follow up to the heat recovery ventilator

[Tim Lamb]

In the end I opted for the *integra* unit from Vent Axia. Basically the only modestly priced (405 ukp+ VAT) unit that could be fitted horizontally in the space available.

Lots of hole boring and duct pulling later..... the suction side seems fine: inlets from two bedrooms, kitchen and bathroom. The bathroom has a simple humidistat to bring in *boost* speed which I am a bit doubtful about as humidity levels vary with temperature.

Noise! The return side, so far a single 4" duct feeding a single 4" outlet in the hall is ***** noisy! How much of this is electrical/fan blade generated or single duct air flow is difficult to determine. I can put in a T and halve the flow rate fairly simply or create a padded discharge box with much bigger outlet vents.

Does anyone have any practical noise reduction suggestions? I suspect the more expensive DC motors would be electrically quieter but I don't think they were an option for this model.

More control sophistication would ease the issue. At present a time clock restricts the trickle use to 6.30am to 10.30pm with boost triggered by the humidistat. It might be better to arrange a further limit on boost operation to a timed interval based on bathroom occupation linked to the light pull switch. Has anyone come across a suitable timer with a relay output?

regards

[NT]

That's easy...

[Onetap]

FFS, Why do you persist in posting bollox about subjects which you know nothing about? I've got 2 books about noise control in ductwork, neither of them mention baffle boxes.

I've never quite understood the subject, but; limit duct velocity (2.5 m/s or less), any restrictions (diffusers, dampers, elbows, duct restrictions) can generate noise. If the noise is still excessive get a sound attenuator (ducts lined with absorbent material. ducts with internal absorbent splitters). Ask the manufacturers, their product is intended for domestic applications and I'm sure it's a subject they will have answered previously.

Are you using flexible ducts? It's crap, only suitable for the final connection to a diffuser and then no more than 150 - 200mm.

[Tim Lamb]

In message , NT writes

I had in mind allowing the air flow to expand into a box with some form of foam absorbent material but the *baffle box* with carpet sounds ideal:-)

regards

[Tim Lamb]

Yes. Job for Tuesday morning.

Flexible. Crap in the sense of restricting airflow, creating noise or delicate?

The installation is tucked away in the ceiling void and the ducts routed through cupboards at high level. The inlet needed to be insulated anyway to avoid condensation on the exterior so, where practical, the ducting is in an insulating sleeve.

The job for today is to disconnect the outlet so the discharge is into the ceiling void and see how much difference this makes to the overall sound level.

regards

[NT]

baffle boxes work excellently, I know that primarily from experience. They're also standard kit in some noise reducing apps.

so?

well I never. I dont claim to be an expert on this either, but baffle boxes do work wonders. Several sets of ears don't lie.

NT

[Onetap]

For example Woods, Practical Guide to Noise Control; Woods are well known throughout the ventilation & AC industry as purveyors of fans and sound attenuators. Their guide doesn't mention baffle boxes. Why would that be? Do you know something they don't? Or maybe they know something you don't? Like your proposal would restrict the air flow and there are better ways of achieving a reduction in the propagated noise levels?

I'm sure they don't; they probably all noticed a huge reduction in the noise. If you'd been able to measure the air flow rate, you'd probably have noticed a huge reduction in the air flow rate. I am not am expert in comparison with people like Woods' employees who have made a career of noise control. I've merely genarlised in HVAC stuff. You know better? Of course you do.

[Onetap]

Yes on all counts. It has a much higher resistance per m compared with smooth ducts. It would probably absorb more noise than the turbulence generated.

[NT]

Fact is baffle boxes are used in domestic air handling, built into fan units used to being air in in noisy situations such as near motorways.

Why one book doesnt mention one method neither of us can know, but seldom do books discuss every possible method of doing things. All it tells us is its probably not the no 1 popular approach.

well, they've been used /a lot/ in various apps, they achieve very high levels of noise reduction, and if sized suitably have little effect on airflow. The one obvious potential downside is that they have to be a fair bit bigger than the duct.

low resistance is one of their plusses.

I doubt it very much, and never claimed such a thing. To suggest I did is a straw man. I'll wager your book doesnt say baffle boxes are a lousy idea.

Nothing you've presented shows they either dont work well, or arent cost effective, arent reliable etc. Your central argument, that one book doesnt mention them therefore theyre no good is simply illogical, and your claim that I claim to know better than Woods is a fantasy.

NT

[Tim Lamb]

Fortunately I have the space.

Disconnecting the duct from the outlet spigot and discharging the air into the ceiling void reduced the noise dramatically.

I will speak to Vent Axia technical tomorrow and get their advice but a lined baffle box is easily do-able. I was thinking cushion type carpet underlay stapled inside for lining.

regards

[NT]

We await the result. So far I've been impressed each time.

NT

[Onetap]

Where? Who makes them? Where can I buy one?

I've been in HVAC for 30+ years (with several minor detours). I've never heard of a baffle box. Fearing that I was missing out on some modern and effective technology, I had a Google for " "baffle box" attenuator". Two of the hits on the first page referred back to this thread. Most of the rest seemed to be about acoustics and speaker enclosures. Google for the alternative "sound attenuator" and there are more real products than you can shake a stick at. I also searched for images of a "baffle box"; the first 4 pages were about drainage and beds. I gave up then.

In short, this is more NT pipe-dream bollox, isn't it?

It isn't used because it is ineffective in comparison with the industry standard sound attenuator/ silencer.

They're used a lot? For ventilation ducts? Where? Any pictures?

What? Four 180 degree turns shown on your diagram; that causes resistance.

What didn't you understand? It does not mention "baffle boxes" at all. It uses the word 'baffle' twice in the context of (1) improving acoustic absorption of a room and (2) reducing noise emission from a plenum chamber; this application uses 1 baffle with air passing around both ends. It is what I know of as a splitter, as used in sound attenuators.

Nothing you've presented shows they either dont work well, or arent

What I said was that no-one makes them and the reference books I have don't mention them. If they are cost effective, why doesn't someone market them?

Acoustic foam on the duct walls, put in foam lined splitters if space allows; i.e., sound attenuator. You need access to the splitters to clean any debris off them. Foam will disintegrate eventually.