Deleted

Mstty

That didn't answer my question but think I stumbled across another thread of yours which states 14oC for outer rooms and 20oC for main and hall others close by 17oC?

For many that's just too low so to try and compare that to a Gas CH 34kwh boiler in a modern house I suspect it would hardly fire up at all 

Quote from your post on fans moving heat about

We have a single 3kw woodburner in the lounge. That's our sole source of heat (of course, we have CH and all sorts of other fires but woodburner is all we're using). Our house is toasty warm with main areas heated to 20+ degrees. Every part of the house gets heated to 14 degrees minimum each day. Rooms where we're active (kitchen, bathroom, office get heated to 17 degrees). 

The key thing is, the rooms aren't heated to these temps all day. We use fans to move the heat around the house so that by the time we want to use a room it's at the ideal temperature. Because every room reaches at least 14 degrees every day it's keeping everything safe from cold/dampness damage and regardless of weather outside temps never get too low indoors.

Mstty

With the hybrid approach we are doing this with our ASHP system right now. I refuse to alter the weather compensation curve and that gets the house up to a certain temp if we want more light the log burner and open all doors.

I do like your idea of using fans though to distribute to the furthest corners.

InvertedVee

It's a combi. I'll look into the possibilities you've suggested. 

It's a worcester Bosch - something to do with 'greenstar' from memory.

Hi @scarter, I've got a Worcester Bosch Greenstar 30i combi boiler too. They're really good.

It's rated at 30kW max for heating water and 24.6kW max for central heating. But that's its peak power. I have limited the maximum heating output to 8kW ('range-rating') and it modulates down to 6kW. But it only burns gas for part of the cycle because after the water reaches the set flow temperature (48C) the boiler continues to pump it around the system until it needs heating again (about 30C). So my boiler gently burns gas on and off throughout the day (far more 'off' than 'on').

I took meter readings yesterday and this morning as I was curious how much it would use in this chilly weather. It was -5C here last night. The boiler used 40kWh to keep the whole 3-bed detached at 19C-20C between 6am and midnight and a little bit of hot water. We heat all rooms to this temperature. Averaging out the 40kWh over the 18 hours gives an average of 2.2kW of gas input. Allowing for 90-95% efficiency means that its output was 2kW, which is about right for a well-insulated smaller home. This is the figure to compare with your 3kW wood-burner.

I paid £3.13 for yesterday's heat. But I have to admit, my white box won't look as nice as your wood-burning stove!

Your hybrid idea might be best achieved by running the gas boiler in the daytime and the stove for the evenings. It sounds like your thermostat in the hall is warmed by the stove in the lounge when the doors are open. Every home is different - enjoy the experiments.

ariarnia

Deleted_User said:

ariarnia said:

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. 

The point is, a 3kw stove is successfully heating a home in sub-zero temperatures - a home deemed to need a 34kw boiler to run the CH system. 

That's what I find interesting.

Boilers are designed to heat houses in a way that was practical when gas was cheap, abundant and we weren't facing net zero targets..

as others have said your house (most houses) probably doesn't need a 34kw boiler (it probably wasnt based on any assessment of your property) and it might never get up to that point. the same as your stove might often be kicking out more than 3kw. that's why knowing if your boiler is a modern one or not matters (because if it modulates then it might only be actually running at 4kw https://www.hvpmag.co.uk/Boiler-oversizing-has-become-an-issue-explains-Martyn-Bridges/11564) 

the advantage of the boiler is with your stove you need to move the heat from one place to another. so the room with the stove will always need to be hotter than the rooms away from the stove that you want to heat. if you dont want that room hot then that's 'wasted' energy (if you want a bedroom to be 17 for some reason then the lounge and path to that bedroom will have to be something like 20+ to let you push the heat along). the boiler is easier to control where the temp goes and is more efficient both in burning the gas and directing the heat in the system (if you have valves on the rads)

but even when a bigger boiler is needed its normally to heat fairly large amounts of hot water quickly not for heating. how long would it take for you to heat water for a bath on your stove? we can just about fit three kettles on the top of ours so maybe we could do it in half an hour if we didn't want the bath to hot (and didn't mind chucking a huge amount of steam into our living space where theres not an extractor fan). which isnt to bad and definitely an option if there was a long power cut or something. but is hardly the hot water on tap in a minute you get with modern boilers.

answer for you would be to not have a bath (or not have a hot bath) and have a solar shower or some alternative. but then its not an apples to oranges comparison. both are good but good for differnt things. iyswim?

