Is this condening boiler condensing?

[stem]

Almost 4 years ago, my mother had a new condensing combi installed to replace her 25 year old glowworm balanced flue cast iron boiler. She had hoped that it would be more efficient. However, after 3 full heating seasons she's noticed that the new boiler pretty much consumes that same quantity of gas as the old one!

Installed in a bungalow, the flue is vertical and goes out through the roof. However we have never seen it "plume" which I believe it should when condensing. Could there be something wrong with the boiler that prevents it operating in condensing mode?

The boiler was installed and is serviced by BG (after they told her they couldn't get parts for the old one) and generally works OK otherwise. It is a Gloworm CX which she chose because her original Glowworm had never failed or needed new parts in 25 years. Unfortunately the new one has failed twice already, once rather spectacularly by filling the kitchen with smoke due to an internal electrical fault.

She has 6 radiators in a 1980's bungalow with insulated cavity walls and has the heating on all day. 21 degrees in the lounge and bathroom 18 everywhere else. Piping is copper microbore in the loft space with runs down to the radiators embedded in the walls.

If you concentrate hard, in very cold weather, there is a slight hint of a "plume" when operating in hot water mode.

We find it strange that a modern high efficiency boiler (A or B rating?) is consuming the same amount of gas as a "G" rated one. any ideas?

 

[kevindgas]

how have you calculated the gas usage? cost or actual readings?

if actual readings are you comparing an imperial reading with a metric one?

 

[kevplumb]

quantity or price

 

[stem]

I've not checked her calculations, but it's based on the meter reading. (It's the same meter throughout and I think it records cubic feet). She doesn't take into account the calorific value. Just the volume.

 

[D_Hailsham]

It is a Gloworm CX

Exact boiler model please; there are several CX models.

Size of rads and type - single (no fins); single condenser (with fins); single+condenser (double rad with fins on one half only); double condenser (fins on both halves).

 

[kevplumb]

never heard of condensing rads

 

[petit_pablo]

never heard of condensing rads

Theres a marketing scam just waiting to be exploited!

 

[D_Hailsham]

never heard of condensing rads

That was a real typo It should, as you all know, have been "convector" radiator.

 

[mehran]

sadly many people think that you can just bung in a new boiler and it will slash the gas bill/usage.

first off if the boiler is a combi(i don't know GW range very well) then most likely you have greater water flow from the taps...so more wasted heat down the drain.

second, if the house was well insulated before the boiler changed then it will make little difference as it does not matter if the old boiler has >70% efficiency if the house is does not lose much heat.

third, condensing boilers work at their best with a return lower then 55 degrees, the lower the better. so a condensing boiler boiler with a 65 degree return may be 80% efficiency but at 40 degree return maybe 95% efficiency .

the plume thing is a little misleading because a boiler working below 55 degrees may condense all the water out of the flue gas(poc)

 

[TicklyT]

As mehran said, monitoring the condensate outlet may give a better idea of whether the boiler is condensing than monitoring the flue terminal.

 

[stem]

Exact boiler model please; there are several CX models.

From memory, I think it's a CX 30, but I'll check when I visit over the weekend. It will have been installed 5 years next summer, (in case its age helps you identify it)

Size of rads and type - single (no fins); single condenser (with fins); single+condenser (double rad with fins on one half only); double condenser (fins on both halves).

They're all singles with fins, I would estimate them to be 600mm high, and lengths as follows:
Lounge / dining area 2 x 1600mm
Kitchen 1 x 800mm
Bathroom 1 x 600mm
Bedrooms 2 x 1200mm

 

[stem]

As mehran said, monitoring the condensate outlet may give a better idea of whether the boiler is condensing than monitoring the flue terminal.

I thought that, but the pipe is plumbed in and solvent welded, so difficult to break into without damage. Also I wouldn't know how much condensate it should be producing over a given period, which I guess would vary depending on several factors.

 

[D_Hailsham]

From memory, I think it's a CX 30, but I'll check when I visit over the weekend. It will have been installed 5 years next summer, (in case its age helps you identify it)

In that case it will be an A rated boiler supplying between 5 and 22kW to the heating system

They're all singles with fins, I would estimate them to be 600mm high, and lengths as follows:
Lounge / dining area 2 x 1600mm, Kitchen 1 x 800mm, Bathroom 1 x 600mm, Bedrooms 2 x 1200mm

That adds up to 7kW. However, due to the way condensing boilers work, the radiators may only be giving out 6kW. This may not matter if the rads are oversized for the house.

You can check this by using the Boiler Sizing Wizard. Deduct 2kW from the result to find out the heating requirement of the house then post the information.

 

[stem]

The calculation came out at 8kW exactly, less 2kW = 6kW

I checked the boiler and it's a 30CXi

 

[D_Hailsham]

The calculation came out at 8kW exactly, less 2kW = 6kW. I checked the boiler and it's a 30CXi

OK so you need 6kw and have 7kW of rads, which is fine. The problem is the boiler. You need 6kw to raise the temperature 21°C (from -1°C to 20°C). So 5kW will raise the temperature by 17.5°C, i.e from 2.5°C to 20°C. Most of the time the temperature will be above 2.5°C (particularly with global warming!), so the boiler needs to produce even less than 5kW. As it cannot modulate any lower, it reverts to on/off operation - just like older boilers. This is less efficient.


What setting do you have the boiler's Heating Temperature control (left hand knob) on. If it is fully clockwise, try turning it back until it is pointing to 12o'clock and see what effect it has on the house temperature. You may have to experiment to find the best setting which will maintain the desired house temperature.