Energy saving via condensing boilers??

[johnb]

Government policy is said to mean that from 2005 or so, ALL replacement and newbuild boilers will be high-efficiency, so presumably condensing types.

But hang on a tick...

To condense effectively, the flue gas temperature at the back end of the boiler has to be 56 degrees C or less (the dewpoint of water under typical conditions in a boiler).

Hands up whoever has their CH or DHW temperatures set at or below this point. Not many hands to be seen, I bet. OK - the idea may be that HE boilers spend at least SOME of the time in condensing mode. Indeed, there are some models already around which boast about specific control logic which MAXIMISES the time in condense mode.

But in reality, how effective is this change actually going to be in achieving significant energy savings?

And what about a new boiler going into an installation where there's an existing heatstore? This won't be much use unless the temperature is up around 80C most of the time. Is there any worthwhile future for heatstores at all?

Informed views, anyone?

best regards,
John
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best regards,
John

[Whiter]

Are there any design guides for condensating boilers?

[billy bob]

As far as I have been informed the radiators do have to be oversized, to allow for the lower radiator temperature, alot of the boiler manufacturers said this was not the case, but, how do they expect to heat a room with lower temperatures without increasing the radiator output, the radiator calculations have to be different

Trouble with the UK is the toucy feely syndrome, people feel cold because they do not burn their hand on the radiator, even though the room is at the required temperature, the public need to be educated as well as the fitters, for the condensing technology, when fitted into existing systems new controls and stats will need to be added to give the best results

[edhall]

I am no expert, but there are two things to consider. 1st, 56 deg. (actually 56 Celsius, degrees are for the Fahrenheit scale as it is based on a circle. That is why 212 deg is boiling and 32deg is freezing. The difference is 180 degrees!) is the output temperature.

Since the input water is heated by the exhaust gasses, the output water temperature could be considerably higher depending on the efficiency of this heat exchanger and the incoming water temperature.

Second, I would assume (but don't know for sure) the boilers are smart enough to modulate themselves so that, once the ideal heating water temperature is reached the boiler could modulate itself for maximum efficiency.

That said, I do not think the efficiency of these boilers offsets the cost difference if maintenance and lifespan is concerned. But if someone is keen to help the government meet IT’s promised targets at THEIR expense or they are eco warriors, then I would not hesitate to recommend one. Mind you, I am not a heating engineer – so I wouldn’t install it anyway.

Finally, modern condensing boilers do not require oversized rads. Though it seems like people today like to keep their houses at 23 Celsius in the winter so they generally require them anyway.


Edward Hall
Advanced Plumbing Systems
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Edward Hall
Advanced Plumbing Systems

[johnb]

'1st, 56 deg. (actually 56 Celsius, degrees are for the Fahrenheit scale as it is based on a circle. That is why 212 deg is boiling and 32deg is freezing. The difference is 180 degrees!) is the output temperature.'
AFAIR, your explanation of why Fahrenheit has 180 degrees between freezing and boiling points is b******s - sorry - 'incorrect'. Anyway, most people in heating and plumbing in UK use Celsius solely and automatically - it's an SI unit and taught uniquely in schools, etc.

'Since the input water is heated by the exhaust gasses, the output water temperature could be considerably higher depending on the efficiency of this heat exchanger and the incoming water temperature.' This unfortunately ignores the First Law of Thermodynamics!!! 'Heat cannot of itself travel from a hot body to a hotter body', if I recall correctly.

My own guess about Government policy is that someone told a Downing Street wonk that adopting condensing boilers across the board would automatically raise the efficiency of the UK installed base from (say) 78 percent to 90 percent, hence achieving a large chunk of the UK energy savings target 'at a stroke'. Unfortunately, this was short on detail and ignored a couple of snage (see above). But that's the way joined-up Government works!! Produce a convincing soundbite, capture a few naive voters and on to the the next problem. No need at all to actually deliver anything concrete. Or am I too cynical?




best regards,
John
_________________
best regards,
John

[billy bob]

I know I am repeating myself, but I think you will find I am correct about oversized radiators, having oversized rads will increase the efficiency of the boiler, by helping to reduce the return temperature, they just recommend not to, on existing systems, because it is not cost effective, but when fitted to an existing system with ordinary sized rads it will not go into condensing mode as often as a system correctly designed for a condensing boiler, although even when not in condensing mode they are still more efficient then ordanary boilers

If you think about it the best system to get the most out of a condensing boiler is underfloor heating, because of the lower temperature required

[thescruff]

You are correct billy bob, for the same reason that condensing boilers are ideal with underfloor heating, you have bigger surface area, therefore the water temperature needs to be lower.

scruff

[n7plumber]

lets face a few facts...the design teperature of central hating systems in London is -1c...may be in london we have that temperature may be 14 nights a year..the rest of the time radiators are oversized....and the system inefficient as the boiler is prgrammed for the worse case scenario..

If condensing boilers dont have outside tmeperature conmpensation, they will only be in condensing mode when firing up...with weather compensation the flow temperature would be related to outside temperature and heat loss of the building...(which is a function of temperature difference between the inside and outside)

Radiators withn TRVS only give out maximum heat output when the temperature is low in the room,(they modulate the rest of the time) eventhough the flow temperature is 82 from the boiler....a complete waste of energy...and an increase in co emisions...

[Plumbob]

The TRV's regulate the flow (or modulate) in order to hold the room at the set temperature. It makes no difference if the flow temperature is 82 degrees, the more the TRV's regulate, the less hot water flowing around the system, so what happens?.. The boiler thermostat cuts in, (if the room stat has not already done so, (if one fitted)) and the boiler shuts down. With all due respect n7plumber, but how is that a waste of energy.?
Regards Norman.

[johnb]

'With all due respect n7plumber, but how is that a waste of energy.?'....
At risk of stating the obvious and defending 'n7' without permission: because if the boiler stat is controlling the LOWEST temperature of the boiler, as would be case here, chances are that this would be ABOVE the 'dew point' at which the boiler could actually condense. Therefore, efficiency at (say) 78 percent instead of (say) 90 percent.

best regards,
John
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best regards,
John

[JAH]

How is a condensing boiler cheaper when it is dearer to buy, dearer to fit, dearer to service and has a shorter life expectancy, in the real world that is !

[billy bob]

JAH

How is it dearer to service a condensing boiler, it is the same as an ordinary boiler, the only exception is you have to flush out the condense trap

[JAH]

Yea well being a pedantic vindictive type I am not prepared to do it for nothing, the Tech guy from Ideal suggested that I remove the gas valve to get to the awkward condense trap on an Eyesore, so it aint that straight forward AND WILL add on time to the job, if you want to do it for free and absorb the cost out of YOUR profit do feel free or should I say work for free, PLUS I dont normally have to supply a gasket kit to service ordinary boilers which I WILL charge for.
Oh I forgot to mention the cost of a day off work and the £53.50 and the extra stress of an assesment to do the Domestic energy certificate
because we are all so thick, my serviceing prices go up by the day plus dont forget if I take any waste away (old gaskets) I will have to fill in a form telling the environment agency where it is from and where it is going to and pay them £130.00 every three years for the privelidge, need I go on it's all dearer and getting dearer all the time.


This message was edited by JAH on 3-11-04 @ 5:01 PM

[billy bob]

Fair points JAH

[n7plumber]

Plumbob, my point is that if it is always a waste of energy heat hot water up, and then cool it down. The case is the same for a bath, shower or radiator...if a radiator is maintaining an average temperature of say 70c and the boiler providing water at 82c that is a waste of energy.


The timed anti cycling on some boilers overcomes this to a cerain extent!