Wasted heat - what can be done
Discussion
I notice many houses with combi boilers spewing out clouds of steam from their boiler and it's especially noticeable in this cold weather. Indeed my own cloud of steam is currently almost covering the garden. BTW boiler is serviced every year so there's nothing wrong with it.
But this steam is really hot to the extent that you can't put your hand in it. So a waste of energy to say the least. Can anything be done to harvest/use this heat rather than just venting it to the atmosphere?
But this steam is really hot to the extent that you can't put your hand in it. So a waste of energy to say the least. Can anything be done to harvest/use this heat rather than just venting it to the atmosphere?
If there is a significant quantity, does it not mean the boiler is turned up too high?
On mine at least the rads are supposed to extract a certain amount of heat, and when the boiler sees that this is not happening (for ex because the TRVs have closed), it actually turns down the burn rate itself.
On mine at least the rads are supposed to extract a certain amount of heat, and when the boiler sees that this is not happening (for ex because the TRVs have closed), it actually turns down the burn rate itself.
Simpo Two said:
That's heat from the hot water, not the combustion. (And if the rad didn't extact any heat they wouldn't be rads!)
No but like I said, when the TRV reaches its set point, it effectively switches that rad off. The boilers then sees minimal temp drop on the returned water, and runs combustion at a lower rate. As said condensing boilers do this and reduce the temperature of the steam - I have never tried but I would be surprised if you couldn't put your hand in the outlet of one.
If you can't, and you're so inclined, you could follow the principle of a combined cycle powerplant and run a steam turbine off the waste heat and generate some electricity.
If you can't, and you're so inclined, you could follow the principle of a combined cycle powerplant and run a steam turbine off the waste heat and generate some electricity.
Ideally condensing boilers will run at a far lower temperature than normal non-condensors, which traditionally have an 82/71 degree flow/return temperature.
A condensing boiler should really be run at about a 48/35 flow/return temp - but if it's been fitted onto an existing system, then your radiators won't give out enough heat.
Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
As for making use of the excess heat - I've never seen it done, simple as that! I'm sure you could rig up something, but as far as I'm aware there is nothing on the market.
When we had a gas condensing boiler fitted to our (wartime) house, with ancient rads, the first morning it went on, the neighbours called the fire brigade as they thought the house was going up in flames, there was that much steam. Add that to the fact that you have a cold morning, so there is a lot more steam (you've probably noticed this on the car exhaust, in cold weather), and you find a very steamy flue.
You can redirect it to somewhere else with a "plume kit" - but you will still get the steam and lost energy if you are running it at "traditional" temperatures.
This is also why condensing boilers work beautifully with UFH - it only wants a very low, constant temperature, so the condensing will take place much more efficiently. One I specced recently for a small public bar has a standard Worcester boiler and UFH throughout - you can't even see if it's running or not, unless it has just started to fire and isn't condensing yet!
A condensing boiler should really be run at about a 48/35 flow/return temp - but if it's been fitted onto an existing system, then your radiators won't give out enough heat.
Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
As for making use of the excess heat - I've never seen it done, simple as that! I'm sure you could rig up something, but as far as I'm aware there is nothing on the market.
When we had a gas condensing boiler fitted to our (wartime) house, with ancient rads, the first morning it went on, the neighbours called the fire brigade as they thought the house was going up in flames, there was that much steam. Add that to the fact that you have a cold morning, so there is a lot more steam (you've probably noticed this on the car exhaust, in cold weather), and you find a very steamy flue.
You can redirect it to somewhere else with a "plume kit" - but you will still get the steam and lost energy if you are running it at "traditional" temperatures.
This is also why condensing boilers work beautifully with UFH - it only wants a very low, constant temperature, so the condensing will take place much more efficiently. One I specced recently for a small public bar has a standard Worcester boiler and UFH throughout - you can't even see if it's running or not, unless it has just started to fire and isn't condensing yet!
