Kitchen Chimney Hood Problem

thehorselady

I recently had a Bosch Chimney Hold installed

It's a bungalow I'm living in and the plastic ducking is taken up into the loft and out into one of the ventilation grills in the fascia board, hope you know what I'm getting at.

What is puzzling me is that if I have the extractor on at full power, and the filters in place it will hardly hold a kitchen towel! Put a saucepan of boiling water on the back zone and parts of the steam, not much mind you, escapes around the front.

My nephew disconnected the aluminium surround and then the white plastic pipe that goes up into loft that is supposed to take the steam outside and it does seem to have a better draw. This was just a test

It occurs to me that two things could be happening here: 1. There is an obstruction some where in the plastic pipe, maybe it is twisted. 2 The extractor fan is not working properly or is not strong enough. It should be, it is a good make and I did do my research

According to the literature the extractor has an Airflow Rate of 390 m³, which means nothing to me. Is there a simple non professional test I can apply which will let me know if the extractor is working properly

Thanks for any advice

Horse Lady

Ectophile

What sort of pipe is it?

Bosch say in their instructions:
■ The device achieves its optimum performance by means of a
short, straight exhaust air pipe and as large a pipe diameter
as possible.
■ As a result of long rough exhaust air pipes, many pipe bends
or pipe diameters that are smaller than 150 mm, the optimum
extraction performance is not achieved and fan noise is
increased.

If it's a long stretch of flexi-hose, then the fan may be struggling to push the air along it.

The_Last_Username

I'm sure an expert will be along who may offer pertinent advice.

From my amateur "fitted mine myself" point of view, my observations are:
1/ 390m³h (390 cubic metres per hour extraction rate, maximum) is not a lot. Though by no means "weak" comparatively, the one I purchased - not by any means expensive I must add - extracts around 600 cubic metres per hour maximum.

2/ How long and wide is the ducting between the hood and the outside vent? Does it contain any sharp bends? Because I'm sure I remember reading that the length of ducting should be as short as possible (though I imagine the recommended length is dependent upon individual hoods and their power) AND have no sharp bends as this creates resistance to airflow.

As an aid, I enclose a handy reference from NEFF who suggest a MAXIMUM ducting length of 5 metres (approx. 16 feet in "English") - which I assume includes the upright portion prior to entry into your loft.
http://www.neff.ie/why-neff/get-the-best-from-your-neff/ventilation.html

I hope that provides some insight.

wrightk

thehorselady wrote: »

I recently had a Bosch Chimney Hold installed

It's a bungalow I'm living in and the plastic ducking is taken up into the loft and out into one of the ventilation grills in the fascia board, hope you know what I'm getting at.

What is puzzling me is that if I have the extractor on at full power, and the filters in place it will hardly hold a kitchen towel! Put a saucepan of boiling water on the back zone and parts of the steam, not much mind you, escapes around the front.

My nephew disconnected the aluminium surround and then the white plastic pipe that goes up into loft that is supposed to take the steam outside and it does seem to have a better draw. This was just a test

It occurs to me that two things could be happening here: 1. There is an obstruction some where in the plastic pipe, maybe it is twisted. 2 The extractor fan is not working properly or is not strong enough. It should be, it is a good make and I did do my research

According to the literature the extractor has an Airflow Rate of 390 m³, which means nothing to me. Is there a simple non professional test I can apply which will let me know if the extractor is working properly

Thanks for any advice

Horse Lady

Sounds to me like the pipe run is too long or too twisted.

Does your cooker back on to an external wall?

If so and if you have relatives who are handy with tools it would be far better to duct the vent straight outside using as bigger ducting as possible for the hood and core through the wall behind the cooker and vent it out that way. Then fit an external grill on the outside wall with hood if needed

ryder72

As you mentioned the duct goes into the roof and then to exit through the fascia. This indicates atleast 1 90 degree turn.

Here are some thumb rule calcs-

If you do a straight round plastic duct pipe run, then 5m is the longest run and deduct 1m for every 90 degree turn. If you go into flexi duct, reduce your max run to 3m and then take 1.5m off per turn so you can see that your air flow losses due to the turn explain why you are getting no throughput.

Air flow is a very complex science and large parts of it are outside the scope of this forum. Your best bets are-

Put in a round 5 in riigid duct from the motor exit through the ceiling into the roof void. Then connect it to a very short length of flexi and get to to leave your roof through a vent tile (the high flow rate type).You are guaranteed to see an improvement. This will involve a fair bit of work so its going to cost a few quid. You have to decide if you want to spend that sort of money.

thescouselander

Flexible pipe is terrible for airflow - use rigid pipe all the way if possible.

thehorselady

Thanks to everyone for your replies. After reading your response it does appear that the flexi pipe is, if not the problem, then certainly a major part.

To address one of the comments. Yes the hob is next an outside wall but as luck would have it the outside boiler pipes are in the exact place where the pipe would go out, so no go there

Looks very much that I'll have to take a closer look at the ducting

Thanks for your replies anyway

HorseLady

phil24_7

Can you move the pipes? If not the straight, solid pipes with swept bends will be best.

Regards
Phil

JP08

Had a problem with ours when the kitchen was refitted. Turned out that the installers had missed out a little spacer between a short length of flexipipe and the extractor itself. This meant that the flexipipe was flexing just enough to stop the little flaps on the top of the fan from opening. With them closed there was lots of noise but very little suck. One 5cm spacer made from ducting later all was back in business and working fine.