It doesn't do the whole building but then we tend not to be in most of the rooms.  

One that has a whacking great duct that has to go to atmosphere? 180mm on the one in my sons attic room, but it's so cold up there atm that you could keep meat; whilst the rest of us squelch about in the ridiculous heatwave.....

 

No wonder my other kids go and kip up there when the sun finally shows its face.

[flanagaj]

Some interesting responses here, but as mentioned by @jack it seems that the best solution is to keep the systems all separate.  

MVHR for air circulation, but with added heat exchange bypass functionality
ASHP system for heating and domestic HW
Separate air con system for key rooms

 

If you then add PV, why not negate all of the solutions in relation to keeping the heat out, eg venetians, solar glass ...  When it's hot, the sun is out and you can simply run the aircon for free.   When it's time to turn in, it's now dark and the sun is no longer heating up the house and you cooled the bedrooms during the day for free.

I had a quote for external roller shutters for 5 windows and the quote came back ~ 8k, that is comparable in cost to installing an air con system.

The other question I have is around whether I could use the same ASHP external unit to perform both the heating and running the air con.   I appreciate that they probably are unable to heat and cool at the same time, but you could maybe have the HW feature come on at 5am to heat the cyclinder and then after a couple of hours you could put the AC on.

M

[Dave Jones]

On 01/07/2025 at 13:34, Nick Laslett said:

Fancoils, A/C or A2A heat pump. 
 

My build isn’t finished, so MVHR not commissioned. But I did install the ComfoPost fed from UFH circuit to provide 2°C to 3°C degrees of cooling. Can’t wait to get it working. This is probably the worst way to address this issue. All the supply ducts need 13mm insulation, and the plenums, manifold etc. The MVHR moves so little air, that this cooling will not be noticeable. But if I hadn’t done this, I would be kicking myself later on. It would not be going to far to suggest that few on BuildHub would endorse this approach, you are just throwing away money. 
 

In any case, right now with just the UFH the house is still at 21.5°C, with it being 32°C outside today. 
 

My strategy for cooling our house. 
 

1. ASHP for UFH cooling

2. Insulated foundation with UFH

3. Insulated poured concrete 1st floor with UFH. (part of ICF build)   

4. Solar control glass on south facing windows

5. 50cm Roof Over hangs on upstairs south facing windows. (Not nearly enough overhang, possibly pointless?)
6. Comfopost for MVHR

 

Other things to consider:

1. Properly designed window shading

2. External Shutters

3. Fancoils

4. Smart glass

 

 

 

 

 

would have sold you my comfopost for half price! It's utterly crap. Zero noticeable difference with it on or off at any of the terminals.

[Dave Jones]

fancoils > Aircon

 

Too many positives vs split system. Just oversize them  if you have a leaky house and/or poor glazing etc

 

Small ones in our place kept bedrooms at 21/2 24x7 with 34c outside. Cost virtually nothing to run as there is huge excess of solar and night-time is cheap rate in any case. If our tesla Powerwall hadn't gone bang that was enough to spin the heat pump all night.

 

[Nick Laslett]

1 hour ago, Dave Jones said:

 

would have sold you my comfopost for half price! It's utterly crap. Zero noticeable difference with it on or off at any of the terminals.

@Dave Jones, my expectations are so low at this point, that it will be hard to be disappointed! 

[Dave Jones]

1 hour ago, Nick Laslett said:

@Dave Jones, my expectations are so low at this point, that it will be hard to be disappointed! 

 

its only saving grace is in the winter it does have a marginal effect boosting the incoming air.....

[JohnMo]

25 minutes ago, Dave Jones said:

 

its only saving grace is in the winter it does have a marginal effect boosting the incoming air.....

Very true. Passivhaus heat demand was designed around what you could effectively heat the air by, so you didn't need expensive central heating. At passivhaus MVHR flow rates, 0.3 ACH you get approximately 10W, per m² of floor area. So a 200m² house gets approx 2kW of heat via MVHR air heating.

[flanagaj]

So why not simply have a single air to water system for downstairs UFH and HW and then an air to air for air conditioning?