HRV Ducting

Slig

I was meeting with a client on-site about a specification for finishes to their house when I noticed the HRV ducting had been installed. It looked like a cheap flexible duct (similar to the stuff that they use at the back of clothes driers) with mineral wool insulation and a reflective foil covering. It was fixed to the rafters by galvanised straps but these straps were not pre-formed and were compressing the duct. Everywhere the ducting turned there was a kink and where it passed through the hollowcore the bend was so so tight the duct was nearly flat at the elbow. To add to this all the straight runs were sagging between the straps and the connections to the HRV unit were put into the wrong room.

I informed the client, in passing, that the building regs require solid ducting and he insisted that I get onto the installer for him. Am I being pedantic by insisting that he take out the ducting and replace it with solid even if the solid has a poorer spec?

I'm not certifying the build but I know the engineer that is and he's not that up to date with the renewable stuff.

therightangle

The solid may have a poorer spec but it will allow him to use the certified MVHR performance figures in his BER - with flexible he is back to BER defaults now matter how good the spec or installation of unit or ducting is.

AMG86

It maybe easier and quicker to install flexible than rigid. However attention to detail is required to do it right. This does not always happen. Flexible should be fully extended i.e. without being compressed or using more than is necessary. Avoid bends at sharp corners. A support is required every 1.5m. Supports and bends should not compromise the duct. In short it is difficult but not impossible to install flexible correctly.

If a flexible duct is crushed etc. its shape will be more than lightly changed. This will impinge on air its flow capacity. The cross sectional area may be reduced. This could happen when some guy is working in the attic. You are unlikely to be told. The inside of a flexible duct is not smooth. It maybe corrugated up and down. Less electrical power will be used to move the same volume of air through a rigid round or oval duct than through a flexible system.

It may help if you brought a professional on site to have a look. It will cost but it could save you grief in the future. The engineer should know about these things. They are not exactly new.

refco

flexible duct should only be used for last metre or so off solid to inlet/outlet as it can affect performance and where possible should not be used anywhere flexi duct is only used by installers where space or angle is too tight for solid duct to be used
also because of the coil in the make up of flexi duct it just gathers dust and holds it and cant be cleaned where it cant be reached causing restriction to air flows and air quality

Carlow52

refco wrote: »

flexible duct should only be used for last metre or so off solid to inlet/outlet as it can affect performance and where possible should not be used anywhere flexi duct is only used by installers where space or angle is too tight for solid duct to be used
also because of the coil in the make up of flexi duct it just gathers dust and holds it and cant be cleaned where it cant be reached causing restriction to air flows and air quality

+1
a bigger worry is condensation in the sagging flexi: perfect for mould etc to propagate and have the spores blown all over the house.

Slig, based on ur post I would have it taken out

esox28

sorry for grave digging!!

question on the pipework configuration in hrv....my duct work is in place fiest fixed I will have two units one for ground and one for first floor, my question is if it is viable or advisable to have extract from ground floor connected to the supply duct for first floor through the same unit and extract from ff suppling the gf in the other unit?

My thinking is the ff is always at a higher temp to gf this imo will give a better whole house cross flow.

would the issue of cross flow between units cause drama?

sinnerboy

All duct runs must connect back to a heat exchanger or exchangers - not other ductwork runs. A basic principle of HRV is that the air volumes must balance - you must replace the same volume you extract. So unless both storeys are exactly the same volume this sounds like a non starter.

Don't complete the installation without expert input. This is something so vital to your health and well being and you don't want to screw it up.

fclauson

esox28 wrote: »

sorry for grave digging!!

question on the pipework configuration in hrv....my duct work is in place fiest fixed I will have two units one for ground and one for first floor, my question is if it is viable or advisable to have extract from ground floor connected to the supply duct for first floor through the same unit and extract from ff suppling the gf in the other unit?

My thinking is the ff is always at a higher temp to gf this imo will give a better whole house cross flow.

would the issue of cross flow between units cause drama?

No the extract temp will be very low eg
External is 5
Supply could be 21
Return could be 20
Exhaust could be 7

You would not want to connect the 7 degree to the supply run of the upstairs

What you should look at is having an extract up stairs (typically warmer than downstairs) being part of the extract of the down stairs unit AND a supply from the down stairs feeding to somewhere up stairs - this will give better temperature management

BUT as Sinnerboy says get expert advice from at least 3 sources to cross check the advice you are getting

esox28

Yea thanks guys will chat to the supplier next week,
Wanted to know if anyone used this method to balance out the temp diff between ffloor and gfloor.
I.e. extracts from ff, supply to gf.
. . . Extracts from gf, supply ff.

