Individual room heat loss calcs and ACH

[WGL]

I'm about to embark on a full upgrade and extension of our 1960s house and I'm looking to carry out fairly accurate heat loss calcs. I want to know myself what size system I need. I am an experienced builder and able to do u value calcs and I feel I understand the pitfalls of building fabric issues well.

 

Having looked at the formulae, ACH rate seems to make a huge difference to the result and is the main unknown variable. I'm aiming to get the place up to what I would call a 'decent quality new build' level of airtightness.

 

The end result will be EWI to existing (has blown EPS in cavity also), fully VCL and taped membrane timber frame with rendered blockwoork skin to extension, new double glazing and a warm roof loft construction. Existing suspended timber floor is the week spot, but going to rip up boards and insulate. We also have an under 5kw log burner which we're hoping to use a lot less.

 

Roughly what ACH value should I be looking at please? One for natural ventilation and another for MVHR would be great. 

 

Apologies in advance, I'm sure this info is on the forum somewhere but I'm struggling to find it.

 

 

[JohnMo]

MVHR will be around 0.5ACH, with an efficiency for heat recovery, of say 80 to 90%.

 

You sound like you will be better than 3 m3/m2 @ 50Pa airtightness, so you will be needing mechanical ventilation of some sort, so the next is MEV or dMEV, so that would be same ACH, but 0% heat recovery. MEV or dMEV would require trickle vents also, within wall or windows.

[WGL]

Thanks for that. Sorry about my wording, I meant some sort of more traditional mechanical ventilation with trickle vents when I stated natural, you're right to point that out though. So how does the heat recovery (or lack of) get incorporated into my heat loss calcs? 

 

I'm calculating the heat loss to ventilation as;

 

m3 of room  X  0.33  X  ACH  X  internal to external DT  =  Watts of ventilation heat loss

 

How do I factor in MVHR vs MEV into this calculation please?

[JohnMo]

So in the ACH part of the equation for MEV, use 0.5. For MVHR use (0.5 x 0.2) this will represent you getting 80% efficiency in the heat recovery or use 0.1 if you think you will get 90% efficiency. That should be close enough.

[WGL]

Thank you for that, makes complete sense now. Much appreciated