Edited by guindilias on Wednesday 1st February 13:38
guindilias said:
Ideally condensing boilers will run at a far lower temperature than normal non-condensors, which traditionally have an 82/71 degree flow/return temperature.
A condensing boiler should really be run at about a 48/35 flow/return temp - but if it's been fitted onto an existing system, then your radiators won't give out enough heat.
Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
As for making use of the excess heat - I've never seen it done, simple as that! I'm sure you could rig up something, but as far as I'm aware there is nothing on the market.
When we had a gas condensing boiler fitted to our (wartime) house, with ancient rads, the first morning it went on, the neighbours called the fire brigade as they thought the house was going up in flames, there was that much steam. Add that to the fact that you have a cold morning, so there is a lot more steam (you've probably noticed this on the car exhaust, in cold weather), and you find a very steamy flue.
You can redirect it to somewhere else with a "plume kit" - but you will still get the steam and lost energy if you are running it at "traditional" temperatures.
This is also why condensing boilers work beautifully with UFH - it only wants a very low, constant temperature, so the condensing will take place much more efficiently. One I specced recently for a small public bar has a standard Worcester boiler and UFH throughout - you can't even see if it's running or not, unless it has just started to fire and isn't condensing yet!
Interesting post. We fitted a Worcester 35 condensing system boiler, big rad (oversized for the rooms) and UFH in the kitchen/conservatory, which is a large (50m2) area. It also has to run a Megaflo.A condensing boiler should really be run at about a 48/35 flow/return temp - but if it's been fitted onto an existing system, then your radiators won't give out enough heat.
Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
As for making use of the excess heat - I've never seen it done, simple as that! I'm sure you could rig up something, but as far as I'm aware there is nothing on the market.
When we had a gas condensing boiler fitted to our (wartime) house, with ancient rads, the first morning it went on, the neighbours called the fire brigade as they thought the house was going up in flames, there was that much steam. Add that to the fact that you have a cold morning, so there is a lot more steam (you've probably noticed this on the car exhaust, in cold weather), and you find a very steamy flue.
You can redirect it to somewhere else with a "plume kit" - but you will still get the steam and lost energy if you are running it at "traditional" temperatures.
This is also why condensing boilers work beautifully with UFH - it only wants a very low, constant temperature, so the condensing will take place much more efficiently. One I specced recently for a small public bar has a standard Worcester boiler and UFH throughout - you can't even see if it's running or not, unless it has just started to fire and isn't condensing yet!
Edited by guindilias on Wednesday 1st February 13:38
The plumbers have jacked the boiler temp up when installing it. What should it be running at, given it has to heat hot water once a day if needed, run the UFH pretty much constantly, and heat up a couple of rooms on demand each day (Honeywell Evohome, so rooms are only heated when needed)?
Sorry for the dumb questions - wish I had known more when speccing the system...
If your rad is sufficiently "oversized", you can try turning down the temperature by say 10 degrees each day, and see if it still heats the place. Your Megaflo will eventually settle to roughly the same temp as the flow temp (or about halfway between the flow/return temps, depending on how it is plumbed in) - which is normally just about right for a shower.
It WILL take longer to heat the room, obviously, as the rad is colder - but you can offset this by having the heating come on earlier.
The other issue, which isn't really an issue in a domestic house, is that the water in the Megaflo should ideally be heated to 60 degrees each night, to prevent Legionella growing - but I don't think I have ever heard of this happening in a domestic house. It is mainly an issue in offices, older shopping centres with "cooling towers" for the a/c, and other large systems.
The turnover of hot water in a domestic property is generally high enough that legionella is not a hazard - I have never even come across a case of Legionnaire's disease in a domestic property, but it may be something to be aware of.
In offices, etc, I would have the heating programmed to heat the hot water to 60 degrees each night, for a short period - this kills off all Legionella that could be present. But a lot of our offices are specced to have a tank large enough to hold enough hot water for a full 24h (sometimes 48h) use, so there is a very minor risk there. It's largely a "defence" precaution, so that if the tank is kept at 40 degrees or so over a fortnight's holiday, the bacteria will be killed each night.
