MVHR is Largely Bogus

[IanR]

1 hour ago, LionessHeart said:

We're seriously looking at NOT installing an MVHR - not only because of the cost, but to [a] minimise energy wastage 

 

Centralised mechanical ventilation is consequence of minimising energy losses, and is only required if you are setting a high energy performance target for your conversion.

 

One significant factor with regards energy loss is uncontrolled ventilation, where the fabric of the building is leaking the warm air that the heating system has been heating up. If you take steps to reduce those energy losses, through the uncontrolled ventilation, then you get to a point, around 3m³/m².h@50Pa on an infiltration test, that you will no longer meet building regs for natural ventilation. At this point the Building Control officer will look for a whole house ventilation system that then brings the property back to meet the minimum ventilation requirements.

If you've plugged up all the leaky holes and gaps to stop the energy losses through uncontrolled ventilation, then an MVHR system allows you to control the ventilation and recover the heat from the internal air in the property, but if you're "only" achieving an infiltration rate of say 2.5 or 3m³/m².h@50Pa then you have to question whether the MVHR will be cost effective.

If you are achieving in the region of 1m³/m².h@50Pa infiltration rate, then MVHR is very likely to be cost effective. But, if you are not setting your energy performance targets at that level it may be better to hit 3m³/m².h@50Pa infiltration rate, go without MVHR, have controllable trickle vents on your windows and use extracts in the wet rooms.

Edited April 12, 2023 by IanR

[Adsibob]

1 hour ago, PeterW said:

you can’t… 

 

you have PV - so stick in MVHR, when you’re not there you’ll be using the significant generation capacity to offload the running costs (we are talking 80w/hr) and then have the comfort of ventilation and energy saving.

Plus 1.

 

 Keep in mind that when the house is unoccupied you can set the MVHR to holiday mode, and it will consume even less electricity.

[SteamyTea]

  

2 hours ago, PeterW said:

we are talking 80w/hr

?

[jfb]

On 23/03/2023 at 10:29, Mike said:

However - leaving aside the (correct) argument that EPCs don't always match reality - without MVHR I'd have been in the middle of band D; with it I'll be in the middle of band C, with no easy upgrade to B.

Interesting - I spoke to an epc producer and they said that technically MVHR counted the epc down (I think about 5 points) which surprised me. 
this is offset in part by the advantage in having a better air tightness test. But he said the reason was the energy gained by heat exchange was generally less than the energy to run the MVHR. Maybe model of MVHR can affect this as well.

[Marvin]

21 minutes ago, jfb said:

Interesting - I spoke to an epc producer and they said that technically MVHR counted the epc down (I think about 5 points) which surprised me. 
this is offset in part by the advantage in having a better air tightness test. But he said the reason was the energy gained by heat exchange was generally less than the energy to run the MVHR. Maybe model of MVHR can affect this as well.

I'm quite surprised by this. I will have to check my calculations. I had calculated a saving in winter for heating and a saving in summer for cooling....

[Adsibob]

16 minutes ago, Marvin said:

I had calculated a saving in winter for heating

If the efficiency of the machine is good enough, then yes, there should be a saving.

 

16 minutes ago, Marvin said:

 

and a saving in summer for cooling....

This i’m less sure about. You won’t get more than 0.5C or 1C cooling at most from MVHR.

Edited April 12, 2023 by Adsibob

[Sparrowhawk]

13 hours ago, LionessHeart said:

- But, are reluctant to bore holes into triple glazed windows for trickle vents

 

My question is, how can we achieve the 1 ach if we're not using mechanically driven ventilation? Should we consider a Positive Input ventilation (PIV) and fans (for the hot days of the summer)?

(emphasis added)

 

All replies so far have overlooked this key line. Under current building regs, if you want to avoid trickle vents what alternatives do you have to MVHR with vents in each room? I don't think PIV covers it, being a central input to the house?

[Marvin]

9 hours ago, Adsibob said:

If the efficiency of the machine is good enough, then yes, there should be a saving.

 

This i’m less sure about. You won’t get more than 0.5C or 1C cooling at most from MVHR.

Interesting. I will have to upload my calculations to find where I have gone wrong.

 

M

[MJNewton]

11 hours ago, jfb said:

Interesting - I spoke to an epc producer and they said that technically MVHR counted the epc down (I think about 5 points) which surprised me. 
this is offset in part by the advantage in having a better air tightness test. But he said the reason was the energy gained by heat exchange was generally less than the energy to run the MVHR. Maybe model of MVHR can affect this as well.

 

According to this page it's not quite as simple as running costs being assumed to be greater than energy savings, but - if I've understood what they're saying correctly - that the SAP calculations assume different infiltration losses with and without MVHR. 

Edited April 13, 2023 by MJNewton

[Gone West]

7 hours ago, Sparrowhawk said:

I don't think PIV covers it, being a central input to the house?

Does it have to be though? Why not fit the PIV output into a manifold and duct to required rooms.

[LionessHeart]

22 hours ago, Gone West said:

Does it have to be though? Why not fit the PIV output into a manifold and duct to required rooms.

@Gone West - can you expand on this thought please? 

 

Would a plumber be able to specify and design such a ventilation system... ? Or, is it a specialist arrangement? Thanks.

[Onoff]

13 minutes ago, LionessHeart said:

Would a plumber be able to specify and design such a ventilation system

 

Unlikely:

 

 

[JohnMo]

On 13/04/2023 at 09:00, Sparrowhawk said:

 

All replies so far have overlooked this key line. Under current building regs, if you want to avoid trickle vents what alternatives do you have to MVHR with vents in each room? I don't think PIV covers it, being a central input to the house?