Andrew_Doran

esox28 wrote: »

Yea thanks guys will chat to the supplier next week,
Wanted to know if anyone used this method to balance out the temp diff between ffloor and gfloor.

MVHR doesn't push enough energy around to have a significant impact. Air conditioning and forced heating systems affect temperature significantly, but they use higher airflow rates and they have a much higher temperature difference compared to ambient = much more energy moving about.

esox28

Thanks for your reply,

I understand what you are saying, but with hrv you only get back a % of what you put in, in other words if the temp gf is 19* outdoor 15* you will have supply duct at say e.g. 17*

There fore the ff is at 22* with external temp@ 15* your supply at 20*

In theory the supply duct using the extract from the ff supplying gf with a temp @ 19* with supply air @20*

Also in theory the supply duct using the extract from gf supplying the ff with temp of 22* and supply of 17*c

Again in theory I think the configuration of the extracts from ff supplying gf and vise versa would balance out the natural warm air convection current associated with two story housing.

Any taughts?

fclauson

I think you need to look at this again
the extract air from the gf will be smelly & damp - why would you want to feed this into the first floor as the fresh incoming ait

as I mentioned in my earlier post you would be better off having an extract on the first floor fed into the ground floor extract network - that would take you 22 degree air and pass it into the heat exchanger which is covering the ground floor

More importantly what you are proposing will not meet Part F compliance

esox28

fclauson wrote: »

I think you need to look at this again
the extract air from the gf will be smelly & damp - why would you want to feed this into the first floor as the fresh incoming ait

as I mentioned in my earlier post you would be better off having an extract on the first floor fed into the ground floor extract network - that would take you 22 degree air and pass it into the heat exchanger which is covering the ground floor

More importantly what you are proposing will not meet Part F compliance

Come on please helpfull comments, obviously the extract is running through the heat exchanger and vented externally, give me some credit....

Part f comp. Don't talk to me these are the same guys granting planning 5 years ago for developments using 9" cavity block cont. And 50mm battons internally insulated with glass wool.

I work under part l wlevery day and its even questionable at times.

fclauson

esox28 wrote: »

question on the pipework configuration in hrv....my duct work is in place fiest fixed I will have two units one for ground and one for first floor, my question is if it is viable or advisable to have extract from ground floor connected to the supply duct for first floor through the same unit and extract from ff suppling the gf in the other unit?

Sorry for miss understanding you - but the above gave the view that you where mixing supply and extract air.

esox28 wrote: »

Part f comp. Don't talk to me these are the same guys granting planning 5 years ago for developments using 9" cavity block cont. And 50mm battons internally insulated with glass wool.

What interesting about Part F as to some of the other TGDs is its based on some well establish international standards. I did a lot of research on this during my build and the advice in Part F is well in line with the thinking from some leading research institute in Europe.

back to your original point if I was building a two storey house and needed two units I would definatly cross-mix as you are suggesting.

Years ago when I visited the welsh Centre for Alternative energy they had a very simple device - a plastic tube you clipped to the high point on the landing which would then drop down to the ground floor and a small electric fan at the bottom which would "suck" the warm air from upstairs and deliver it down stairs. Simple but very effective

esox28

fclauson wrote: »

Sorry for miss understanding you - but the above gave the view that you where mixing supply and extract air.


What interesting about Part F as to some of the other TGDs is its based on some well establish international standards. I did a lot of research on this during my build and the advice in Part F is well in line with the thinking from some leading research institute in Europe.

back to your original point if I was building a two storey house and needed two units I would definatly cross-mix as you are suggesting.

Years ago when I visited the welsh Centre for Alternative energy they had a very simple device - a plastic tube you clipped to the high point on the landing which would then drop down to the ground floor and a small electric fan at the bottom which would "suck" the warm air from upstairs and deliver it down stairs. Simple but very effective

Yes, I have two story with two units located side by side.

Don't get me wrong im 100% behind new reg standards, but im my own build I have blown most of them out of the water.