Tipper, I trust thats made it clearer! I've given up the will to live. 20/25% will achieve the results you are after.
Condensing boiler radiator sizing - CHIC
- Thread starter Tipper
- Start date
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The return temperature needs to be below the dew point for natural gas, which is approximately 57C but, to be on the safe side let's assume 55C. So the system needs to be designed so that under worst conditions (-1C or lower) the boiler is still able to condense.
I agree that a boiler's efficiency increases as the flow gas temperature decreases because the ratio between the actual volume of condensate in a condensing boiler and the theoretically possible volume of condensate increases as the flue gas temperature drops.
So if the boiler temperature, and consequently the flue gas temperature, reduces as the outside temperature increases more condensate will be available to convert to useful energy and the boiler efficiency will increase.
I suspect that the plume of steam emanating from a condensing boiler flue is the water vapour which has not been converted into useful energy.
Ha, you got me! I finally found your bit of message hiding inside the quote
The reason for "over-sizing" is because the flow temperature should be as low as possible for best condensing, but conventionally sized radiators will not output enough heat at a low flow temperature. It isn't really an issue of how much water is inside the radiators, although that is another issue which might be worth talking about. You can just as effectively increase output by adding fins without making the radiator contain more water. Conventional sized radiators will also very often not output enough heat to allow a 20C temperature drop even at a low pump setting, which isn't ideal for the boiler.
So you will have to whack up the flow temperature in cold weather to get enough heat into your rooms, then the return temperature will be near or above the dewpoint and you will get little or no condensing. You may even have to whack up the flow temperature to achieve the 20C drop. Quite easy to run at 90C/70C even with conventional sized radiators but that sort of defeats the object
Long ago sizing was for 60C above ambient, so in practice about 85C with a 11C or 12C temperature drop (to 75C for an average of 80C over the whole radiator). More recently, radiators seem to be sized to run at 50C above ambient which still means 75C with an 11C drop, or 80C with a 20C which isn't even starting to condense.
Absolutely. I'm sure more than one installer has proudly pointed their customer to a huge plume as evidence of efficient condensing when in fact it is the opposite. Every bit of water vapour you see is wasted energy, and a boiler "fully condensing" emits very little visible water vapour.
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The reality is that most older rads were sized for the typical insulation at that time, little loft, single glazed windows etc. When those have been improved the rads are oversized and so currently will often be adequate because the heat loss of the house has been lowered.
There is little wrong to increase the boiler output temperature on those very few days when its below zero outside because whilst the eficiency may be lower its only for a very few days.
Tony
There is little wrong to increase the boiler output temperature on those very few days when its below zero outside because whilst the eficiency may be lower its only for a very few days.
Tony
Ha, you got me! I finally found your bit of message hiding inside the quo
The reason for "over-sizing" is because the flow temperature should be as low as possible for best condensing, but conventionally sized radiators will not output enough heat at a low flow temperature. It isn't really an issue of how much water is inside the radiators, although that is another issue which might be worth talking about. You can just as effectively increase output by adding fins without making the radiator contain more water. Conventional sized radiators will also very often not output enough heat to allow a 20C temperature drop even at a low pump setting, which isn't ideal for the boiler.
So you will have to whack up the flow temperature in cold weather to get enough heat into your rooms, then the return temperature will be near or above the dewpoint and you will get little or no condensing. You may even have to whack up the flow temperature to achieve the 20C drop. Quite easy to run at 90C/70C even with conventional sized radiators but that sort of defeats the object
Long ago sizing was for 60C above ambient, so in practice about 85C with a 11C or 12C temperature drop (to 75C for an average of 80C over the whole radiator). More recently, radiators seem to be sized to run at 50C above ambient which still means 75C with an 11C drop, or 80C with a 20C which isn't even starting to condense.
So ianniann are you saying that it is impossible to have a desired room temperature ( say 21 degrees) in winter while making the most of a condensing boiler?
Absolutely. I'm sure more than one installer has proudly pointed their customer to a huge plume as evidence of efficient condensing when in fact it is the opposite. Every bit of water vapour you see is wasted energy, and a boiler "fully condensing" emits very little visible water vapour.
I think you means it to look like this! (The preview button may help. )
The first thing you have to do is select the lowest outside temperature. Traditionally this is -1C, but you could select any temperature you like. The lower the outside temperature, the larger the rad has to be, as the heat required is directly proportional to the difference between outside and inside temperature, so the larger the rad will be required.
