TEMP BEWTEEN FLOW AND RETURN???
- Thread starter fronnie
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Convention for heating systems that don't include a condensing boiler is 82 deg. flow, 71 deg. return. Although, in order to keep pipe sizes down people often design on 80/60 (makes the radiators a bit bigger though).
Just by way of explanation, the 82 deg comes from a BS requirement to maintain at least 17 deg. between the normal operating temperature and the boiling point at atmospheric pressure. On larger installations the system pressure can be increased to allow higher operating temperatures (at 150 psi. the flow temperature can be 170 deg.C, and with a return temperature of 120 deg. distribution pipes can be kept relatively small).
For condensing systems the return temperature needs to be much lower, with a corresponding reduced flow temperature. There are other instances where reduced temperatures are used such as some underfloor heating systems where pipes are buried directly in the screed.
Just by way of explanation, the 82 deg comes from a BS requirement to maintain at least 17 deg. between the normal operating temperature and the boiling point at atmospheric pressure. On larger installations the system pressure can be increased to allow higher operating temperatures (at 150 psi. the flow temperature can be 170 deg.C, and with a return temperature of 120 deg. distribution pipes can be kept relatively small).
For condensing systems the return temperature needs to be much lower, with a corresponding reduced flow temperature. There are other instances where reduced temperatures are used such as some underfloor heating systems where pipes are buried directly in the screed.
PTH what are views regarding installation of condensing boilers onto existing systems where heat loss is calculated for flow of 85 degrees and drop of 11.
At a trade exhibit, I asked same question to one of the boiler people. He told me their boiler stayed condensing all the time regardless the flow temperature. I cannot see how. I get the impression a lot of people are being hoodwinked.
At a trade exhibit, I asked same question to one of the boiler people. He told me their boiler stayed condensing all the time regardless the flow temperature. I cannot see how. I get the impression a lot of people are being hoodwinked.
DP,
I don't see how condensing can take place if a system flow temperature is at 85 deg. The return temperature will be entirely dependant on the heat given up by the radiators, heat transfer to the domestic hot water (if the boiler isn't a combi.) and heat loss from pipework. Now, unless people start installing radiators the size of football pitches the combined return temperature is going to be no less than about 60 deg. and in most cases it will be higher.
The boiler manufacturers are generally careful with their words when asked these sort of questions and will reply that the boiler will still operate at a high efficiency when used in these situations......which isn't the same as operating at the design efficiency when condensing. In other words, unless the whole system is designed to operate at the temperatures required for condensing to occur, you might as well keep your old Ideal Concord - the operating efficiency won't be improved enough to give you a payback within the life of the new boiler.
Of course, in new build situations where building insulation is up to current part L standards, designing an 82/71deg system results in postage stamp sized radiators which can look a bit daft. (I know Oilman would say that if a rooms heatloss is that small, don't put a radiator in it, but when people buy a house from a developer they expect to see radiators in every room.) So, with a condensing system you can use radiators that look ok.
I think that was a long winded way of saying that I don't like condensing boilers, but they are here to stay and as they are advertised, people will want them. Good luck to them.
I don't see how condensing can take place if a system flow temperature is at 85 deg. The return temperature will be entirely dependant on the heat given up by the radiators, heat transfer to the domestic hot water (if the boiler isn't a combi.) and heat loss from pipework. Now, unless people start installing radiators the size of football pitches the combined return temperature is going to be no less than about 60 deg. and in most cases it will be higher.
The boiler manufacturers are generally careful with their words when asked these sort of questions and will reply that the boiler will still operate at a high efficiency when used in these situations......which isn't the same as operating at the design efficiency when condensing. In other words, unless the whole system is designed to operate at the temperatures required for condensing to occur, you might as well keep your old Ideal Concord - the operating efficiency won't be improved enough to give you a payback within the life of the new boiler.
Of course, in new build situations where building insulation is up to current part L standards, designing an 82/71deg system results in postage stamp sized radiators which can look a bit daft. (I know Oilman would say that if a rooms heatloss is that small, don't put a radiator in it, but when people buy a house from a developer they expect to see radiators in every room.) So, with a condensing system you can use radiators that look ok.
