Can you hack a heat-recover system if you have PIV?

[Garald]

As many of you know, I'm now tinkering with improvements after a major renovation. My place is the greater part of a house from the early 1930s. 

 

I have PIV (positive-input ventilation), with pre-heating. I am not finding the ventilation outlets (or rather, I've found two outlets, outside the bathroom and the kitchen, but they look a bit abandoned; the contractor is being his usual laconic self). Perhaps the idea (not mine) was to just let air leave through imperfections in the air envelope - which of course I will be minimizing as I make things more airtight. (Then there's also the chimneys, which I haven't sealed - but they are in the first floor; shouldn't there be outlets in the (inhabited) attic as well?)

 

And then there's what is really a separate question: if I succeed in making things fairly airtight, would it not make sense to install extractors with heat-recovery, with heat being "fed" into the PIV? This makes sense at a physical level, but I'm not sure it's a thing that is done at all at the practical level. (Not to mention that all the walls have already been painted and I would hesitate to rip everything apart.)

[Savage87]

In my opinion, you should have been tinkering with ideas before a major renovation. You most likely have extractor grills for the bathroom and kitchen and possibly some in line fans.

 

Answer to your first question: chimneys need to vent at the top and bottom, not necessarily in the middle.

If you success in making the house more airtight you will need to think of a solution to introduce more ventilation(with heat recovery) either decentralised or centralised.

 

Good luck!

 

[Garald]

On 15/12/2023 at 23:57, Savage87 said:

In my opinion, you should have been tinkering with ideas before a major renovation. You most likely have extractor grills for the bathroom and kitchen and possibly some in line fans.

 

I agree. The long story is that this was a major renovation at the beginning of which I had pretty terrible pseudo-professional advice. Thanks partly to the help of people in this forum (who helped me see just how terrible that advice was...), I've managed to get a pretty good overall outcome (according to official diagnostics). Now I'm wondering what can be improved.

 

But yes, the contractor got back to me: there are vents in the kitchen, bathroom and a couple of other places, as well as mechanical ventilation in the tiny laundry room.

(The vents were preexisting, as simple outlets; of course more air goes out of them now because of the PIV.)

 

He recommends not locking the skylights in the attic - that's presumably a place where there aren't enough outlets (if any).

 

One change I've thought about for the medium run is to install a thermodynamic water heater in the attic. Surely one should be able to have a switch so that:

- in summer (when the attic tends to get especially hot) the air cooled by the heater can be mainly recirculated to the attic,

- in winter, the air cooled by the heater can be extracted to the outside?

This would be in addition to my current water-heater (part of the main heat pump). Does this sound like a good idea?

 

 

[Ferdinand]

Do you actually need the PIV?

 

In answer to your first question, my view is that Yes, you can create a sort-of-MVHR. You will not get the same performance, but you get improved performance in so far as you got towards matching the conditions in a modern well sealed, well insulated house. And over a trad house I reckon one can get more than half way there.

 

It has been my standard approach across renovating properties, dated 1850-1970+, in the last decade.

 

My current habit is to install a PIV, which is set at the appropriate level (so it is background not blowing a hoolie) at the top, and put in a background HR extract fan at the other end (eg utility room downstairs). I have been using Vent-Axia Lo Carbon Tempra units since about 2014, which state an efficiency of 78% for their heat exchanger.

i call it poor man's MVHR.

 

My reno scheme is now typically good double glazing and doors, under floor insulation, backdraft shutters on extract fans, proper sealing of penetrations, air sealed loft hatch, and all the other detailed stuff I can get at. Plus good -> very good insulation in other respects - aiming for say a good C in EPC terms.

 

To help internal air circulation you may need to do something like trimming the bottom of doors.

 

TBH if you have open chimneys to the rooms then that is likely significantly or entirely to undermine any attempt to control your ventilation or environmental conditions well. IMO seal and ventilate. 

 

PIVs have always been almost a magic bullet for fixing condensation (assuming leaks etc sorted), and imo this is the next level.

 

If your house fabric is still full of holes and leaks, then I'm not sure about heating incoming air on the PIV. That's direct electric heating, which imo is not perhaps a good idea if you are about low energy bills.

