Green Building Forum - Kitchen with only MHRV extract duct

Kitchen with only MHRV extract duct

wholaa — Fri, 03 Sep 2021 21:49:55 +0100

Hi all,
Is it commonplace to have an MRHV system without any input duct? Just an extract duct? The house in particular has a small enough kitchen with low occupancy but it is in a three-story house and its MRHV is not beautiful so I wonder about pressure drop.

Kitchen with only MHRV extract duct

Mike1 — Fri, 03 Sep 2021 23:56:19 +0100

No.

Kitchen with only MHRV extract duct

Jonti — Sat, 04 Sep 2021 08:09:44 +0100

I presume you mean a MHRV and if it is just removing then what is it doing with the heat it recovers?

Kitchen with only MHRV extract duct

Ed Davies — Sat, 04 Sep 2021 09:34:39 +0100

Do you mean the whole house has only extract ducts or that the kitchen only has an extract duct and there are ~~input~~ supply ducts in other rooms (living room, bedrooms ?)?

Having only extract ducts in â€œwetâ€ rooms (kitchens, bathrooms, shower rooms, WCs) and supply ducts in other rooms is the usual MHRV arrangement. A tiny amount of care is needed to ensure that there's enough airflow between the rooms even when the doors are shut - e.g., leaving a gap under the door clear of the carpet.

Alternatively you could have a house with only extract ducts feeding a heat pump which heats hot water, either for space heating or for DHW (washing, etc). Called an EASHP (extract air source heat pump).

Edit: change â€œinputâ€ to â€œsupplyâ€; had a mental block on the right word earlier.

Kitchen with only MHRV extract duct

djh — Sat, 04 Sep 2021 10:58:49 +0100

Pedant mode on: It's MVHR - Mechanical Ventilation with Heat Recovery

Kitchen with only MHRV extract duct

Ed Davies — Sat, 04 Sep 2021 12:09:05 +0100

It can also be MHRV: mechanical heat-recovery ventilation. The two forms seem to be about equally commonly used. I tend to use this form just because it follows the usual English pattern of putting the adjectives before the noun, but this is a very mild preference. Also, I've seen HRV used to mean heat-recovery ventilation in general (mechanical or otherwise (e.g., passive)) but never VHR.

Kitchen with only MHRV extract duct

djh — Sat, 04 Sep 2021 13:09:00 +0100

MVHR is English MHRV is American

Kitchen with only MHRV extract duct

wholaa — Sat, 04 Sep 2021 13:55:31 +0100

Posted By: JontiI presume you mean a MHRV and if it is just removing then what is it doing with the heat it recovers?

Thanks for the feedback, pardon the typo. Yes it is a MVHR system, Vent axia. The house has various positive outlets, such as the living room on the same floor. I have heard people suggesting that kitchens really should have a positive and a negative supply but I am just wondering is it is important. The ducting isn't great. It has a fair few sharp bends and flat channels so with the MVHR unit in the attic, I was worried if collectively these issues are combining to make a subpar installation that is over ventilating upstairs and under ventilating downstairs. So far the house is fine to live in but at the moment is is under-occupied.

Kitchen with only MHRV extract duct

djh — Sat, 04 Sep 2021 15:43:37 +0100

Posted By: wholaaThanks for the feedback, pardon the typo. Yes it is a MVHR system, Vent axia. The house has various positive outlets, such as the living room on the same floor. I have heard people suggesting that kitchens really should have a positive and a negative supply but I am just wondering is it is important. The ducting isn't great. It has a fair few sharp bends and flat channels so with the MVHR unit in the attic, I was worried if collectively these issues are combining to make a subpar installation that is over ventilating upstairs and under ventilating downstairs. So far the house is fine to live in but at the moment is is under-occupied.

It's perfectly normal to just have an extract in the kitchen and a supply in a nearby room - dining room or living room for example. That's what we have. Sometimes there's also a supply in a different area of a kitchen, but only if the shape is unusual or the supply needs balancing for some reason.

Whoever designed the system should have calculated the flow resistance of all the ducts, taking into account such things as bends etc, to make sure the flowrates will be sensible. When the system is installed it should be balanced so that the actual flowrates match the design rates for each terminal. There's supposed to be a check list that shows the measured flow rates. It's easy enough to do the balancing - we did ours ourselves. The exact details depend on what type of duct system you have.

Kitchen with only MHRV extract duct

wholaa — Sat, 11 Sep 2021 14:17:04 +0100

@djh Thanks very much!

I must get your thought on this duct (extraction) joint. Is airtightness normally ok to do joints?
https://i.ibb.co/94sHjSs/original-d2d45f7b-5bfd-4f05-894c-614609863109-20210307-223219.jpg

Kitchen with only MHRV extract duct

djh — Sat, 11 Sep 2021 20:22:56 +0100

Not sure I understand your question? And I'm not sure I understand what I'm looking at in the photo. Plus it's blurred so I can't read what it says on the tape.

But it looks like what I'm looking at is the duct above an extraction terminal that is then connecting to some type of horizontal duct and the junction is covered in some kind of sticky tape. What do I think? I don't think junctions in ventilation ducts normally need to be wrapped in sticky tape. But if there was a problem with the airtightness of a particular junction then it might be possible to seal it with an appropriate sticky tape. But whether there was a problem that couldn't be resolved in a better way and whether that is a suitable type of sticky tape I cannot say, I'm afraid.

It looks most like the points where my main intake and exhaust ducts pass through my airtightness barrier. There's a multitude of short lengths of Siga Rissan tape that seal each of the ducts to the airtightness barrier (lime plaster). And there's similar, although less impressive, around each of the main service penetrations - water, electricity and communications. So the technique can be useful, but I don't know whether this is a sensible use case.

Kitchen with only MHRV extract duct

wholaa — Sun, 12 Sep 2021 17:35:51 +0100

Thanks. I was just wondering is assembling a joint like that with airtightness tape ok? The tape work looks really messy to me. It is an extract on a flat channel duct on my ground floor to the unit in the attic. The house is three stories. As a whole, the system is rather noisy so I am wondering is there is poor detailing causing pressure loss and a noisy system.

Kitchen with only MHRV extract duct

djh — Sun, 12 Sep 2021 19:55:41 +0100

Posted By: wholaais assembling a joint like that with airtightness tape ok

As I said, I don't know what I'm looking at so I don't know what type of joint it is, let alone whether it's OK!

The tape work looks really messy to me

Tape tends to look messy. That's not the issue. Should tape have been used is the question?

It is an extract on a flat channel duct on my ground floor to the unit in the attic.

Ah we're narrowing it down, but still. What does the joint look like without the tape? Is it a proprietary system or something generic? Where can I see examples of the duct parts?

As a whole, the system is rather noisy so I am wondering is there is poor detailing causing pressure loss and a noisy system.

Something's definitely wrong then. What's the design flowrate in each of the ducts? Excessive flowrate is one common cause of noise. Another would be restrictions in the flow path, and poorly designed changes of direction.

You'll either need to get some indpendent expert take a look, or post lots and lots of detail here including specs, design drawings and photos of the reality if we're to help.