I think that was a long winded way of saying that I don't like condensing boilers, but they are here to stay and as they are advertised, people will want them. Good luck to them.
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ask the same lot how it works with thermal stores if they can keep it condensing i'll eat my hat 
Shaggy - People with existing systems will not be required to change their boilers. There are plenty of existing boilers that can be kept running for many years without much effort. Unfortunately there are also plenty of service engineers, plumbers and well meaning forum contributors who will recommend a new boiler at the first sign of a malfunction.
DP - I don't think we are alone : hands up any installers who would swap their existing boiler for a condenser (unless they're getting as freebie)
Kev - Thermal stores in particular need a reasonable mean 'primary temperature to function with any appliance needing a good flow rate such as a bath, shower, kitchen sink etc. but any stored water will be a problem. If water is to be stored at a decent temperature, say 60 deg.C. then the mean primary temperature must exceed it by a reasonable margin to allow heat transfer, say 65 deg.C. With that mean temperature the return is unlikely to be below 60 deg.C. So, no condensing in the summer... or when you want hot water, or when you want heating.... so condensing only during start up and a very low demand...bit like a conventional boiler really!
DP - I don't think we are alone : hands up any installers who would swap their existing boiler for a condenser (unless they're getting as freebie)
Kev - Thermal stores in particular need a reasonable mean 'primary temperature to function with any appliance needing a good flow rate such as a bath, shower, kitchen sink etc. but any stored water will be a problem. If water is to be stored at a decent temperature, say 60 deg.C. then the mean primary temperature must exceed it by a reasonable margin to allow heat transfer, say 65 deg.C. With that mean temperature the return is unlikely to be below 60 deg.C. So, no condensing in the summer... or when you want hot water, or when you want heating.... so condensing only during start up and a very low demand...bit like a conventional boiler really!
Sorry Shaggy, I wasn't taking the p*ss. The Consultation document issued by Mr. Prestcott's office states that "In some specific boiler replacement situations it is acceptable to install a non-condensing boiler.
It goes on to say that "about 20% of the replacement boiler market will continue to be non-condensing for good technical and safety reasons" It doesn't elucidate on those technical reasons but I would think that a statement backed by return temperature calculations would be a good starting point.
The document also states that it estimates the difference in cost between inatalling a conventional boiler and a condensing boiler is "in the range of £150 - £200".....mmmm.
It goes on to say that "about 20% of the replacement boiler market will continue to be non-condensing for good technical and safety reasons" It doesn't elucidate on those technical reasons but I would think that a statement backed by return temperature calculations would be a good starting point.
The document also states that it estimates the difference in cost between inatalling a conventional boiler and a condensing boiler is "in the range of £150 - £200".....mmmm.
No, but sadly it's just more b*ll*cks foisted on the general public via the idiots at the ODPM who know f*** all and accept the lies told to them by industry lobbyists whose only interest is in making more money.DP said:
Believe it or not....
I'm an installer with a Condensing boiler (Keston Celsius 25) which (again believe it or not!) can support a 20-rad system in a Victorian house with double glazing but also solid walls that bleed heat like there's no tomorrow. Furthermore, my gas consumption (when I measured it quite accurately over a month) was HALF that of the very similar house next door and 15 percent less than the similar house over the road with a (larger) condensing boiler.
So I'm happy with a condensing boiler. But I must admit I do see many examples of wrongly installed and badly maintained condensers which have definitely not justified their extra cost.
I'm an installer with a Condensing boiler (Keston Celsius 25) which (again believe it or not!) can support a 20-rad system in a Victorian house with double glazing but also solid walls that bleed heat like there's no tomorrow. Furthermore, my gas consumption (when I measured it quite accurately over a month) was HALF that of the very similar house next door and 15 percent less than the similar house over the road with a (larger) condensing boiler.
So I'm happy with a condensing boiler. But I must admit I do see many examples of wrongly installed and badly maintained condensers which have definitely not justified their extra cost.
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