Plumbers fit condensors because
a)They are legally obliged to, for efficiency
b)Hence that is pretty much all that is on the market these days.
They crank the heat up because
a)Customers want instant heat when they turn on the heating
b)Most are being fitted to an existing system, which needs the high temps to keep the place warm
c)A lot of plumbers simply don't understand the principle of a condensing boiler, and the energy savings it can produce - so they set it to the "old" flow and return temperatures - thus reducing the effective efficiency of an "A" rated boiler to a "C or a "D" - but hey, they fitted an "A" rated boiler for efficiency, and they aren't paying your gas bills...
It WILL take longer to heat the room, obviously, as the rad is colder - but you can offset this by having the heating come on earlier.
The other issue, which isn't really an issue in a domestic house, is that the water in the Megaflo should ideally be heated to 60 degrees each night, to prevent Legionella growing - but I don't think I have ever heard of this happening in a domestic house. It is mainly an issue in offices, older shopping centres with "cooling towers" for the a/c, and other large systems.
The turnover of hot water in a domestic property is generally high enough that legionella is not a hazard - I have never even come across a case of Legionnaire's disease in a domestic property, but it may be something to be aware of.
In offices, etc, I would have the heating programmed to heat the hot water to 60 degrees each night, for a short period - this kills off all Legionella that could be present. But a lot of our offices are specced to have a tank large enough to hold enough hot water for a full 24h (sometimes 48h) use, so there is a very minor risk there. It's largely a "defence" precaution, so that if the tank is kept at 40 degrees or so over a fortnight's holiday, the bacteria will be killed each night.
Plumbers fit condensors because
a)They are legally obliged to, for efficiency
b)Hence that is pretty much all that is on the market these days.
They crank the heat up because
a)Customers want instant heat when they turn on the heating
b)Most are being fitted to an existing system, which needs the high temps to keep the place warm
c)A lot of plumbers simply don't understand the principle of a condensing boiler, and the energy savings it can produce - so they set it to the "old" flow and return temperatures - thus reducing the effective efficiency of an "A" rated boiler to a "C or a "D" - but hey, they fitted an "A" rated boiler for efficiency, and they aren't paying your gas bills...
guindilias said:
A condensing boiler should really be run at about a 48/35 flow/return temp - but if it's been fitted onto an existing system, then your radiators won't give out enough heat.
Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
Radiator ratings, and the sizing calculators various firms make available, usually assume the average temp through the rads is 70C (flow 75C return 65C). This allows the use of pretty tiny radiators.Plumbers ignore this, (because they have to fit condensing boilers), and then just wind up the flow temperatures - so not a lot of condensing takes place, which is why you see the steam.
If it was a "perfect", fully condensing system, boiler, rads, flow/return temperature et al, you wouldn't see much steam, if any. But that's pretty rare - even in newbuilds, bigger rads cost more and are harder to fit in smaller spaces, so not much condensing takes place.
Dropping to the temps you talk about would mean much (like 3x) bigger rads would be needed.
I had this "discussion" with the guys replacing the whole system in my daughter's house. Fully accreditated WB firm, fit lots of systems etc etc, but they just didn't get it. They did fit larger rads but I think only to shut me up. The daft thing is they suggested fitting the Worcester Wave controller and one of the things it does is it takes over the boiler stat (which they thought should be on max) and tries to keep the boiler in condensing mode.
Harry Flashman said:
Another silly question - is it not equally valid to run a higher temperature but heat for less time, as the room warms up more quickly?
It is, but the condenser in your boiler won't condense much, hence it'll blow out the steam, wasting heat. The whole idea of a condensing boiler is that it reclaims as much of that heat as possible and puts it back into the system, but the fins on it will not be cool enough for the steam to condense fully if it is banging out full temperature steam - most of it will just blow right past, hence the plume. If you have a modulating boiler that can be set to run at full crack for x minutes, then return to a lower flow/return temperature, that would be grand - but this is generally only available on large boilers.
