BG Expensive

[catmando]

Hi all,

Further to my recent posts I have had a private individual to quote for new central heating (awaiting quotation) and last night I had British Gas.

I want a condensing boiler & 7 new radiators to replace my old back boiler.

British Gas quoted me £4300 last night and that is without a fire in the living room! I was quite shocked and did not expect it to be that much, or is that about the going rate? It will be interesting to see what my other quote comes in at!

 

[Circuit]

If your system works, don't change it. Most back boilers will outlive a modern condensing boiler many times over.

 

[catmando]

Hi there,

But my back boiler is expensive to run.....£45 per month for 2 people and we dont even cook on gas!

Plus I need new radiators.

 

[Circuit]

A corectly running back boiler should be around 75 - 78% effinceint. It may just be sooted up, in which case a CORGI engineer could restore it back to its new condition for around 1 hours work.

It may be that it is not being controlled properly by room and cylinder stats, again an experienced CORGI bod could advise.

Be aware that a sooted up back boiler would not only be very dangorous to run since carbon monoxide might seep into the room and cause death etc, but the boiler would give out about say half as much heat and therefore your bills would double

 

[catmando]

Hi Circuit,

I did have the boiler completely serviced last year and it was 'spitting' soot ! This has not made any difference to my heating bills. I do have a CO detector on the fireplace as well.

I may just do one step at a time and replace my old radiators first, would that make any difference to running costs.

FYI my back boiler is a Glom Worm I think.

 

[Circuit]

I refer you to my previos post, a correctly running boiler such as you have should be 75 - 78% efficent, a condensor will be 90%, the savings per year will be small comapred with the £4500 outlay you are considering.

Wait a few days and see what the pros think on this forum. Please bear in mind that I am just a humble home owner as yourself not a CORGI bod.

A last thought for the CORGI bods to consider, a correctly running boiler should never really soot up, the fact that it did might indicate that the flue and or air suppply to it are not correct. Catmando wait for others to comment on this.

 

[catmando]

The BG man reckons that my bills will half with a condensing boiler, saving me about £250 a year according to his sales speel!

 

[croydoncorgi]

a correctly running boiler should never really soot up, the fact that it did might indicate that the flue and or air suppply to it are not correct.

Stronger than that! A gas boiler should NEVER soot up!!! If it does, the air supply and / or fluing may be incorrect (a very serious a safety issue which you must get looked at ASAP and turn off the boiler until you do).

Alternatively, boiler settings may be wrong - which would be surprising if a CORGI engineer serviced it!

Get it fixed, now.

A point to bear in mind about condensing boilers: they CAN ONLY achieve the quoted high efficiency (90 - 92%) when they are actually 'condensing'. That is, when condensation is occurring on the heat exchanger (HX). For this to happen, the temperature of (at least part of) the HX must be less than 56 degrees C (the 'dew point'). In practice, this means that the Return water temperature must be LOWER than (say) 54 degrees. If these conditions cannot be achieved, the boiler will still be quite efficient (maybe mid-80s percent) but it won't achieve the quoted max efficiency.

A particular problem will arise with 'heat store' systems - these usually need to be held at more than 70 degrees C so driving a heat store with a condensing boiler and expecting optimal efficiency is a no-no.

In most cases, best that can be expected with a condensing boiler is that it will condense some or most of the time, but not all the time.

Just because a steam plume appears out of the flue does NOT NECESSARILY mean the boiler is condensing. The condensation may be occurring in the flue but not in the HX. (Could be argued, I suppose, that because the flue is mostly inside the building the heat is being recovered 'usefully' but I reckon we're getting into 'angels on heads of pins' territory!)

 

[Circuit]

When pluming is occuring, the flue gasses should be 100C, I asume this from the laws of pysics since I do not know anyone localy who has such a boiler.

When the gasses are at 100C and pluming, some condencing will be occuring on the secondary HE and some will not.

If the seconday HE corrosion problem is solved on current models, then condensing boilers will be fine, only time will tell.

The 90% figure is suppost to be a rough figure for normal rad system averaged over the year.

Remember when your boiler cuts in in the morning and your rads are quite cool / luke warm, condesers will give about 95%. Perhaps a reason to fit very heavy cast iron rads that can be obtained on the scrap market?

I saw some at my local hospital recently free for the taking with permission

 

[catmando]

Perhaps I misled you guys....

Before I had the boiler serviced last year it was occassionally spitting soot out the front of the fire, since it had its service it no longer does that.

It is probably ready for another service however I have held back till I decide what I am going to do!

 

[ChrisR]

Back boilers are often the least efficient types, especially woith a bit of cat fur in the air intake - seriously they suffer really badly from that. Condensers are used widely on the continent so the technology ought to be mature, but even with Bosch boilers there are concerns over primary heat exchanger corrosion - the jury in this country still seems to be out.

As for "seconday HE corrosion problem " , eh??

As for the BG quote - expect maybe a third off from an independent. Saw a BG quote recently which specified "adding 2 metres of insulation, £34", and " 3 metres of 15mm gas pipe, £84"

 

[ollski]

Energy efficieny guide rates most bbu's at 65% efficient, so realistically you could expect to cut your bill by about a third with no other efficiency upgrades, but you say you need new rads which will almost certainly be smaller convectors and more efficient controls so your BG man won't be far out.

 

[croydoncorgi]

It all depends which 'Law of Physics' you're referring to. Boiling point of water ain't the one! Dewpoint, relative humidity, air pressure and temperature are all inter-related and control condensing performance.

If you look on the Web, there are LOTS of references to 'dewpoint'. If you can follow the maths, good luck.

Doesn't really matter anyway. Look at ANY condensing boiler documentation. It should reference 55 or 56 degrees as the 'critical temperature' for Return water to allow condensing to happen.
For example, Keston Ceslius 25:

When the return water temperature is below 55C, part of the water vapour in the combustion products will condense inside the heat exchanger, thus increasing the boiler efficiency further by releasing the latent heat of condensation.

 

[Circuit]

croyden corgi you are right of course, my mind was rather on my worcester cbi melting heat exchanger when I wrote that and I had regressed few years when I incorrectly thought that was the case.

Good job I don't do this for a living

But to be correct for a change, the 55 - 56 critical temp is not a "magic" temperature, since it depends on the % excess air in the flue gasses.

If the excess air was reduced to around 20 % the "critical temp" would be greater than 56 C and the savings from condensers would be greater.

But that would never be allowed on a domestic boiler would it?

This weekend's booby prize goes to the first person to post the % CO2, % oxygen and hence the % excess air on a named condenser as measured on their flue gases meter

 

[croydoncorgi]

I'm not sure what you're getting at. Are you assuming that REDUCING 'excess' air would increase saturated vapour pressure of water in the flue gas? or something else?

Anyway, as far as condensing boilers go, 55 - 56 degrees DOES seem to be a 'magic number'. Most seem to quote it.

I think that to achieve complete (efficient) combustion boilers have to run with a high percentage of excess air, anyway. As soon as you start creating more CO versus CO2, leave unburned gas in the fluegas, etc., efficiency falls over for those reasons too.