From my O level physics and 15 years working on chemical plants, this is what I understand about condensing boilers.
When a hydrocarbon such as natural gas burns it produces carbon dioxide and water vapour. hard fact.
The latent heat of vapourisation is the large amount of energy it takes to turn a liquid into a gas / vapour and the similar amount of energy released when a gas / vapour turns into a liquid. hard fact.
The following are what seem like facts to me
So, in a condensing boiler, if you provide a heat exchanger surface cold enough to cool the combustion gases below the dew point, the water vapour condenses on this surface, transfering the latent heat of vapourisation to the heat exchanger and the heating water it contains rather than it being lost out of the exhaust. Water from the combustion leaves the boiler as a liquid, not a gas
About 54'c is the dew point when condensation starts, more water vapour will condense and more latent heat will be extracted from the exhaust gas at even lower temps.
The temperature of the heat exchanger is governed by the temperature of the water returning to the boiler from the radiators / hw tank / hw heat exchanger. The cooler the return water, the better.
These are some bits I don't understand / agree with
Boiler manufacturers and plumbers say that a large plume is a sign that the boiler is condensing well, BUT the plume is made of water vapour condensing when it hits the cold outside air so it has left the boiler as a gas with all of its latent heat so I can't see how a large plume is a sign of good condensation in the boiler, very much the opposite I would say.
Most conventional or system (non combi) boilers have only one temperature control so if the temperature is set high enough to heat the tank, then the return water from the heating is to hot for condensation to occur and efficiency is reduced especially if trv's or zone valves have reduced the amount of the system being heated.
If the boiler flow temperature is reduced to ensure that the return temp is cool enough to condense the water vapour, the tank will take forever to heat up, if at all, and undersized radiators may not emit enough heat. Does anyone else see this or have I missed something??
I designed the heating system that feeds our two holiday flats and our own house, 300sqm of floorspace, 7 bedrooms, 4 bathrooms and accommodation for up to 15 people with two unvented cylinders and a mixture of directly fed ufh and oversize radiators running at ~50-60' supply temp. All fed from a single 24kw Ravenheat condensing system boiler that had two output controls.
Unfortunately it was not noted that the wrong boiler had been supplied untill the gas fitter had mounted it on the wall (and it had laid in the shed for 3 months) so not returnable. some rather dubious methods were used to make this boiler work in the manner required.
When the Ravenheat boiler worked, the system worked very well, gas bills are about £1200 /yr which I think is good for all of the above accommodation which is all single glazed and not particularly well insulated in an exposed location.
Pluming from the boiler was only noticable when the tanks were being heated at high output temp
After 5 years of exhaust fans seizing, output thermocouples failing and now the ignition board has got fried, coupled with the fact that it's the wrong boiler in the first place I want to replace. BUT apart from Ravenheat and Keston I cannot find any system boilers that have separately controllable output temps for heating and hot water.
From my own experience and it seems every gas fitter I speak to, Ravenheat boilers are unreliable and the posts on various websites suggest that Keston boilers are no better, Are the more recent boilers from these manufacturers any better??. Does anyone know of other boilers that have the dual output feature
Regards
Gary
Anyo
When a hydrocarbon such as natural gas burns it produces carbon dioxide and water vapour. hard fact.
The latent heat of vapourisation is the large amount of energy it takes to turn a liquid into a gas / vapour and the similar amount of energy released when a gas / vapour turns into a liquid. hard fact.
The following are what seem like facts to me
So, in a condensing boiler, if you provide a heat exchanger surface cold enough to cool the combustion gases below the dew point, the water vapour condenses on this surface, transfering the latent heat of vapourisation to the heat exchanger and the heating water it contains rather than it being lost out of the exhaust. Water from the combustion leaves the boiler as a liquid, not a gas
About 54'c is the dew point when condensation starts, more water vapour will condense and more latent heat will be extracted from the exhaust gas at even lower temps.
The temperature of the heat exchanger is governed by the temperature of the water returning to the boiler from the radiators / hw tank / hw heat exchanger. The cooler the return water, the better.
These are some bits I don't understand / agree with
Boiler manufacturers and plumbers say that a large plume is a sign that the boiler is condensing well, BUT the plume is made of water vapour condensing when it hits the cold outside air so it has left the boiler as a gas with all of its latent heat so I can't see how a large plume is a sign of good condensation in the boiler, very much the opposite I would say.
Most conventional or system (non combi) boilers have only one temperature control so if the temperature is set high enough to heat the tank, then the return water from the heating is to hot for condensation to occur and efficiency is reduced especially if trv's or zone valves have reduced the amount of the system being heated.
If the boiler flow temperature is reduced to ensure that the return temp is cool enough to condense the water vapour, the tank will take forever to heat up, if at all, and undersized radiators may not emit enough heat. Does anyone else see this or have I missed something??
I designed the heating system that feeds our two holiday flats and our own house, 300sqm of floorspace, 7 bedrooms, 4 bathrooms and accommodation for up to 15 people with two unvented cylinders and a mixture of directly fed ufh and oversize radiators running at ~50-60' supply temp. All fed from a single 24kw Ravenheat condensing system boiler that had two output controls.
Unfortunately it was not noted that the wrong boiler had been supplied untill the gas fitter had mounted it on the wall (and it had laid in the shed for 3 months) so not returnable. some rather dubious methods were used to make this boiler work in the manner required.
When the Ravenheat boiler worked, the system worked very well, gas bills are about £1200 /yr which I think is good for all of the above accommodation which is all single glazed and not particularly well insulated in an exposed location.
Pluming from the boiler was only noticable when the tanks were being heated at high output temp
After 5 years of exhaust fans seizing, output thermocouples failing and now the ignition board has got fried, coupled with the fact that it's the wrong boiler in the first place I want to replace. BUT apart from Ravenheat and Keston I cannot find any system boilers that have separately controllable output temps for heating and hot water.
From my own experience and it seems every gas fitter I speak to, Ravenheat boilers are unreliable and the posts on various websites suggest that Keston boilers are no better, Are the more recent boilers from these manufacturers any better??. Does anyone know of other boilers that have the dual output feature
Regards
Gary
Anyo
