New boiler uses more gas than 16 yrs old boiler.

Road_Hog

HTC wrote: »

Suppose the boiler's efficiency is 90% but it uses more gas. We understand this theory but then it is a con!

But don't you feel better, because you've gone green?

I'm sticking with my old boiler.

oldskoo1

I'm going to stick with my older 78% efficient boiler as long as I can

C_Mababejive

I must admit that i have not dismantled any HE boilers recently. I presume they all use the downfiring fan induced model with a wet sump. How is the recovered latent heat utilised? Presumably its used to preheat incoming combustion air?

HappyMJ

C_Mababejive wrote: »

I must admit that i have not dismantled any HE boilers recently. I presume they all use the downfiring fan induced model with a wet sump. How is the recovered latent heat utilised? Presumably its used to preheat incoming combustion air?

Preheats the water that's why the water needs to be as cool as possible when it's returning to the boiler so it can recover some heat.

C_Mababejive

HappyMJ wrote: »

Preheats the water that's why the water needs to be as cool as possible when it's returning to the boiler so it can recover some heat.

So there is a secondary heat exchanger then which contains system water and the condensate delivers heat to it?

I wonder if any domestic boilers use the pre heat combustion air model? I guess not as there would need to be additional controls.

rogerblack

C_Mababejive wrote: »

So there is a secondary heat exchanger then which contains system water and the condensate delivers heat to it?

I wonder if any domestic boilers use the pre heat combustion air model? I guess not as there would need to be additional controls.

It would do little.
preheating the input air uses very little of the available energy. (And is done to some small degree by the concentric exhaust) this is simply because to heat air from 0c to 50c takes very little of the power of the burner.
Certainly lots less than the amount available by condensing water.

Possetjohn

Most of the heat exchangers these days are one piece but with a long gas pass. The condensation takes place at the gas exit end of the heat exchanger where the exit gas contacts the return water to the boiler via this stainless steel. It is important to remember the condensation only happens if the exit gas is cooled below its dew point, which is about 53°C. Hence it is important the return is as cool as possible. Hence these boilers work very well with underfloor heating systems where the return water is quite cold. In effect the majority of the condensation will take place when the system starts from cold or when the system switches off on the room stat and then restarts a while later after the radiators have cooled.
Balancing is very important as a small radiator which has too much water flowing through it will heat up the return water prematurely.
Changing an inefficient wall hung cast iron boiler for a modern Valiant combi we saw a drop in gas consumption of 25-30%. Changing from an 80% efficient boiler to a 90% efficient should give you a decrease in gas consumption if everything else is the same. However you have changed some radiators and as someone else pointed out September, October and November were colder than last year, though not 2010!

I would suggest the following.

Check the radiators are balanced and there are no cold spots. This is best done with the TRVs wide open as the system starts from cold.

I assume that model of the new boiler is modulating. If it is, when the system starts up it will run at a low rate for about 90 seconds, then turn up to maximum heating output. A the system heats up it will modulate down the heat input to the boiler. You can hear the fan slowing down and the burner running quieter. Check this is happening.

Run the boiler set temperature as low as you can and still get comfortable heating

Best of luck

HTC

I can't find how to use "quote"on ipad so just copy and pasted below.

7.71 - 8.56 /day is 11% not 88%, which could easily be explained by a slight variation in average temperatures outside or a variety of factors inside.

88% increase was worked out from this data.
22/08/2011- 13/11/2011 (83 days) gas used 134 cM=1.61cM/day
28/08/2012- 25/11/2012 (89 days) gas used 270 cM=3.03cM/day
1.61/(3.03- 1.61)=0.88

Soon after the installation of this new boiler we had bath a few times a week for two weeks or so but it shouldn't make such a huge difference of gas consumption.

However, the difference is not that big now. But the water temperature fractuates during shower, both upstairs and downstairs (so it is not the shower fault) . It had been sorted by Bosch engineer twice so far but it comes back again.

No we don't live in a mansion! Ours is a 3 bedroom detached house but downstairs had been extended and we have a open plan dining/lounge/kitchen so understand that it will take some time to heat the whole room. When TRV is 3 on upstairs radiator, it is so hot that I can't keep my hand on it but downstairs radiator TRV3 is not hot at all. I have to make it 5 to feel the same as upstairs. Downstairs radiator 1 is single and 180cm, radiator 2 is double and 90cm wide.

Perry525

Your boiler has to lift the cold water temperature from just above zero at this time of year to perhaps 35 -40C possibly more, depending on how warm you like to shower and how high the shower head is - the water loosing its heat as it falls through the air, before contact with your skin.
The temperature of the cold water will also vary depending on how long the pipe run is indoors, how well insulated and how much heat the water picks up as it travels once out of the ground. I find running my shower at 65C for a count of 20 and then turning it down for a count of 8, gives the pipes a chance to settle down and me a perhaps slightly too warm shower.
Of course a combi is a dreadful thing to have when showering, you do have to live in hope that other people will not turn a tap on or off, or flush a toilet somewhere to alter the water pressure into the boiler and thereby alter the shower pressure and temperature.

dogshome

I'm not a techie on Gas Boilers, but a few months ago there was a thread on this site about a new boiler and hugely increased gas bills - The problem in the end was traced to the gas supply pipe from the meter being too smaller a bore to cope with the Boilers demand, which resulted in the boiler constantly oscilating ON/OFF and never reaching it's working temperature.