Exactly! Even the "Plumber's handbook" in my office still wants you to calculate rad and pipe sizes at 82/71, because for about 40 years it's been "the norm".
Bigger rads, much bigger rads, or UFH let the boiler run in full condensing mode all the time (except if you do need an "overnight boil" for legionella, or a quick blast at full power in the morning to get the house warm), and that is what the Sedbuk "A" rating will be based on.
Trying to trick the boiler, or just running it at a far higher temperature, results in bucketing heat out the flue and sending your gas bills up.
I've been an energy consultant alongside my main role as Mechanical design engineer for many years, and a lot of plumbers still work by what they were taught - which with modern technology boilers and controls, is simply wrong!
Bigger rads, much bigger rads, or UFH let the boiler run in full condensing mode all the time (except if you do need an "overnight boil" for legionella, or a quick blast at full power in the morning to get the house warm), and that is what the Sedbuk "A" rating will be based on.
Trying to trick the boiler, or just running it at a far higher temperature, results in bucketing heat out the flue and sending your gas bills up.
I've been an energy consultant alongside my main role as Mechanical design engineer for many years, and a lot of plumbers still work by what they were taught - which with modern technology boilers and controls, is simply wrong!
Sheepshanks said:
Radiator ratings, and the sizing calculators various firms make available, usually assume the average temp through the rads is 70C (flow 75C return 65C). This allows the use of pretty tiny radiators.
Dropping to the temps you talk about would mean much (like 3x) bigger rads would be needed.
I had this "discussion" with the guys replacing the whole system in my daughter's house. Fully accreditated WB firm, fit lots of systems etc etc, but they just didn't get it. They did fit larger rads but I think only to shut me up. The daft thing is they suggested fitting the Worcester Wave controller and one of the things it does is it takes over the boiler stat (which they thought should be on max) and tries to keep the boiler in condensing mode.
Dropping to the temps you talk about would mean much (like 3x) bigger rads would be needed.
I had this "discussion" with the guys replacing the whole system in my daughter's house. Fully accreditated WB firm, fit lots of systems etc etc, but they just didn't get it. They did fit larger rads but I think only to shut me up. The daft thing is they suggested fitting the Worcester Wave controller and one of the things it does is it takes over the boiler stat (which they thought should be on max) and tries to keep the boiler in condensing mode.
guindilias said:
Harry Flashman said:
Another silly question - is it not equally valid to run a higher temperature but heat for less time, as the room warms up more quickly?
It is, but the condenser in your boiler won't condense much, hence it'll blow out the steam, wasting heat. The whole idea of a condensing boiler is that it reclaims as much of that heat as possible and puts it back into the system, but the fins on it will not be cool enough for the steam to condense fully if it is banging out full temperature steam - most of it will just blow right past, hence the plume. If you have a modulating boiler that can be set to run at full crack for x minutes, then return to a lower flow/return temperature, that would be grand - but this is generally only available on large boilers.
I think my boiler is a modulating one - Worcester Greenstar 35CDi Classic system boiler. Hoping that this will help. Will do some experimentation...
so as I'm about to get a new heating system I may as well ask as it's a minefield.
I'll have ufh downstairs, rads upstairs and an unvented cylinder
for a 5mx4m bedroom 1st online calc says 2300BTU, second 3200BTU.
1200 600 single convector is 4000BTU so I thought that would suffice as the consensus seems to be overspec
I'm guess the rad BTU is for a large temp difference, how do you size them so that everything works efficiently?
I'll have ufh downstairs, rads upstairs and an unvented cylinder
for a 5mx4m bedroom 1st online calc says 2300BTU, second 3200BTU.
1200 600 single convector is 4000BTU so I thought that would suffice as the consensus seems to be overspec
I'm guess the rad BTU is for a large temp difference, how do you size them so that everything works efficiently?
Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff