Does MVHR affect heat loss & calculations much?

[ruggers]

I've been using some heat loss calculation software that many heat engineers use & I've accurately included all of my data & exact U-values. 

I've tried altering certain room temperatures from 21°C to 19C to see how it affects things room by room and the total. 

 

The software defaults the ACH per hour but I don't see any option to include the MVHR unit I'm choosing to use & include its efficiency. Zehnder Q350.

Looking at an MMVHR estimate I received, it states some nominal air changes 1/h. I input one of them to a small second bathroom, overriding the default and i had to increase the ACH from 2 to 3.47 if this is correct? to see how it would affect the heat loss. It increased it much more than expected, so I'm now wondering how much this is going to negatively affect my overall loss. The standard heat loss for 235m2 is 6.4kw with 10% extra added on due to exposed area setting.

 

How should MVHR be included into the ACH section once I have a detailed design? We know with UFH/radiators and weather compensation that its very important to get accurate heat losses to size the emitters. I was also hoping for a good heat loss result. 

[MikeSharp01]

Which software are you using? MVHR should improve your heatloss as it warms the incoming air from the heat embodied in the outgoing air.

[Andehh]

MVHR are not 100% efficient so do reduce things, they are just "less bad" then trickle vents and extractor fans.

[nod]

I was surprised that  our sap guy said MVR had a detrimental effect on the sap He said while it’s cheap to run and efficient So is opening a window

[joth]

5 minutes ago, nod said:

I was surprised that  our sap guy said MVR had a detrimental effect on the sap He said while it’s cheap to run and efficient So is opening a window

Silly SAP. It's very expensive and inefficient to open the windows in winter, as doing so lets all your expensively generated heat out the building. (Likewise in a summer heatwave opening windows allows all the expensively cooled air out).

 

This was the first thing that got me looking into MVHR: I hate the wastage of open windows in winter, my wife hates the humidity and damp of closed windows. 

 

[joth]

6 hours ago, ruggers said:

i had to increase the ACH from 2 to 3.47 if this is correct?

Can you explain why you had to do this?

It doesn't sound right. 

 

[ProDave]

6 hours ago, ruggers said:

The standard heat loss for 235m2 is 6.4kw with 10% extra added on due to exposed area setting.

There is no such thing as a standard heat loss per square metre.  It will be individual to each building depending on insulation levels and air tightness.

 

Once you get to a certain level of insulation, then ventilation heat loss starts to dominate, and that is really where MVHR works best.

 

At this stage in a new build you should be striving for maximum insulation, detailed well, and very good air tightness, and then mvhr will get you a low energy house.

[Marvin]

1 hour ago, nod said:

I was surprised that  our sap guy said MVR had a detrimental effect on the sap He said while it’s cheap to run and efficient So is opening a window

What a crock of ......t!

 

I don't know where to begin! Sap man may be right that SAP calcs are about as good as EPC calculations and assumptions (don't get me started about EPC's) but as efficient as opening a window? That's utter bollix. 

 

Hi @ruggers

 

In my world, when you open a window to let air in you let other air out, Our home tends not to expand like a balloon. In doing so the temperature of the air going out is replaced with the temperature of the air coming in. So when we open a window on a very cold winter's day, the outside air at, say, 5C replaces the warm air in our home.

 

However on the same day the air supply from outside via our MVHR is usually around 20C.

 

This sounds like a small thing but when the maths is done over 24 hours and 365 day a year its a lot!

 

If the outside temperature at our home is -1C we will use about half a kWh replacing the temprature of the air using the MVHR in an hour.  If instead we opened windows and replaced the air in the house at the same rate as the MVHR we would use about 2.5kWh an hour. Our MVHR uses 25 to 33 Watts an hour.

 

I am completely satisfied that using an MVHR helps to keep our home at a more stable temperature (resisting both hot and cold outside temperatures) and saves us energy and saves us money. Its the gift that keeps on giving... 

 

Good luck with your project

 

M 

[ADLIan]

A badly designed MVHR system can have a negative effect on the SAP outputs - if the cost (money or energy) of running the fan is greater than the energy saved.

[SteamyTea]

Is part of the problem that a house leaks air when there is no forced ventilation, then, adding forced ventilation just adds to the losses, even if it reclaims an optimistic 80% of the energy in the expelled air.

 

So let us assume that at 5 ACH and a delta T of 15°C.  

1 tonne of air has 15 MJ of energy, that is 4.2 kWh, every hour.

Add on another 0.5 ACH for forced ventilation, that is 4.62 kWh every hour.

Even if an MVHR system can reclaim 80%, that is 4.284 kWh every hour.

 

Change the natural looses ACH figure to 1, and that becomes 0.84 kWh every hour, add on the 0.084 kWh for the MVHR, then 0.924, call it 1 kWh, or 24 kWh a day.

 

Get the house airtight, more airtight than you can imagine or you are, at current gas rates, throwing away 15 quid a day.

[ruggers]

A good few replies from a late night post. I'll answer below. I believe MVHR to be good and having to have an air tightness test on new builds means MVHR or trickle vents, ones cheap, one isn't, but i hate trickle vents with a passion. So going down the MVHR route, I'm fitting a quality unit that can recover up to 94%. I have a good draft design that i will self install and have narrowed it down to two really good companies to complete the design, install & commission after speaking to a few of them. (Will select once overall quotes are in for all trades). So I've decided to make sure I apply a high level of air tightness & insulation detail myself after researching a lot on here & speaking to people.

5 hours ago, MikeSharp01 said:

Which software are you using? MVHR should improve your heatloss as it warms the incoming air from the heat embodied in the outgoing air.

Heat engineer app followed by sending the report to the desk top version to fine tune everything. It will warm the incoming but would it not also be dumping some of the warm air outdoors continuously, 20°C extracted at 90% recovery would mean the supply air is 18°C? I sometime see quotes saying MVHR can lower your heating bills by 20%. In winter I probably wouldn't open my windows & never use trickle vents so it would be less than MVHR. I'd crack them open on catch in bedrooms for an hour or so in the morning or when cooking.

2 hours ago, joth said:

Can you explain why you had to do this?

It doesn't sound right. 

 

It's probably not correct what i done, i was just trying it based on the attached screen shot, the heat loss software defaults all ACH to 0.5, 1.5 or 2 for various room types. 

2 hours ago, ProDave said:

There is no such thing as a standard heat loss per square metre.  It will be individual to each building depending on insulation levels and air tightness.

 

Once you get to a certain level of insulation, then ventilation heat loss starts to dominate, and that is really where MVHR works best.

 

At this stage in a new build you should be striving for maximum insulation, detailed well, and very good air tightness, and then mvhr will get you a low energy house.

Bad choice of words from me sorry, shouldn't have said standard. My calculations are now completed & showing as whole house loss of 6.11Kw based on all of my materials to be used with correct U-Values. This is 10% more than it would be because I've ticked the exposed area option being 125m elevated, 2 miles from the West coast in a windy area.

What I'm surprised at, is by changing my 0.17W/m2K cavity wall insulation to a different U-Value to simulate how adding some internal insulated plasterboard at different thicknesses would improve things, it made hardly any difference at all and they cost 3x more than a standard plasterboard. Adding 37.5mm to the inner skin gives a value of 0.14W/m2K, adding 52.5mm gives a value of 0.13W/m2K.

My 6.11Kw house heat loss only improves to 5.97kw and 5.92. It seems a waste of money adding the insulated board to the inner walls unless I've done something wrong?

[ruggers]

I think MVHR is a good idea, and I'm happy with the heat loss calculation although would like to improve it further. What I don't know how to do, is get the most accurate results from the heat loss software when installing MVHR as this could/will affect the air changes beyond their defaults. This can through the calculation out massively & I could end up under or over sizing my heat emitters which defeats the object. I'm aiming to get them as accurate as i can, so when fitted with weather comp the house is sized perfectly & feels good. 

 

Question is, what do i set the ach to in the software to reflect the MVHR install? 

[SteamyTea]

1 minute ago, ruggers said:

Question is, what do i set the ach to in the software to reflect the MVHR install? 

Try 20% of the value.

 

[ruggers]

1 minute ago, SteamyTea said:

Try 20% of the value.

 

Of each room value? Why that figure? I'm hoping it will be answered at a later date once i pay for a detailed design with the MVHR company who can advise me what to add but MVHR won't know much about heat loss calculations or care so I need to make sure I can input the correct data to get my results accurate. 

As mentioned, a whole house of adding insulated plasterboard made very little difference but changing some parallel rooms lowest temperatures from 18°C to 20°C to match the 21°C design temperature shaved off 0.4kw total.

[SteamyTea]

10 minutes ago, ruggers said:

Of each room value? Why that figure

If the MVHR unit is 80% efficient (optimistic in reality) that means 20% is not recovered.

Room by room or whole house probably does not make much difference, but the Building Regs specify flow rates. Just convert from litres per second to kWh per day.

[joth]

33 minutes ago, ruggers said:

It's probably not correct what i done, i was just trying it based on the attached screen shot, the heat loss software defaults all ACH to 0.5, 1.5 or 2 for various room types. 

 

(I don't see the screen shot?)

 

This sounds very similar to the standard MCS heat loss calculator template. We used the "Cat C" columns below, which assume 0.5 of 1.5 ach with a special note about extract ventilation. The issue with their spreadsheet is less the specific ACH used but the fact they have no calculation for heat recovery of any sort.  The net result was it overestimated our heatlosses by 100% vs PHPP.

 

[JohnMo]

Not sure why you are not just using the heat loss calculations in the boffins corner, it lets you set the ACH and MVHR efficiency.  It free easy to use and pretty good representation of what to expect from your house.

[JohnMo]

In a close to airtight house an exposed windy location, shouldn't have any undue effect on the heat loss calculation as the wind cannot get into the house.

 

Unlike a humans, objects such as house do not suffer wind chill.  The air temperature is what it is.

[joe90]

That sounds counter intuitive to me! I have a west facing wall and believe the wind chill will lower the temperature of that wall which will “suck” more heat through the insulation , hope to be proved wrong!

[Marvin]

36 minutes ago, JohnMo said:

In a close to airtight house an exposed windy location, shouldn't have any undue effect on the heat loss calculation as the wind cannot get into the house.

 

Unlike a humans, objects such as house do not suffer wind chill.  The air temperature is what it is.

Except for the wind chill effect on the external temperature of the building... 

[Marvin]

4 hours ago, ADLIan said:

A badly designed MVHR system can have a negative effect on the SAP outputs - if the cost (money or energy) of running the fan is greater than the energy saved.

 

True. 

 

Using the data over the year 2020 we had an average temperature of 12.1 centergrade here.

The average temperature in the home was 21C (19 for about 8 hours over night and 22 the rest of the day.

We would require about 1157 Watts to heat up the same amount of air taken directly from outside, as apposed to, the 231 Watts of heating required and 32 Watts to run the MVHR. So a saving of about 926 Watts an hour. 

 

926 Watts by 24 hours by 365 days is 8111 kWh a year! 

 

Note: In this example both ways of calculating do not take into account heat produced in the building, from people, heat all electrical appliances or solar gain. Estimating these brings the difference between air inside to air outside to only about 4.1C and the saving down to about 2000kWh a year for us.

 

FYI our home is about 105m2 on plan!

 

In my humble opinion, you have to have some really really poor MVHR design for the running of the MVHR to cost more than the saving on heating. 

[SteamyTea]

46 minutes ago, JohnMo said:

Unlike a humans, objects such as house do not suffer wind chill. 

There are evaporation losses from wet walls, these can be quite high, especially at night.

The latent heat of vaporisation is 2257 kJ.kg^-1. 

[joe90]

5 minutes ago, SteamyTea said:

There are evaporation losses from wet walls, these can be quite high, especially at night.

The latent heat of vaporisation is 2257 kJ.kg^-1. 

Yes I had to “treat” my west wall with waterproofer as it was sodden last winter (ha, when we had rain!). So hopefully that will cut down my evaporation losses?

[ruggers]

@joth This is my concern, that the software only detects a change of defaults resulting in more air exchanges per hour so it estimates that i require more heat than needed because it doesn't factor in the heat recovery percentage. I will look into this in more detail because a tutorial video i recall a mention of 50% default for MVHR but an option to increase the %.  Is PHPP the passive house detailed calculation, do you have a passive house? I've included some attachments, the MVHR report is just one of the 3 companies who provided me with some details & a quote. 

[ruggers]

2 hours ago, SteamyTea said:

If the MVHR unit is 80% efficient (optimistic in reality) that means 20% is not recovered.

Room by room or whole house probably does not make much difference, but the Building Regs specify flow rates. Just convert from litres per second to kWh per day.

 

1 hour ago, JohnMo said:

Not sure why you are not just using the heat loss calculations in the boffins corner, it lets you set the ACH and MVHR efficiency.  It free easy to use and pretty good representation of what to expect from your house.

I will take a look at them John, always good to compare because sizing emitters can only be done once with UFH. I just imagined the sheet would require me doing a lot of long hand calculations with formulas which isn't my forte. Some experienced heat engineers I've been speaking to had said about using the heat eng. software as it was good, although they can do it via others means it would provide me with good results and use it themselves. I'm not sure if they have considered MVHR before though. My concern is how the software correlates with heat losses vs ach & heat recovery.

1 hour ago, JohnMo said:

In a close to airtight house an exposed windy location, shouldn't have any undue effect on the heat loss calculation as the wind cannot get into the house.

 

1 hour ago, joe90 said:

That sounds counter intuitive to me! I have a west facing wall and believe the wind chill will lower the temperature of that wall which will “suck” more heat through the insulation , hope to be proved wrong!

I think I'm with Joe on this one to a degree (No pun intended). The chilled wind could act as a wicking effect at drawing the heat away from the dwelling, especially if wet mortar between face bricks, I may be wrong because the inner and outer skins are independent, but we do know that poorly fitted PIR in a cavity can have the same effect as a radiator in reverse. Air passes between the gaps from poor install and draws heat away from the inner cavity wall. Effectively no wind should be able to get in other than tiny weep hole vents but its possibly just from the difference of temperatures.