Having sized you rads (say a total of 15kW) you then know the size boiler required. (Some size the boiler first and then the rads, but that's not relevant to the explanation.) The important thing is that there's no point buying a boiler which is larger than the total rad output.
So you need a 15kW boiler and 15kW of rads, but you want the boiler to condense even when the outside temperature is at the lowest you have chosen. This means that you boiler has to run at max temperatures of 65/55 (or 75/55). But if you do this your rads will not produce the 15kW you require, they will only produce 10.75kW (or 12.4kW). To overcome this you install rads which are nominally larger, i.e 21kw (or 18kW).
If the outside temperature drops below the design outside temperature, you have two options: turn the boiler temperature up so it no longer condenses - this will mean the house gets up to temperature; or, leave the boiler alone, in which case the house will not get up to temperature.
The old days of 11C have disappeared. They were a throwback to the BC (before Celsius) days, when boilers were designed for a drop of 20F, which is equivalent to 11.111C.
)And, no ianniann is not saying that. If you want the system to condense in winter the system has to be designed so it will condense.
The first thing you have to do is select the lowest outside temperature. Traditionally this is -1C, but you could select any temperature you like. The lower the outside temperature, the larger the rad has to be, as the heat required is directly proportional to the difference between outside and inside temperature, so the larger the rad will be required.
Having sized you rads (say a total of 15kW) you then know the size boiler required. (Some size the boiler first and then the rads, but that's not relevant to the explanation.) The important thing is that there's no point buying a boiler which is larger than the total rad output.
So you need a 15kW boiler and 15kW of rads, but you want the boiler to condense even when the outside temperature is at the lowest you have chosen. This means that you boiler has to run at max temperatures of 65/55 (or 75/55). But if you do this your rads will not produce the 15kW you require, they will only produce 10.75kW (or 12.4kW). To overcome this you install rads which are nominally larger, i.e 21kw (or 18kW).
If the outside temperature drops below the design outside temperature, you have two options: turn the boiler temperature up so it no longer condenses - this will mean the house gets up to temperature; or, leave the boiler alone, in which case the house will not get up to temperature.
There's no "seem" about it. BS EN 442 states that the output of a rad must be measured at 75C flow, 65C return and 20C room temperature. The connections have to be top and bottom, same end.ianniann said:
The old days of 11C have disappeared. They were a throwback to the BC (before Celsius) days, when boilers were designed for a drop of 20F, which is equivalent to 11.111C.
It's certainly possible although I suspect a fairly high proportion of people don't do it. My conventional radiators run just fine with the boiler set at 60C, so almost whatever the return there will be some condensing. The return typically runs at between 45C and 50C depending how many radiators are open at the time. Luckily, the room stat is in a large open plan area with four double radiators so the boiler is never stuck on driving through a tiny hall radiator that is little more than a bypass while the TRVs on all the other radiators are shut off.
It probably helps that the house is kept closer to 15C than 21C and is fairly well insulated. So the radiators are probably about 30% over-sized if you were to calculate for those exact conditions, but then many people are in the same situation as Tony described.
Anyway, most people aren't in a situation to pick the size of their radiators and it certainly isn't cost effective to replace them all just to eke out an extra couple of percent from the boiler, so just set your boiler to the lowest temperature that effectively heats the house for you. Ideally this will be 60C or even less for most of the year, and then maybe higher for a while in winter. Or get a controller that does that for you, although again it is doubtful how cost-effective it is to spend a couple of hundred quid for a fairly small gain. Might be worth it if your wife keeps whacking the boiler up to 90C
M
mysteryman
Well, that's all been very interesting - and theoretical - hasn't it?
You need the flow temperature below 53C for full condensation on natural gas. This will reduce pluming, which indicates partial condensation.
Weather compensation, properly set up, will achieve this automatically for as much of the time as possible, which is most of the time. You only need full output in severe weather, and the flow temperature will be higher then. Current advice is that you do not need to oversize rads when you fit weather comp.
You need the flow temperature below 53C for full condensation on natural gas. This will reduce pluming, which indicates partial condensation.
Weather compensation, properly set up, will achieve this automatically for as much of the time as possible, which is most of the time. You only need full output in severe weather, and the flow temperature will be higher then. Current advice is that you do not need to oversize rads when you fit weather comp.
