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IanR

Are Heat Pumps a possible fire hazard?

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The Grenfell Tower thread got me thinking about potential fire hazards around the exterior of my house.

 

I have timber cladding with a vent gap over a timber structure, similar to many on this forum and was quite happy about the fire resistance of the structure from an internal fire, and the protection offered from internal fire alarms.

 

I'd never considered a fire starting on the outside of the property. The only item close to the facade of our house, that could potentially cause a fire, is an ASHP, placed 500mm from the external wall.

 

A quick Google search for "heat pump fire" brought up more incidence than I expected, although it is a little dominated by current Carrier/Bryant recall.

 

My assumption is that it is a low risk, however if an external fire did take hold It could be well established before the house occupants became aware of it.

 

What's the consensus? Does the risk need mitigating, and if so by what actions?

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The simple answer is YES. An internet search lists a number of recalls and reports of incidents involving heat pumps.

 

Which did a report based on Government information, here is a simple graphic of the results

productsafety_large-406055.jpg

Edited by Triassic
Source added

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I would have thought that being a min of 500mm from wall and that there is not a great deal of combustible material in a heat pump means they are low risk. 

 

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11 minutes ago, dogman said:

I would have thought that being a min of 500mm from wall and that there is not a great deal of combustible material in a heat pump means they are low risk. 

 

 

They also have a pressurised gas and a pressurised water system - water would potentially limit a fire however some refrigerants are flammable.  R410a will burn under pressure however the amount of pressure in the systems means it would flash over pretty quickly.

 

 

productsafety_large-406055.jpg

 

What is missing from this is one very important device that causes more than the top 5 in that list - portable device chargers..!!! They account for a significant number of fires, and even more so at night as people charge devices overnight. Advice is use it and then switch it off or unplug it !

 

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A quick look inside ours shows that it is mainly of metal construction, with some neoprene pipe insulation, a plastic fan and a flammable refrigerant gas.  There are no heat sources inside the case, and no flammable materials adjacent to the circuit board that I can see.  The only real fuel source within the unit seems to be the refrigerant, and a refrigerant leak will trigger the low primary circuit alarm and turn the unit off.

 

Frankly, I suspect that heat pumps are a pretty low risk, as long as they don't contain either heating elements, flammable material in the case and structure, or flammable insulation.  Some certainly do have electric heating elements, both to assist during defrost and as flow temperature boosters, and these may pose a greater risk.

 

As @dogman rightly says, there is a requirement to space them away from walls so that they can get a good intake air supply, and this, combined with the fact that the fan blows outwards, away from the building, should reduce the risk a lot.

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^ but many do have a sump/ condensate tray heater. I'd contend tho tha it's likely to be a sightly better irem than the defrost heaters used by Beko et al.

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22 minutes ago, dpmiller said:

^ but many do have a sump/ condensate tray heater. I'd contend tho tha it's likely to be a sightly better irem than the defrost heaters used by Beko et al.

 

 

Which is precisely why I wrote this:

 

1 hour ago, JSHarris said:

.....................as long as they don't contain either heating elements, flammable material in the case and structure, or flammable insulation.  Some certainly do have electric heating elements, both to assist during defrost and as flow temperature boosters, and these may pose a greater risk.

 

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I note conventional wisdom is to place the ASHP so the unit (and the fan) is parallel to the wall and half a metre or so out from the wall, drawing air in between the unit and the wall,. and expelling air perpendicular to the wall.

 

Is that really the best orientation?

 

I am giving consideration to mounting it perpenducular to the wall, so it takes air in once side and expells air parallel to the wall the other side.  My main reason for this thinking is the direction of expelled air will be away from the habitable rooms of the house and therefore less likely to be audible. Also most of the time here that would mean the flow of air through the unit is going with the prevailing wind.

 

In this scenario the unit may be closer than half a metre to my rendered, wood fibre clad wall.

 

I guess thee is nothing stopping us fitting a heat alarm inside the top of the ashp and linking it to the house alarms?

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4 minutes ago, ProDave said:

I am giving consideration to mounting it perpenducular to the wall, so it takes air in once side and expells air parallel to the wall the other side. 

 

Could that have a chilling effect on your external wall? The expelled air will be below outside air temp.

 

6 minutes ago, ProDave said:

I guess thee is nothing stopping us fitting a heat alarm inside the top of the ashp and linking it to the house alarms?

 

I still have my ducts open to the ASHP position and am just considering whether to run another cable to some sort of sensor/fire alarm, but not sure what would be best.

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Ours has a wrap-around heat exchanger at the rear, that draws in air from the side and the rear.  Some are circular, and draw air in from all around and expel it from the top.  I think the main thing is to leave enough room for air to be able to freely flow into and out of the unit, and there are minimum distances in the installation instructions to allow this.

 

If placing one out in the open, then I think the only thing to be concerned about is the effect of wind.  Ideally, the unit needs to be in a fairly sheltered location, away from strong winds, I think.  There doesn't seem to be a practical problem with just having them fitted parallel to a wall, spaced away from it by around 500mm, as far as I can tell.  The intake area is pretty large, larger than the front elevation area of the unit, because of the wrap-around design, so the intake flow velocity is pretty low.  Even the outlet air flow velocity is fairly low.

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31 minutes ago, IanR said:

 

Could that have a chilling effect on your external wall? The expelled air will be below outside air temp.

 

 

I still have my ducts open to the ASHP position and am just considering whether to run another cable to some sort of sensor/fire alarm, but not sure what would be best.

It's the garage wall so not bothered about any chilling effect.

