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EssexHebridean

Deleted_User said:

We have a 4 bed bungalow. A new boiler was fitted about 4 years ago - it's 34KW

This year we're (so far, and despite days of freezing weather; -5 last night and STILL not above freezing)  managing to heat the whole house comfortably with a 3kw woodburner.

If I run the woodburner flat out for 13 hours that's 13 x 3kw = 39kw.

But it's not running flat out - sometimes is spends a few hours damped down and burning slowly. Every so often I'll put a home made paper brick on it and leave it for an hour - so that it almost drops out of the green zone - before stoking it up again. 

The point is, and I don't really know what to make of it yet, my woodburner is running all day using less energy than my boiler would use in an hour.

The pattern of heating is different - more really cosy rooms with the woodburner, but more cooler areas in the house too. We definitely get up to colder rooms, but they quickly get EXTREMLY cosy. Swings and roundabouts - both work well...I prefer the woodburner. 

This isn't about cost as I get most of my wood for free. But I'm just trying to get my head around all the factors involved in switching to a more energy efficient way of running the house.

This isn't right though is it, as you boiler wouldn't be running flat out for 13 hours. My understanding is that the boiler will run at capacity for the first little while until the temperature in the house increases, then would cycle on and off as needed to maintain that temperature. Heating via a stove also of course is significantly more labour intensive, and wouldn't as a sole source of heating suit the average working household. In your situation it may well be more cost effective, but not based on "it costing less to run all day than your boiler would use in an hour".

daveyjp

Thinking a boiler uses 31kw all the time is like thinking a car engine uses all its power all the time, neither do. 

My boiler is currently on and ticking over to maintain circuit water at 55 degrees to maintain 18 degrees in doors.  It fires occasionally and uses 10- 20% of its 31kw capacity for a few seconds to do so.

FreeBear

Deleted_User said:

We have a 4 bed bungalow. A new boiler was fitted about 4 years ago - it's 34KW

This year we're (so far, and despite days of freezing weather; -5 last night and STILL not above freezing)  managing to heat the whole house comfortably with a 3kw woodburner.

If I run the woodburner flat out for 13 hours that's 13 x 3kw = 39kw.

A couple of points - Your wood burner may well have a nominal rating of 3KW, but "flat out", I would expect it to be producing quite a bit more heat than just 3KW.

My stove has a nominal output of 6.4KW, but will produce anything between 1.5KW and 9KW when using wood.

Your gas boiler, even although rated at 34KW, is probably not pumping that much out when running the central heating. The amount of heat that can be extracted from your boiler will be limited by the number & sizes of the radiators. If you have a smarter thermostat, it may well be modulating the boiler down to 7KW as the property gets up to temperature.

brewerdave

If we ran our woodburning stove for 13 hours every day , our woodstore would be empty in less than a week! And dry logs are getting more expensive by the hour ! You must have a very large barn !

alleycat`

We aren't using the gas CH this year. But out of interest (and in case we do things differently in future) can you expand on 'modulating the boiler down to 7kw as the property gets up to temperature'? That just went right over my head!

The output of your boiler is the theoretical maximum it will use when working "hard". In most instances it's "cycling" on and off to keep the "temperature" at the value set by the thermostat.

So as it gets closer to say a target of 19 degrees it will "slow down" the amount of gas it consumes and in effect use "less" - so it'll modulate down how hard it's running and how much fuel it's using (in essence using 7 as opposed to 34).

Think of it as changing from a 70mph road to a 30mph road as you get closer to home. At 70 you're getting to your destination faster but using "more" fuel to get there. You don't charge up to your front door at 70mph. You effectively modulate down to arrive at the destination. Your boiler does the same (not a perfect analogy but hopefully a rough visual idea).

In this instance the boiler and it's controls are intelligent enough to work it all out to arrive as requested in the most efficient manor.

alleycat`

You may also find this interesting (or not) but it discussed boiler flow / return temperatures.

https://www.theheatinghub.co.uk/articles/turn-down-the-boiler-flow-temperature

Olly_J

Deleted_User said:

When you heat the house from cold (say for example you've been at work all day) how much of the 34kw will be used and for how long? (ballpark figures).

It depends to a large degree on how many radiators you have, and what output they are rated at, and to an extent what flow temp you have it set to.