NO!! It's the return temperature which has to be below the dew point temperature of natural gas. You could have the flow temperature at 90C if you wanted, provided the return is below dew point (approx 57C).
And where did this advice come from?
M
mysteryman
Calm down, fellas!
Look at the manufacturers' data sheets and compare outputs at 80/60 and 50/30. The whole of a cross flow heat exchanger has to be below dew point for full condensation. You get partial condensation with pluming otherwise.
The advice not to oversize rads is from the DoE.
Look at the manufacturers' data sheets and compare outputs at 80/60 and 50/30. The whole of a cross flow heat exchanger has to be below dew point for full condensation. You get partial condensation with pluming otherwise.
The advice not to oversize rads is from the DoE.
D_Hailsham thank you really very much for your time and your explanation. However there is a point that i most certainly must be missing out thus you are saying that for example a 15 kw boiler will not manage to get 15 kw output out of 15 kw output rads ( but only 12 kw) if the boiler is to condense, and if i wanted a 15 kw output i will have to over-size the rads.My question is if the boiler can not make the 15 kw rads output produce 15 kw while in condense mode, why and how should it manage to get say over-sized rads produce 15 kw? I mean over-sized rads contain more water and if the boiler is to condense they will just take longer to output the same temperature as smaller rads ( 12 kw).Thank you very much in advance for your clarification!!
The first thing you must understand is that a hot water radiator is not like an electric fire. A 1kW electric fire always produces 1kW. But a 1kW hot water radiator only produces 1kW when specific conditions are met, i.e 75/65/20 (Flow, Return, Room). Change one of these parameters and the rad output changes.
(I know the electric fire analogy is imperfect. If you take a UK 1kw fire to the USA it will only produce 500W because the voltage in USA is half that in UK)
Secondly a radiator is not like a kettle, where the water is stationary, the water in the radiator is continually flowing through the radiator. The amount of heat given off by a radiator is determined by the flow rate. So a rad which produces 1kW will have a flow rate of 0.024 litres/second, irrespective of the size of rad. The volume of water in the rad is largely irrelevant.
A flat steel radiator 600 x1000 without fins will produce 610 watts and contains 3.25 litres of water. Stick fins on the back and the output increases to 1kW, without any change in the water volume. That's a 64% increase in output.
(I know the electric fire analogy is imperfect. If you take a UK 1kw fire to the USA it will only produce 500W because the voltage in USA is half that in UK)
Secondly a radiator is not like a kettle, where the water is stationary, the water in the radiator is continually flowing through the radiator. The amount of heat given off by a radiator is determined by the flow rate. So a rad which produces 1kW will have a flow rate of 0.024 litres/second, irrespective of the size of rad. The volume of water in the rad is largely irrelevant.
A flat steel radiator 600 x1000 without fins will produce 610 watts and contains 3.25 litres of water. Stick fins on the back and the output increases to 1kW, without any change in the water volume. That's a 64% increase in output.
M
mysteryman
To summarise the advice I would give:
Do not oversize the radiators.
Fit weather compensation and save yourself a further 15% over and above what you save merely by having a condensing boiler.
However, I must correct something.
The electric fire analogy above is incorrect. USA electricity is 110 volt and ours is 240 volt. Thus their voltage is 46% of ours.
Reduce the voltage by 46% and you reduce the current by 46% as well. The power output is therefore 46% of 46% which is 21%.
Whilst this is correct in theory, the resistance of the heating element may vary with different operating temperatures, and the actual heat output may be different from the predicted output.
And finally - the output from a radiator can be calculated by measuring the mass flow rate of the water and the temperature difference. You would need to know both values.
Do not oversize the radiators.
Fit weather compensation and save yourself a further 15% over and above what you save merely by having a condensing boiler.
However, I must correct something.
The electric fire analogy above is incorrect. USA electricity is 110 volt and ours is 240 volt. Thus their voltage is 46% of ours.
Reduce the voltage by 46% and you reduce the current by 46% as well. The power output is therefore 46% of 46% which is 21%.
Whilst this is correct in theory, the resistance of the heating element may vary with different operating temperatures, and the actual heat output may be different from the predicted output.
And finally - the output from a radiator can be calculated by measuring the mass flow rate of the water and the temperature difference. You would need to know both values.
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