House fire - how to control mvhr

[warby]

In the event of a fire, there is a need to extract smoke on BOOST and stop supplying fresh air - how can this be done when a smoke alarm goes off?

Can there be a form of Spring closing flap or fire damper on the inlet, triggered by the fire alarm?

[Gone West]

I was told it was best to automatically turn off the ventilation system when the fire alarm goes off. I haven't yet looked into how simple that might be.

[warby]

I believe the biggest problem during a fire is smoke not flames.

I guessed that removing smoke with the MVHR might help, hence why I thought putting extract on boost was required but not to feed air to make the fire worse.

Is this wrong?

Is this covered by building regulations?

[Nickfromwales]

44 minutes ago, warby said:

I believe the biggest problem during a fire is smoke not flames.

I guessed that removing smoke with the MVHR might help, hence why I thought putting extract on boost was required but not to feed air to make the fire worse.

Is this wrong?

Is this covered by building regulations?

Whatever the mvhr extracts has to be replaced by incoming fresh air. That fresh air is worse than the stagnant air in the property so you'd be effectively 'fanning the flames' if you boosted the mvhr during a fire. 

The only correct thing to do when there is a fire is get out of the building, simple. 

Setting the extract to operate and stopping fresh air being subsequently drawn in will be a very difficult task in an airtight house, so my opinion is to have as early a warning as possible and evacuate accordingly. 

Smoke detectors in every room and connecting hallways etc, smoke / heat detection in the mvhr exhaust ducting, and additional sounders wherever deemed advantageous. 

You should aim at stooping a fire at the smoulder / smoke stages, rather than discover it when it's already involved ( flames ). 

 

[warby]

Nick -  you say "Whatever the mvhr extracts has to be replaced by incoming fresh air" - It will create a vacuum but we are only talking about a short time?

Surely you do not want to be feeding the flames with fresh air and the smoke will kill unless it removed.

If someone is unconscious/deaf; smoke alarms do not help.

Totally agree with your other comments about early warnings but the MVHR will still operate and create a worse fire unless a control is established.

[Nickfromwales]

If you shut the inlets, in an airtight house, then you stop the flow of air. No air in = no extract air out ( smoke removal ) other than the value of the ventilation loss as ascertained by the blower test.   

[Nickfromwales]

For deaf people an entirely different discipline is adopted including strategically positioned strobe lights. 

[warby]

47 minutes ago, Nickfromwales said:

If you shut the inlets, in an airtight house, then you stop the flow of air. No air in = no extract air out ( smoke removal ) other than the value of the ventilation loss as ascertained by the blower test.   

In a fire, solids change into gases, many toxic and therefore extra gas is accumulating into the house and needs to be and can be extracted even in an airtight house. Cutting off the air inlet is the required option, I just need to know how this should be best done

[mike2016]

I'm interested in this also,

 

If you're integrating home automation you can link the smoke alarms with the power to the MHVR or separately cut off power to MHVR when the indoor temperature reaches a critical point. The indoor air pressure will increase initially and oxygen levels drop in an airtight house. The pressure increase makes inward opening doors and windows a barrier to exiting as experienced 3 years ago in a German Apartment fire. You should aim as already said to make early detection a priority, smoke detectors in every room and consider a water mist system to cool the ignition point and buy you more time to evacuate. There are good articles online that discuss fire in airtight houses in more detail.

 

Note: Reversing the MVHR flow is risky as one report stated it can lead to the pollution of the other rooms in the house. 

 

For people with disabilities, there are smoke detection systems that trigger wrist worn vibration alerts and also a pillow shaker that goes under a pillow for nighttime alerting. 

 

Ideally I would program my home automation to cut power to everything except any fire suppression system & all lighting leading to escape routes. I'll ensure the rear door and all windows are outwards opening also, except for the front door as the postman will be annoyed if I keep breaking his nose.....!!

 

I don't know if I'll manage to get all of the above in my house - I hope I never encounter the need to use it! 

[warby]

This product is suitable to close off the inlet: http://www.advancedair.co.uk/products/fire-fire-smoke-dampers/2530-electrical-release-fire-smoke-damper

In the product information it states "When the temperature is reached the actuator power is disengaged and then the actuator closes the blades within 15 seconds".

I am confident that this could be modified so that the actuator power is disengaged if the smoke detector is triggered.

I am staggered the MVHR manufacturers don't offer something similar.

Business opportunity for someone?

Thoughts?

[Nickfromwales]

I brought this up many moons ago on Ebuild. 

Click here and see if the content helps out   

[warby]

55 minutes ago, Nickfromwales said:

I brought this up many moons ago on Ebuild. 

Click here and see if the content helps out   

Great minds think alike.

This is fine example for lobbying government. 

I do not agree with the view that: " MVHR system needs to have a means of being switched off in the event of fire "

The requirement is definite in a fire: 1. boost MVHR extract to remove smoke, 2. stop MVHR inlet feed with a damper. 3. Control mechanism detecting smoke to trigger 1 and 2.