Obviously you've got to heat all the water up that is in the heating system first, so the boiler will be flat out for a period, but then once up to temp it will depend on how fast the heat can be output by the radiators

BUFF

Deleted_User said:

FreeBear said:

Deleted_User said:

We have a 4 bed bungalow. A new boiler was fitted about 4 years ago - it's 34KW

This year we're (so far, and despite days of freezing weather; -5 last night and STILL not above freezing)  managing to heat the whole house comfortably with a 3kw woodburner.

If I run the woodburner flat out for 13 hours that's 13 x 3kw = 39kw.

A couple of points - Your wood burner may well have a nominal rating of 3KW, but "flat out", I would expect it to be producing quite a bit more heat than just 3KW.

My stove has a nominal output of 6.4KW, but will produce anything between 1.5KW and 9KW when using wood.

Your gas boiler, even although rated at 34KW, is probably not pumping that much out when running the central heating. The amount of heat that can be extracted from your boiler will be limited by the number & sizes of the radiators. If you have a smarter thermostat, it may well be modulating the boiler down to 7KW as the property gets up to temperature.

Fair comment re the woodstove - but for most of the winter it's probably idling at 1kw for most of the day. Although yes - right now it's going full pelt.

We aren't using the gas CH this year. But out of interest (and in case we do things differently in future) can you expand on 'modulating the boiler down to 7kw as the property gets up to temperature'? That just went right over my head!

Is it a 34kW combi or a 34kW conventional or system boiler?
If it is a combi that 34kW rating is probably for hot water demand & it won't be rated at quite the same output for central heating (your installer may even have range rated it - limited it's output to likely demand).

Expanding on alleycat's posts most modern boilers can do 5:1 modulation but the very best/latest can do 10:1 or more - Viessmann even has a 35kW model that can do 19:1 (minimum output 1.8kW).
Again taking his car analogy you could think of it as pressing the accelerator to the floor to get up to your required speed from rest but once you get there you don't need to keep your foot hard to the floor, you just need enough accelerator to keep it at that speed.

ariarnia

it doesn't help that with a stove the heat output is set by the manufacturer and doesn't actually mean all that much. our 4.9kw stove has the same size firebox and baffle as several 8kw models. but less than 5kw is easier to install (so a selling point).  if we wnated our stove going 'full pelt' for 13 hours we'd be loading it with coal and it would give out a darn site more than 34kw. 

TESTING STOVES

Stoves in the UK are tested against British and European standards and while there is considerable flexibility with regards to the circumstances of each test they do give a useful indicator. For the record it is worth noting that there is flexibility on:-

The heat output at which the test will be carried out 
The amount of fuel being burned during the test
How often the stove is refuelled – although it is no less than every 45 minutes

The actual process is straightforward; the appointed tester will measure the temperature and carbon content of the flue gases. As the amount of fuel added to the machine is precise, these three elements are used to calculate the amount of heat and fuel which has been waste (i.e. not been retained within the stove) hence the efficiency calculation.

DIFFERENT MEASUREMENT CRITERIA

As we touched on above, there are certain elements of the efficiency rating test which can vary from stove manufacturers to stove manufacture. In some circumstances this can make comparisons between stoves manufactured by different companies not worthless but a little less meaningful. It is also worth noting that during the testing period the air supply (impacted using the stove vents) can also be manipulated although this is perfectly within the rules. In theory it is therefore possible for stove manufacturers to “hit” a certain level of efficiency by using their own settings within the measurement process protocol. However, their machines still need to hit this level, so they are true readings.

https://www.bowlandstoves.co.uk/blog/stove-advice-and-maintenance/measure-efficiency-wood-burning-stove/

modern boilers are more efficient than stoves. stoves can't make heat from no where. so what makes the heat is how much fuel of what type is in it. how quickly its burning. and how much heat is moving away from the stove into the room vs up the flu.

the same as with boilers its gas in hot water out and what matters is how many radiators of what size and how much heat is moving away from them into the room.

all of that is how the thing is used and house layout not boiler vs stove directly. 

Mstty

You must be lucky if your log burner is situated in the middle of your house for this to work. Otherwise the furthest away would be too cold for us (however people have different tolerances). What is the temperature in the room furthest away from your log burner and does it have the benefit of solar gain? All factors.

Going to have to say considering where you live in Scotland a single log burner not in the centre of the house burning at an estimated 1-2kwh max all day is unrealistic for most people. Firstly many won't have free wood and secondly other parts of the house would be too cold.

However nice experiment and you never know next year we might have to try the same depending on what cards are dealt.