MVHR: 'tain't easy to get yer 'ed round it.....

[ToughButterCup]

This post is a deliberate attempt to expose my thinking on MVHR in the hope that someone will see a flaw in what is planned or be able to make a suggestion which -as has happened before- saves us from unnecessary errors and maybe even saves us a bit of money.

 

What have I learned about MVHR before I got here?

Avoid standing hot water pipe heat loss. Get the house air tight. Forget cat flaps. Visit lots of houses with MVHR (thanks @VIPMan among several others). Don't allow your postie anywhere near the front door. Forget normal keys. Open the windows if you like, and switch the system off if you like. Many people have MVHR and don't use it or worse, haven't been told how to use it. In the passiv or nearly passivhaus sector use PHPP (again thanks @VIPMan) to estimate your heat requirement (19 Watts per square meter per annum) - in our case 5 Watts short of a picnic. Watch for overheating (10% risk in  our case)

 

Getting from numbers on a spreadsheet to buying a system

As I've said elsewhere, here be dragons. Let's be kind and say that MVHR isn't well understood. By sellers sometimes but  by Jo Public in particular. I've been digging round for two or three years now, and frankly, my heart's still in my mouth. But  I have moved on from looking at a house and thinking - oh yeah, lets put a bigger log burner in there... it'll be lovely. Poor life decisions in relation to pensions taught me that. Useful,  relative penury sometimes. Makes me think more, and more deeply.

 

So, I start with a PHPP print out, our plans and send it to a company that says 'We'll do a heat-loss calculation for you'

Thanks very much 

Here is one claculation.

 

 

Hmmm, much to explain there (for me anyway)

Take the Total Column: that's the total in Watts of heat that is used: Living Room uses 518 Watts

If you add a 50% factor for 'safety' you get 776Watts

And (next column) it takes 642 Watts to heat it, ERGO, the MVHR recovers (776-518) Watts

With a Heat pump inline (MVHR and HP) I'd recover (661-518) Watts

With a heat pump and a Duct heater, I'd recover (679-518 ) watts

 

Not sure what the percentages refer to..... (help @SteamyTea

Bottom line, I'll need just over 3 kW of additional heat  ; 3077, 3169, 3261 to heat the house to 21/ 22 ish (bottom line in grey)

 

So, what does this tell you?

There's two of us, a cat and two dogs: cat =10 Watts, dogs 100 Watts between them, @MrsRA 1kW (hot stuff ) me a bit less, so 2 and a bit kW.

Put the hoover on, the oven and  a few lights (no hot water see above) and we get close to just right- or needing to open the windows?

 

Answers on a postcard please...............

 

 

 

 

[SteamyTea]

50 minutes ago, recoveringacademic said:

Not sure what the percentages refer to

Me neither, where are they and what do they refer to?

51 minutes ago, recoveringacademic said:

Take the Total Column: that's the total in Watts of heat that is used: Living Room uses 518 Watts

If you add a 50% factor for 'safety' you get 776Watts

Watts is power, kWh is energy (or heat).

Are you saying that it needs somewhere between 518 and 776W of power to hold the temperature where you want it (the 21/22°C).

That would be 14.43 to 18.62 kWh/day.

[joe90]

Well, I never was one for maths ?, I have not done a PHPP , insulation will be very good for a temperate climate down in Devon , air tightness will be good (I hope), ASHP should be adequate ( cheap from EBay) , MVHR will be oversized ( and cheap from  EBay) so it should run quietly and efficiently, we are having a wood burning stove 1/ because we both love the caveman feel ( and er indoors gets cold) 2/ emergency heating in case of power failure or arctic conditions. ( and we are very rural and surrounded by free wood).

 

Some on here may think me bonkers, some have worked out their heat load etc to one degree, I don't fancy a house that clinical. I have planned a house with lots of thermal mass ( sorry Jeremy!) to iron out the temperature changes.

 

Time will tell.

[ToughButterCup]

12 minutes ago, SteamyTea said:

Are you saying that it needs somewhere between 518 and 776W of power to hold the temperature where you want it (the 21/22°C).

 

No; I think I'm saying (think I'm saying....) it takes 518 Watts to raise the temperature in the living room (in this instance) to 21 degrees when the outside temperature is -1 degrees Celcius and 776 Watts to do the same if you use  a factor of 0.5 f 'reheat value' Quite what 'reheat value' is, I'm not sure.

 

 

[SteamyTea]

Is re-heat the power you would need to raise the temperate up (from some arbitrary temperature), rather than just hold the temperature steady?

Edited December 13, 2016 by SteamyTea

[Guest Alphonsox]

1 hour ago, recoveringacademic said:

 

Not sure what the percentages refer to..... (help @SteamyTea

 

 

The percentages along the bottom are all relative to the "total plus margin" figure. The one under the "total plus margin" column is misplaced and should be under the "total column.

I read the re-heat factor as some measure of MVHR efficiency.

[ToughButterCup]

5 minutes ago, Alphonsox said:

The percentages along the bottom are all relative to the "total plus margin" figure. The one under the "total plus margin" column is misplaced and should be under the "total column.

 

@Alphonsox, demystify the percentages for me, please!

 

30 minutes ago, SteamyTea said:

Is re-heat the power you would need to raise the temperate up (from some arbitrary temperature), rather than just hold the temperature steady?

 

No, my understanding of re-heat is a safety margin (in this case of 50%) rather than making the bloody thing go faster than the speed of sound.

[Guest Alphonsox]

Your total heat loss plus margin is 3740W the percentages are trying to show you where it's going. You are loosing 29.9% (1117W) of this through your fabric, 25%(946W) through ventilation, 2.7% (9W) through your leaky structure and 8.9% (331W) through thermal bridging. The numbers in the right three columns are showing what different systems will provide in terms of supplying the required "total loss plus margin" figure - I guess they have varieties of MVHR with in built in heat pump and/or duct heating. What is less clear is where the numbers in these columns come from.

[SteamyTea]

Maybe I should have looked at the spreadsheet and realised it was about MVHR.

But as I was unable to read it clearly, I did not bother.

[ToughButterCup]

3 hours ago, Alphonsox said:

[...] What is less clear is where the numbers in these columns come from.

 

First of all, thanks very much. Time to go and re-read the numbers. 

And thanks for the prompt to ask '...where (they) come from.

I appreciate it.

Ian

[MikeSharp01]

Ian, just working our MVHR design and wondering if you have closed this one out yet?