 

Does insulation in that attic (and the other attics?) require some attention?

[Garald]

3 hours ago, Ferdinand said:

Do you actually need the PIV?

 

Filtering is a must. I'm not so far from a motorway going around a city centre.

 

3 hours ago, Ferdinand said:

 

My current habit is to install a PIV, which is set at the appropriate level (so it is background not blowing a hoolie) at the top, and put in a background HR extract fan at the other end (eg utility room downstairs).

 

I have a PIV entry in every floor. Wouldn't it be best to have the extract fan in the top floor (since used air tends to rise)? The ground floor would actually be more convenient (since that is where my utility room is). How do you use the recovered air?

 

3 hours ago, Ferdinand said:

 

 

I have been using Vent-Axia Lo Carbon Tempra units since about 2014, which state an efficiency of 78% for their heat exchanger.

i call it poor man's MVHR.

 

Thanks! This is what I am aiming for.

 

3 hours ago, Ferdinand said:

 

My reno scheme is now typically good double glazing and doors, under floor insulation, backdraft shutters on extract fans, proper sealing of penetrations, air sealed loft hatch, and all the other detailed stuff I can get at. Plus good -> very good insulation in other respects - aiming for say a good C in EPC terms.

 

Went from an F to a B rating (French standards, which are harsh) :).

 

3 hours ago, Ferdinand said:

 

To help internal air circulation you may need to do something like trimming the bottom of doors.

 

Had that done.

 

3 hours ago, Ferdinand said:

 

TBH if you have open chimneys to the rooms then that is likely significantly or entirely to undermine any attempt to control your ventilation or environmental conditions well. IMO seal and ventilate. 

 

They are not open; they are closed by old-fashioned iron shutters (which are not hermetic). Is sealing still a must?

 

3 hours ago, Ferdinand said:

 

If your house fabric is still full of holes and leaks,

 

Doing my best to eliminate that.

 

3 hours ago, Ferdinand said:

 

then I'm not sure about heating incoming air on the PIV. That's direct electric heating, which imo is not perhaps a good idea if you are about low energy bills.

 

The heater there is fed by the same heat pump as everything else. But does it really make a difference? It's just a strategically placed radiator, after all.

 

3 hours ago, Ferdinand said:

 

Does insulation in that attic (and the other attics?) require some attention?

 

It's fine - blown-in cellulose in the eaves, hemp and reflective on the sides, insulation installed by previous owner in ceiling.

[Ferdinand]

1 hour ago, Garald said:

 

5 hours ago, Ferdinand said:

 

then I'm not sure about heating incoming air on the PIV. That's direct electric heating, which imo is not perhaps a good idea if you are about low energy bills.

 

The heater there is fed by the same heat pump as everything else. But does it really make a difference? It's just a strategically placed radiator, after all.

 

I'll try and come back to the other points in detail later.

 

Remember that I am speaking from my experience in the English Midlands with traditionally built houses, and English climate (which is may be very different to a typical climate in your part of France) and gas boilers / radiator heating.
 

But on this one is the way to find out just to switch that bit off and see if it makes any difference over say a year?

It may be that a low steady heat distributed throughout the house via slow background airflow helps with keeping it all fresh, and that is worth the investment of keeping it running. You can get humidity / thermometer max/min meters off Amazon for under £10 that you can move around over a period of time to see where your peaks and troughs are. I have about three; one routine use I make of them is to tell me when washing is dry in the drying closet I use in winter (which is my small bathroom with the fan left on and a dehumidifier running).

 

There are I think others on Buildhub based in France who may be able to advise.

[Garald]

8 hours ago, Ferdinand said:

 

I'll try and come back to the other points in detail later.

 

Remember that I am speaking from my experience in the English Midlands with traditionally built houses, and English climate (which is may be very different to a typical climate in your part of France) and gas boilers / radiator heating.

 

 

This is the Paris area - not really that different from Southern English weather - and a brick-and-masonry house from the early 1930s. I'm running a heat pump at medium temperature (45 C so far) on radiators.

Edited December 29, 2023 by Garald