Trickle vents in the wall instead of window trickles? MEV, dMEV all require vents in the rooms.  You don't need trickle vents in the rooms with the vent fan, but in this case you need vents in the dry room.  PIV would require vents in all rooms. You can install humidity activated trickle vents.

[Gone West]

29 minutes ago, LionessHeart said:

can you expand on this thought please? 

We now live in an old solid stone walled bungalow which we are going to improve, after having lived in a Passivhaus which we designed and built ourselves. Having had MVHR in the PH we don't want trickle vents here, but we are going to need some form of ventilation. My thoughts, for our less than airtight house, is to fit a PIV unit in the cold ventilated loft with the output connected to a manifold which would supply air to the living quarters. We would have outlet vents in the kitchen and bathrooms to evacuate stale air. We would fit a heater to the manifold to ensure that the supply air was the same temperature as inside the house. This would obviously use more energy than a MVHR system but would be offset, to a degree, by the temperature increase in the loft during Spring and Autumn. The MVHR system we had in the PH had a small ASHP built into it which warmed the supply air so there was no, colder than room temperature air, going into the house as there is with a conventional MVHR system. We are also deciding on whether to use A2A ASHP heating or an oil fired wet heating system. This will also impact on how we design the ventilation system.

[Mike]

On 12/04/2023 at 23:25, jfb said:

I spoke to an epc producer and they said that technically MVHR counted the epc down (I think about 5 points) which surprised me. 
this is offset in part by the advantage in having a better air tightness test.

As noted in my post, this particular apartment is in France. What I'd forgotten is that UK EPCs are still based on a hypothetical predicted fuel cost; in France they are based on primary energy use (kWh/m²/year) & CO2 emissions (kg CO2/m²/year) - the lowest of the two scores determines the rating.

 

I guess the UK may eventually catch up.

 

On 12/04/2023 at 23:25, jfb said:

But he said the reason was the energy gained by heat exchange was generally less than the energy to run the MVHR. Maybe model of MVHR can affect this as well.

My French EPC is calculated on a saving of 2,000kWh when using MVHR - too round to be unit-specific.

 

However, the free Fiabishop.com MVHR calculation spreadsheet (in French only) does calculate based on specific models. It forecasts that my Zehnder unit will use approximately 148 kWh to run for the year, and cut space heating by 1,687 kWh, and overall saving of 1,539 kWh.

 

Assuming gas heating, at the current UK average rates of 10.3p/kWh for gas, and 34p/kWh for electricity, that's a saving of £124/year (or £523 with electric space heating).

 

 

Edited April 15, 2023 by Mike

[IanR]

28 minutes ago, Mike said:

My French EPC is calculated on a saving of 2,000kWh when using MVHR - too round to be unit-specific.

 

However, the free Fiabishop.com MVHR calculation spreadsheet (in French only) does calculate based on specific models. It forecasts that my Zehnder unit will use approximately 148 kWh to run for the year, and cut space heating by 1,687 kWh, and overall saving of 1,539 kWh.

 

Do either of the calculations consider the MVHR heat recover gains within the whole house ventilation losses (incl. natural ventilation)? The calculation would need the infiltration rate for the house, in order to do so, and an assumption of the minimum air changes per hour (controlled and uncontrolled) required for "healthy" internal air.

 

As an example, a property that has a UK Building Regs infiltration rate of 8m³/m².h@50Pa has sufficient natural ventilation to not require an MVHR to achieve healthy internal air, so the additional ventilation that an MVHR would bring to this property is ultimately an energy loss, even though it may be recovering 80% - 90% of the energy within that ventilation at the cost of energy to run the MVHR.

What seems to be missing in the UK is a simple way of calculating the cost-benefit of MVHR within both the property's controlled and uncontrolled ventilation.

[Mike]

4 hours ago, IanR said:

Do either of the calculations consider the MVHR heat recover gains within the whole house ventilation losses (incl. natural ventilation)? The calculation would need the infiltration rate for the house, in order to do so, and an assumption of the minimum air changes per hour (controlled and uncontrolled) required for "healthy" internal air.

I don't have the EPC software, and a air-change value isn't mentioned in the report, so can't elaborate on what's included in that.

 

However the Fiabishop spreadsheet gives air change options for 4 building standards (5, 3, 1.5, or 0.6 air changes / hour), or a custom figure, in addition to the controlled ventilation of the MVHR system. It also allows you to compare 2 different MVHR units*, or one MVHR unit against regular mechanical ventilation (i.e. kitchen & bathroom extractors). It also allows you to input the m² area, m³ volume, location, expected occupancy, lengths and insulation of the main air supply and extract ducts, whether or not you're using hygrostatic controls, whether or not you intend turning it off in summer (and open and close the windows instead), takes into account the local climate, wind sheltering, orientation, the use (or not) of earth warming tubes (puits Canadian / Provençal, with or without pre-heaters), and a few other factors.

 

The figures I gave above are for 0.6ACH, comparing the Zehender unit with regular mechanical  ventilation.

 

*The spreadsheet is pre-loaded with data for 32 MVHR units from 12 manufacturers, with the option to add more entries yourself. So, comparing the Zehnder unit to a Brink Renovent Exellent 300, it forecasts that my energy bills would be €8/year higher.

 

Edited April 15, 2023 by Mike