a cautionary tale

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a cautionary tale

This topic has 15 replies, 11 voices, and was last updated 6 September 2022 at 20:06 by Steve Skelton 1.

Viewing 16 posts - 1 through 16 (of 16 total)

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28 July 2022 at 00:07 #607596
duncan webster 1
Participant
@duncanwebster1
I have to record my meter gas and electricity readings every month and send them to the supplier. Being a Yorkshireman I plot them in a spreadsheet. The gas goes quite high in winter and low in summer as you'd expect, but this summer the gas hasn't gone as low as usual, which is odd because the boiler has been on the blink, so all hot water has been from kettle or electric shower, only the cooker on gas. Decided to investigate. Read the meter last thing at night and first thing in the morning, using no gas in between. First night I used 0.3 cu.m, so I isolated the gas fire in the room we hardly ever use. The second night I used 0.001 cu.m, which I reckon is the pilot light on the other much newer gas fire in the kitchen. I've turned that off as well now. The old fire has be be partly dismantled to light the pilot, and it doesn't have a flame failure device on the pilot, although it won't fire up the main jets if the pilot isn't on. I reckon the gas valve must be leaking, but not enough to keep the main jets burning, and as it is a semi sealed system and gas is lighter than air the gas just goes up the chimney, I've certainly never smelt gas in the room. When the gasman tests the system for leaks he has to isolate the old fire, so a leak doesn't show up

The moral of this story is every now and again check your meter overnight, and get rid of any appliances which don't have flame failure device on the pilot. The fire in question is probably 35+ years old, so it owes me nothing

Edited By duncan webster on 28/07/2022 00:09:56
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28 July 2022 at 00:07 #36927
duncan webster 1
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@duncanwebster1
28 July 2022 at 08:49 #607604
John Haine
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@johnhaine32865
In my misspent youth as a consultant I did a project on microprocessor controlled gas cookers. A problem was that people like gas cookers because they work through a power cut, so it would have to be battery powered to keep it going. Ideally it wouldn't have a mains connection at all. I wondered how much power one might extract from a pilot light, apparently there was a British Standard that specified the minimum power of a pilot as the BThU equivalent of 500 Watts! 1% efficiency in a thermoelectric generator would yield 1 amp at 5 volts.
28 July 2022 at 09:32 #607609
Brian Wood
Participant
@brianwood45127
On our old gas boiler that used a pilot light to ignite the main burner and keep the flame failure device operative at the same time I measured the demand that piece of kit was making on the gas supply during a holiday break with nothing else running.

The figure was a rather surprising 500 kWh per year.

Brian
28 July 2022 at 10:38 #607616
Brian Wood
Participant
@brianwood45127
I should have added that to put that into perspective with the average demand on the boiler at the time it represented about 2.5% of that value. Not a lot maybe but as Tesco are so keen to tell us " Every little helps"

Brian
28 July 2022 at 10:42 #607617
SillyOldDuffer
Moderator
@sillyoldduffer
Posted by duncan webster on 28/07/2022 00:07:27:

… I reckon the gas valve must be leaking, but not enough to keep the main jets burning, and as it is a semi sealed system and gas is lighter than air the gas just goes up the chimney, …

Duncan's hypothesis sounds good to me, apart from the comment 'gas is lighter than air'. Coal gas was lighter than air, natural gas isn't. Pedantic point, but I expect the leaked gas is carried away up the chimney because warm air inside the house is lighter than the cold air outside.

Chimneys are heat engines obeying the same thermodynamic laws as steam and internal combustion engines. John's comment that a pilot light is about 500W, which is a lot, made me wonder what the power consumption of Duncan's chimney might be?

Let's see if I can get the estimate right:
- Duncan reports 0.3 cubic metres being wasted overnight.
- Assuming natural gas is methane, 0.3 cubic metres weighs 0.17kg
- 0.17kg (weight on earth) is 1.667 Newtons (actual mass)
- Assume overnight is 28800 seconds (8 hours)
- Assume Duncan's chimney is 10 metres tall (distance)
- Power is the rate of transfer of energy. The transfer is moving the mass of gas over a distance (up the chimney), and the time taken is also known.
- Watts = Newtons * distance / time
W = 1.667N * 10m / 28800s

or 0.0006W

The estimate is too optimistic because the sum assumes Duncan's Chimney is 100% efficient and it will be much less. In comparison the very best full-size steam locomotives achieve about 5% efficiency. If Duncan's chimney were that good, it would need a heat source of about 0.01W to remove the gas.

Modelling steam engines is great fun, but we skip over most of the engineering. A full-size mill-engine was carefully designed to get the most useful energy out of a given weight of steam. Boilers were carefully designed to maximally convert heat from combustion into steam. The furnace and tubes were designed to burn fuel efficiently, wood requiring very different conditions from hard coal.

Last, but not least, was the Chimney. These were tall, partly to disperse concentrated pollution well above the population, but also to force the draught through the furnace. Locomotives use waste steam for the same purpose, but a blast pipe is inefficient compared with a tall chimney, which allows energy to be recovered from 'waste' steam with a condenser, feed-water heating, or process work.

As tall chimneys are expensive to build, their design must have been matched to the size of the furnaces. Anyone know how efficient chimneys and blast pipes are as heat engines? I'm guessing low compared with a blowing engine, because they replaced furnaces and tall-chimneys as a way of ventilating coal-mines.

Dave
28 July 2022 at 11:21 #607621
Steve Skelton 1
Participant
@steveskelton1
Dave, Natural gas IS lighter than air, I think you are confusing it with bottled gas, propane and butane, which are both heavier than air.