BUFF

Deleted_User said:

BUFF said:

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.

It's a combi. I'll look into the possibilities you've suggested. 

It's a worcester Bosch - something to do with 'greenstar' from memory.

And again, based upon what you've said about getting up to required speed - it's heating the house from cold with gas CH that's going to really cost - especially with a 34kw boiler! 

The 34kW boiler was almost certainly chosen based on anticipated hot water demand, not CH demand.
If you do decide to run it at all then presuming that you would also want to do so in the most efficient manner what is the flow temperature set to?

I think that you can be fairly confident that it won't run at 34kW for heating demand but  what controls do you have (programmer/room 'stat/TRVs)? 

Air pollution may not be an issue for you where you are but wood burners are not a solution for urban areas https://www.theguardian.com/environment/2022/dec/08/eco-wood-burners-produce-450-times-more-pollution-than-gas-heating-report

BUFF

Deleted_User said:

BUFF said:

Deleted_User said:

BUFF said:

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.

It's a combi. I'll look into the possibilities you've suggested. 

It's a worcester Bosch - something to do with 'greenstar' from memory.

And again, based upon what you've said about getting up to required speed - it's heating the house from cold with gas CH that's going to really cost - especially with a 34kw boiler! 

The 34kW boiler was almost certainly chosen based on anticipated hot water demand, not CH demand.
If you do decide to run it at all then presuming that you would also want to do so in the most efficient manner what is the flow temperature set to?

I think that you can be fairly confident that it won't run at 34kW for heating demand but  what controls do you have (programmer/room 'stat/TRVs)? 

Air pollution may not be an issue for you where you are but wood burners are not a solution for urban areas https://www.theguardian.com/environment/2022/dec/08/eco-wood-burners-produce-450-times-more-pollution-than-gas-heating-report

  
I plan on running it for 15 minutes one morning. So I'll find out exactly what it uses to heat the house from very cold. Will it use more power to heat a bowl of water for washing up (boiler doesn't know whether you're running a bath or washing up)  than it does to pump hot water through the entire CH system? I doubt it somehow as the CH heats the house from cold VERY quickly. But I'll report back when I know.

That may depend upon how you use it e.g. does it have a preheat & do you leave it on or switch it off? It's unlikely that your CH system has a smaller capacity than you would be heating for a washing up bowl of hot water so it almost certainly won't use more power for that than pumping hot around your entire CH system.  But if all you want is enough hot for a bowl of water use a kettle :P

BUFF

Deleted_User said:

BUFF said:

Deleted_User said:

BUFF said:

Deleted_User said:

BUFF said:

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.

It's a combi. I'll look into the possibilities you've suggested. 

It's a worcester Bosch - something to do with 'greenstar' from memory.

And again, based upon what you've said about getting up to required speed - it's heating the house from cold with gas CH that's going to really cost - especially with a 34kw boiler! 

The 34kW boiler was almost certainly chosen based on anticipated hot water demand, not CH demand.
If you do decide to run it at all then presuming that you would also want to do so in the most efficient manner what is the flow temperature set to?

I think that you can be fairly confident that it won't run at 34kW for heating demand but  what controls do you have (programmer/room 'stat/TRVs)? 

Air pollution may not be an issue for you where you are but wood burners are not a solution for urban areas https://www.theguardian.com/environment/2022/dec/08/eco-wood-burners-produce-450-times-more-pollution-than-gas-heating-report

  
I plan on running it for 15 minutes one morning. So I'll find out exactly what it uses to heat the house from very cold. Will it use more power to heat a bowl of water for washing up (boiler doesn't know whether you're running a bath or washing up)  than it does to pump hot water through the entire CH system? I doubt it somehow as the CH heats the house from cold VERY quickly. But I'll report back when I know.