Where are the engineers, when you want them?

 

[Roundtuit]

12 hours ago, warby said:

 

The requirement is definite in a fire: 1. boost MVHR extract to remove smoke, 2. stop MVHR inlet feed with a damper. 3. Control mechanism detecting smoke to trigger 1 and 2.

Where are the engineers, when you want them?

 

Sorry, can't understand how mvhr can be a benefit in a fire no matter how you configure it.  Assuming a standard set up extracts only from kitchens and wet rooms, there would be a risk of spreading smoke through the house with your proposal if a fire started in a bedroom.  If your focus is preserving life, go with Nick's detector advice and practice your fire/escape drills. If you want to preserve property, fit sprinklers.

[Nickfromwales]

Pretty much what it boils down to. 

Hear the smoke detectors wailing, get up and get out. 

[warby]

Smoke kills: moving smoke to bathrooms/kitchens and away from escape routes is required. 

Protecting escape routes is essential; my project plan includes for sprinklers on escape routes.

Smoke detectors although an essential alert does nothing to protect you and MVHR inlet air will enhance the fire.

 

I was only 14 when I was in a house fire alone, with my clothes on fire, but was not personally burnt, hence my obsession. I caused the fire in the kitchen by leaving the chip pan on the gas cooker. Melting Polystyrene ceiling tiles were falling on me but I managed to put the fire out. The only reason I was not overcome by smoke was the fact that the kitchen window Xpelair was moving the smoke out of the house and only when I saw smoke passing the lounge window did I realise the kitchen was on fire. These were no smoke detectors in those days.

[mike2016]

Lucky Escape warby!

I was coming home one Sunday morning and the whole main street of the town I was living in was covered in smoke, I was shocked when I got to my driveway that it was my family home (I was a teen at the time) causing it! The chimney had gone on fire! At least that's one element I don't have to worry in my proposed new build, although I wonder if wood burning stoves could have similar issues? I had a washing machine go on fire more recently, so while I was thinking of locating mine under the stairs this makes me more wary as it could block the easiest egress point. Good case for positioning the trampoline outside the back window!!

[Nickfromwales]

5 hours ago, warby said:

Smoke kills: moving smoke to bathrooms/kitchens and away from escape routes is required. 

Protecting escape routes is essential; my project plan includes for sprinklers on escape routes.

Smoke detectors although an essential alert does nothing to protect you and MVHR inlet air will enhance the fire.

 

I was only 14 when I was in a house fire alone, with my clothes on fire, but was not personally burnt, hence my obsession. I caused the fire in the kitchen by leaving the chip pan on the gas cooker. Melting Polystyrene ceiling tiles were falling on me but I managed to put the fire out. The only reason I was not overcome by smoke was the fact that the kitchen window Xpelair was moving the smoke out of the house and only when I saw smoke passing the lounge window did I realise the kitchen was on fire. These were no smoke detectors in those days.

 

Whilst I clearly hear your concerns and unpleasant previous experiences, it's simply not comparable to the safety you'd be afforded in a new 'regs-compliant' home. 

Kitchens will have heat detectors, and as the question relates largely to properties with mvhr, I doubt there will be issues from things as catastrophically bad in a fire as polystyrene tiles. 

Burnt toast in the toaster or a well done steak will set off my smoke detector by the front door, some 12m's away, let alone flames or a fire.  

 

[jack]

I don't think you'll get very much airflow at all if you close the inlet and boost the outlet, as you're largely extracting against a sealed box.  All you'll do is lower the interior air pressure slightly and draw in small amounts of air from around windows and doors (the main relatively leaky points in most airtight houses).  That's why standard advice is to shut off the MVHR rather than boosting etc.

 

I've seen inlets with intumescent collars, the idea being that the duct will close when the heat gets high enough.  But frankly by that stage your smoke detectors should have been going off for several minutes and you and your family should have left the house.

 

Have a look at the different types of detector.  Some are better at detecting different types of fires (eg, smouldering).  Given your apprehension (natural given your experiences), consider going belt and braces with both types in or just outside each bedroom, plus kitchen, utility room and anywhere else you need.   Consider whether you want a sensitive smoke detector in the kitchen though - too many false alarms. 

[warby]

1 hour ago, jack said:

I don't think you'll get very much airflow at all if you close the inlet and boost the outlet, as you're largely extracting against a sealed box.  All you'll do is lower the interior air pressure slightly and draw in small amounts of air from around windows and doors (the main relatively leaky points in most airtight houses).  That's why standard advice is to shut off the MVHR rather than boosting etc.

 

I've seen inlets with intumescent collars, the idea being that the duct will close when the heat gets high enough.  But frankly by that stage your smoke detectors should have been going off for several minutes and you and your family should have left the house.

 

Have a look at the different types of detector.  Some are better at detecting different types of fires (eg, smouldering).  Given your apprehension (natural given your experiences), consider going belt and braces with both types in or just outside each bedroom, plus kitchen, utility room and anywhere else you need.   Consider whether you want a sensitive smoke detector in the kitchen though - too many false alarms. 