Steve
28 July 2022 at 13:02 #607640
Martin Johnson 1
Participant
@martinjohnson1
As tall chimneys are expensive to build, their design must have been matched to the size of the furnaces. Anyone know how efficient chimneys and blast pipes are as heat engines? I'm guessing low compared with a blowing engine, because they replaced furnaces and tall-chimneys as a way of ventilating coal-mines.

Dave

A blast pipe (or jet pump) has a maximum effy. Of arond 30 to 33%. Typical locomotive configuration will be well bolow 25%. Cant help with natural draught chimney.

Martin
6 September 2022 at 13:28 #612567
John Doe 2
Participant
@johndoe2
By coincidence, I have also recently started recording daily, our gas and electricity meter readings. Easy to do by standing in front of the meter boxes and putting the readings straight into a spreadsheet on my phone and programming the 'sheet to calculate the daily usage.

This is to see how much energy we use daily and if we can economise any further.

Anyway, we spent the last two days away so I turned the gas boiler timer/programmer to off. We left about five table lights on timers.

I have just checked the meter readings today, and both meters show a significant amount of energy consumed, even though we were not in the house at all. The meters show about 0.3 m3 of gas was consumed per day, and about 2.5kWh of electricity per day.

At the moment, we normally use about 0.70 m3 of gas per day, but no gas device was used while we were away, so something is clearly wrong – we must have a gas leak downstream of the gas meter, (but no smell of gas). Only four candidates; the boiler, the gas fire, the hob – or an actual pipe leak somewhere.

The electricity usage also seems high, but might be about right – 5 lights at about 40W each, on and off for 12 hours overnight, plus the fridge/freezer, the TV, recorder and computer on standby, the Wi-Fi hub and phone, and the house alarm. Time to get more LED bulbs and don't leave things on standby.

Makes you think though…..I am glad I started recording the meter readings – especially the gas – otherwise I would have just assumed my gas bill was right, even though I obviously have a sizeable leak, which I need to locate. Could be a leaky gas valve.

.

Edited By John Doe 2 on 06/09/2022 13:40:29
6 September 2022 at 15:38 #612579
Steambuff
Participant
@steambuff
Does your boiler or fire have a pilot light which is on all the time regardless of the timer settings?
6 September 2022 at 16:00 #612582
bricky
Participant
@bricky
when working on an extention I damaged a gas pipe under the kitchen floor .After repairing the pipe and relaying the floor the extention was completed.The gas board checked the meter before this and it was according to them sound.We ran an external gas pipe to a new gas fire arounnd the bungalow.The annual check from the gas board plugged off the meter as there was a gas leak, I chopped up the floor and the joint that was repaired was found to be sound.The gas fitter I had used then set about blanking off all our new pipework at the meter and shut everything off,but the meter was running this proved there was a leak under the floor and no one had smelt gas.The gas board clearly had not been doing their original test properly.We then blanked off the pipe under the floor and ran new external pipework to the rest of the bungalow.The meter was no longer running, So a leak is possible under the floor without you knowing.

Frank
6 September 2022 at 16:46 #612586
Frances IoM
Participant
@francesiom58905
nearly 40 years ago when I moved into a late Victorian house with cellar plus a large but unusable space under the floor of the living room – I needed the gasboard to remove unwanted pipes in the kitchen fed by steel pipes from the meter in the cellar – the statutory leak check found a slight leak in a corner joint that ran into this void – I couldn’t smell gas but there always was a strange smell in this room that I put down to the previous owner’s cat peeing on the carpet
6 September 2022 at 17:28 #612590
John Doe 2
Participant
@johndoe2
No, no permanent pilot light – the boiler is a Potterton Netaheat, which electronically lights the pilot light every time the boiler comes on. The gas fire's pilot light is also only on if you are actually using the fire, but we will be taking that fire out and replacing it with a wood-burner with a back-boiler.

But the boiler gas valve might have a small leak, or the gas fire valve, or a ring on the gas hob. Or a pipe under the floor to any of these. Just moved into this house, and I don't know, nor have schematics of where the gas pipes run. They are all behind plasterboard walls or in concrete floors. I hope they didn't put bare copper pipe into the concrete – that would cause the pipe to form holes.

I am facing having to rip open plaster board walls to trace the gas pipes, and probably need a natural gas 'sniffer' probe. Is there such a thing I could buy or are they only pro devices costing thousands? Maybe I could rent one.

Edited By John Doe 2 on 06/09/2022 17:31:04
6 September 2022 at 17:54 #612596
Dave Halford
Participant
@davehalford22513
If you have a 30's house and still use part of the old steel pipe system near the meter it may still have the old town gas water drain, which can pin hole.

All your appliances should have a gas tap so you can isolate them one at a time from your piping.

Doing much else involves breaking the rules.
6 September 2022 at 19:48 #612610
Steambuff
Participant
@steambuff
If you think you have a leak, then you should call British Gas and explain, I'm sure they have to equipment to trace a leak.
6 September 2022 at 20:06 #612613
Steve Skelton 1
Participant
@steveskelton1
John, I suggest you should call a Gas Safe registered plumber and ask them to do a gas tightness check. They will put a manometer on your meter, turn off the supply at the meter and monitor the pressure in the house gas line. If it drops with all appliances isolated that will tell tou that you have a leak in the pipework system. I would caution against contacing the supplier as they will insist you turn off the gas at the meter and not allow you to use any gas appliances until all the system has been checked. The gas tightness check will take about 5 minutes to perform.

Steve

Edited By Steve Skelton 1 on 06/09/2022 20:07:33
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