That may depend upon how you use it e.g. does it have a preheat & do you leave it on or switch it off? It's unlikely that your CH system has a smaller capacity than you would be heating for a washing up bowl of hot water so it almost certainly won't use more power for that than pumping hot around your entire CH system.  But if all you want is enough hot for a bowl of water use a kettle :P

It's just instant hot water - turn on a tap and hot water comes out. (a bowl full, a bath) turn off the tap and the boiler goes off.

Turn on the central heating and hot water pumps through the radiators.

We use the woodburner to heat water for washing up for free - but I was just using that example to make the point that the boiler doesn't know whether you're running a bowl of water or a bath. It seems to me unlikely that it would use more power to heat water (for the taps) than it uses to heat water for the central heating system. As I say, when the CH is completely cold the radiators are warm within about a minute of CH going on. So I think the boiler probably does run at full pelt when starting up from cold at least.

But I'll find out over the coming days. I'll run it for 15 mins when the house is cold and see how much it uses.

It will do from cold but it won't run at 34kW as it is only rated at 30kW for CH output (& that is assuming that your installer didn't range rate it) & once it reaches operating temperature it will start modulating down to probably ~6 or 7kW.
Similarly the boiler does know by the flow rate of hot water demanded & will react accordingly.

I have looked up the manual & it does have a preheat function for hot water so if you wanted to save a little gas you could switch that to ECO (this would however give you a slight delay from opening your tap & hot water issuing from it - what I do is run in ECO 99% of the time & when I know that I am going to need hot water I take it off ECO slightly beforehand to allow preheat, then switch back once I have used whatever it was)

70sbudgie

When we replaced our combi boiler with a system boiler, it reduced from ~30kW to ~24kW. I questioned this because one of the reasons for moving away from a combi boiler was because it didn't perform well enough for us, so I didn't want to install a new undersized boiler. The heating engineer explained that the higher rating is so the boiler can provide instant hot water at the same time as heating. So if you only ever use one or the other, you wont need the full rating and why the rating was reduced when taking the instant hot water out of the equation. The gas consumption seems to indicate that the boiler only works hard for 30-60mins from cold and then scales down. I've yet to work out whether this is due to modulation or reduced cycling.

How hard your boiler works will be dependent on what settings you have it on - a modern boiler will have different DHW and CH temps. The higher the temp, the quicker the CH / DHW will heat up, but the more gas it will burn. 

The boiler may not know if you're running a bath or washing up bowl, but it will work to produce hot water at the temperature you've set for as long as you leave the tap running. 

We have a woodburner and although I don't think we use ours anywhere near as much as you, I have set our system up as hybrid. The radiators in the room where the burner is are turned off with the TRVs (so in reality, on the frost protection settting). And the CH is set to come on in the morning for a couple hours, then the programmable room thermostat reduces the temperature for the rest of the day so we're only using the residual heat from the morning burst. The temp on the boiler is set a smidge under 'eco' (I've always guessed this is it's most fuel efficient temp.) We do have UFH in a concrete floor in one room, which acts as a heat store that lasts all day. If the room with the wood burner feels cold, we light the fire earlier in the day, but if we're not in there to notice the temperature, the fire stays unlit. And I have a couple high efficiency electric heaters that I use to heat me, either when WFH, or if I don't plan to be in the room with the woodburner long enough to justify lighting it.

The main bedroom is the one furthest from the woodburner and is probably the only one that never benefits from its heat. But I actually prefer to sleep in a cooler room with extra blankets if needed. My OH isn't unhappy enough yet to learn how to use the heating controls.

What sort of fans do you use to move hot air around your house? Do you use a stove top fan?

InvertedVee

@Deleted_User Wow, 14.6kW for an hour is certainly a blast!

InvertedVee

Deleted_User said: ...I took a quick look at the boiler display (haven't looked at the manuals yet) and it said 'eco' and 64 degrees (no idea yet what that means).

64C is your boiler flow temperature. It's set well outside the range for efficiency and it won't be running in condensing mode. But if you're going down the short hot blast route I guess you don't have much choice but to set it so high.