Thank you jack.

However, extra gases will be produced in the house as solids burn and form gases, hence  pressure will increase in the house, smoke will accumulate at height and then fall and will escape wherever it can, in particular via the MVHR outlet, hence smoke can be extracted even in a sealed box.

[PeterW]

The problem you face is that most of the MVHR systems don't have enough fan pressure to quickly clear smoke, and in the way they operate - with the sole exception of a kitchen - the risk of fire breaking out in the extract rooms is minimal. Increasing the flow into the bedrooms etc will not give a huge boost to clearing smoke etc.

 

Smoke extraction systems use vastly more powerful fans to clear rooms. The fans in MVHR units tend to be balanced cage type fans, not axial fans which give a higher pressure and volume flow. The other issue would be smoke particles clogging the heat exchangers very quickly, reducing the effectiveness of any extraction. 

 

You could potentially "bridge" the main extract and intake ducts after the MVHR and before the main plenums and use one large fan and a manifold of some description - this could vent straight to the outside air however it would need to be balanced to ensure it didn't draw more air into a fire than clearing the smoke.

 

I think a cheaper and better option is a range of smoke / fire detectors and sprinklers in exit routes. 

[jack]

1 hour ago, warby said:

 

However, extra gases will be produced in the house as solids burn and form gases, hence  pressure will increase in the house, smoke will accumulate at height and then fall and will escape wherever it can, in particular via the MVHR outlet, hence smoke can be extracted even in a sealed box.

 

I don't know all the ins and outs of combustion, but if something's burning to produce gas, surely it's consuming gas (ie, oxygen) to do so?  Is there a net increase in the volume of gas, other than via expansion due to heat?

[warby]

Blimey Jack haven't had to work out things like this since I got a cCemist degree at university in 1974.

When gases in a container (house) are heated, their molecules increase in average speed. This means that they exert a greater force when they collide with the house walls, and also collide with the walls more frequently. Oxygen molecules are changed in a fire for larger (more chemically complex) 'smoke' particles. Also, the gas volume in a house is under greater pressure as temperature rises. Consequently, I think MVHR extract volume will automatically increase, presumably up to the maximum capacity of the ducting. Therefore, I think leaving MVHR extract on is sensible but switching off MVHR inlet feed ought to be mandatory to stop feeding the fire with fresh air.

[Nickfromwales]

8 hours ago, warby said:

Thank you jack.

However, extra gases will be produced in the house as solids burn and form gases, hence  pressure will increase in the house, smoke will accumulate at height and then fall and will escape wherever it can, in particular via the MVHR outlet, hence smoke can be extracted even in a sealed box.

 

If gasses are being produced in those quantities, it'll be from combustion created by a fully involved fire. 

Smoke will have been produced far, far sooner than this combustion will have occurred and will have subsequently been detected by the installed, linked, detection measures. 

By that stage there should be no real reason why you wouldn't be (1) alerted to the smoke generation, (2) up and exiting the property, and (3) removed from a position where such created gasses are a risk to life. 

IMO the focus here is on the wrong thing, hence why I'm splitting hairs. If there is a remit to tackle that situation, the talk should be of fire suppression and emergency / terminal escape measures. 

Its one of the reasons that 3+ storey domestic dwellings send a shiver down my spine. 

[warby]

We seem to be losing the reason for this thread,

I agree with you Nick that " fire suppression and emergency / terminal escape measures " is very important and should be a separate thread. 

However, the MVHR will continue to feed and accelerate a fire unless a solution is found.

[jack]

 

3 hours ago, warby said:

Consequently, I think MVHR extract volume will automatically increase, presumably up to the maximum capacity of the ducting. Therefore, I think leaving MVHR extract on is sensible but switching off MVHR inlet feed ought to be mandatory to stop feeding the fire with fresh air.

 

If that happens, then I imagine the pressure would completely overwhelm the MVHR fans.  If you're right about pressure, then closing the inlet at this point will cause a further increase in pressure, and the smoke will therefore potentially work its way through the rest of the house seeking further exit routes as the outlet in the area of the fire (most likely the kitchen) is overwhelmed by the volume of exiting gases.

 

1 hour ago, warby said:

We seem to be losing the reason for this thread,

I agree with you Nick that " fire suppression and emergency / terminal escape measures " is very important and should be a separate thread. 

However, the MVHR will continue to feed and accelerate a fire unless a solution is found.

 

I know you asked how you can achieve your proposal.  I think Pete's offered one option there.  

 

But turning off the MVHR (the standard proposed approach) will do most of what you're asking.  You've said that pressure increases greatly.  If that's right, then nothing the MVHR does is going to make much of a difference.  

 

I'm not saying any of this is fact, because I simply don't have the necessary knowledge to do so.  There are some interesting principles at play though, and I suspect there must be a reason that MVHR manufacturers recommend simply turning off the MVHR in the event of fire.