 

Re a heat alarm, just use a domestic heat alarm normally fitted to a kitchen ceiling, then it can be linked with a 3 core cable to all the other smoke / heat / CO alarms in the house.

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If you look at the risk of a fire from an ASHP you may wish to move your washer and drier into an external

 utility room, as we have just done.

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The issue is that there is nothing monitoring the ASHP environment, unlike the inside of the house, rather than the risk level. 

 

It's a much lower risk, but if there was a fire you may not know about it until your internal fire alarms alerted you, by which time the fire has penetrated the fabric of the building. 

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There is a fair bit of internal monitoring in the ASHP we have, plus an internal alarm in the house that operates when there is a heat pump problem.  There seem to be several temperature sensors inside the unit, along with an over-pressure cut out switch in the refrigerant circuit, so I feel reasonably confident that there would be an effective alarm if there was a problem.  This probably varies a bit from one manufacturer to another, though.

 

I suppose it's also worth mentioning that we have three other air source heat pumps inside the house, the fridge freezer in the kitchen, the air-to-air heat pump inside the MVHR unit and the small beer fridge in my study.................. 

Edited by JSHarris

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1 hour ago, JSHarris said:

Ideally, the unit needs to be in a fairly sheltered location, away from strong winds, I think.  There doesn't seem to be a practical problem with just having them fitted parallel to a wall, spaced away from it by around 500mm, as far as I can tell.  The intake area is pretty large, larger than the front elevation area of the unit, because of the wrap-around design, so the intake flow velocity is pretty low.  Even the outlet air flow velocity is fairly low.

 

No such thing as a sheltered location where I am....so far doesn't seem to be affected performance.

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Isn't the biggest risk the wiring to the ASHP (or any other inductive load i.e. a motor).

If the circuit breaker is faulty, or the wiring damaged, that could cause a fire.

 

When I was in Australia they were experiencing some huge forest fires (could see the smoke 2 hours before we landed).

Visiting a work mate at his house one day, he asked me if I could help 'water his roof'.

Apparently most house fires (near a forest fire) are caused by leaf debris in the gutters being set alight by wind carried embers.

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I suspect the greatest risk may be the heating element fitted to some heat pumps, including frost-free fridge freezers.  There seems to have been a few problems with the controls for defrost heating elements having stuck on, causing the element to overheat and melt the surrounding plastic lining/insulation in some frost-free fridge freezers, as well as some fires caused by certain models.

 

The idea of a heating element that can overheat in the event of the switch or relay that controls it getting stuck on seems to be a fairly significant risk.  Relays do get stuck on; one of our PIR switches failed a few months ago in just this way, the relay stuck in the on position.  It was only switching a few tens of watts of LED lighting, too, so shouldn't have failed, as it was working well within its rated current.

 

The wiring in a new installation shouldn't pose a risk, as there should be both earth leakage protection and over-current protection on the circuit, so if the cable is sized correctly I doubt this would be a problem.  A heating element internal failure may well cause an earth leakage fault, too.  The kettle element failures, and one immersion heater failure, that we've had over the years have tended to trip the RCD from earth leakage.

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On one of the sunbeds we used to supply, the contactors had a habit of sticking on.  These were large commercial sunbeds, 7 to 9 kW ones.

We redesigned the power circuit to include 2 contactors in series to activate the that side of things.  This reduced the probability of the timer being unable to turn the lights off as the chance of two contactors sticking on was greatly reduced.

I have seen this done with relays in quite a few control systems, but probably less now with solid state relays.

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17 minutes ago, SteamyTea said:

On one of the sunbeds we used to supply, the contactors had a habit of sticking on.  These were large commercial sunbeds, 7 to 9 kW ones.

We redesigned the power circuit to include 2 contactors in series to activate the that side of things.  This reduced the probability of the timer being unable to turn the lights off as the chance of two contactors sticking on was greatly reduced.

I have seen this done with relays in quite a few control systems, but probably less now with solid state relays.

Being pedantic, that on it's own was not necessarilly an improvement. Contactor 1 cold stick and you would not know, until contactor 2 also stuck and the sunbed stayed on.

 

Such a circuit with two contactors is a recognised safety circuit used in machine controls, but the important addition is they are controlled by a safety relay that monitors the auxilliary contacts on each contactor and will not energise them if it detects that one of them is stuck on already.

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Yes you are right, I failed to mention that side of it as I was in a rush.

 

Purely out of academic interest, do solid state relays have this problem?

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7 minutes ago, SteamyTea said:

Purely out of academic interest, do solid state relays have this problem?

 

Yes and no.  Some are designed to always fail open, some cheaper ones can fail closed, it depends on the failure mode.  An over-current event tends to blow the bond wires, making the SSR go open circuit, a dV/dT failure tends to make the SSR fail closed, due to punch-through.  High reliability SSRs are supposed to fail open, and the very few SSR failures I've ever seen have all failed open, so my guess is they may be safer in this respect than conventional relays.

Edited by JSHarris

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And these days it is probably cheaper to use 1 decent and safe SSR than two or three mechanical relays.

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9 minutes ago, SteamyTea said:

And these days it is probably cheaper to use 1 decent and safe SSR than two or three mechanical relays.

 

Without a doubt.  The only caution I would give is that there are a large number of fake Chinese SSRs around, and they look near-identical to the brand name products that they are imitating.  One way to tell is the weight of them, as the fakes often aren't potted, so feel lighter, but unless you're familiar with what the real product is like you can easily be fooled by the Chinese fakes. 

 

Not a problem for a reputable brand of product, but it may be an issue if any repairs are carried out.

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