Massive gas bills!17yr old boiler - to replace, or not to replace? That is the question.

PBD113

Hi all,

I've recently moved into a 2,800 sq. ft. 1920s detached house in the Midlands. The house has an older boiler, and I'm seeking advice on whether replacing it would help reduce my high gas consumption. Currently, I’m averaging 100 kWh/day with 3 hours of central heating and 1 hour of hot water usage. The thermostat is set to 19°C.

Current Boiler Setup:

• Model: Potterton Promax 30 SL (installed in 2007)
• Output: 30.18 kW / 102,980 BTU/hr
• Gas rate: 2.95 m³/hr
• Efficiency: SEDBUK (2005) 90.9%
• Type: System boiler, connected to a recently upgraded unvented tank and well-maintained system
• Heating Zones:
  • Main zone controlled by thermostat and programmer in the hall
  • Extension with 4 radiators controlled by a mobile programmer (Center EHE0200361)

House Characteristics:

• Mix of wall types, mostly solid brick with some cavity insulation
• Double glazing throughout
• Insulated eaves with varying levels of insulation (Celotex and loft rolls)
• 21 radiators, including 3 towel rails, equally split between upstairs and downstairs

Energy-Saving Steps Already Taken:

• Draught-proofing
• Replacing all TRVs
• Turning off heating in unused rooms
• Balancing and bleeding radiators
• Optimizing CH/HW schedule (currently: CH 05:00-07:00 and 16:30-18:00, HW 04:30-05:00 and 16:30-17:00)

Potential Replacement:

I'm considering the Ideal Vogue Max 32 with a 32 kW output and a gas consumption rate of 3.314 m³. Its SEDBUK efficiency is 89.3% (91.1% based on 2005 standards).

My Question:

Would replacing the boiler help reduce gas consumption? Although the new boiler has a higher gas consumption on startup, it should modulate down—could this lead to quicker heating and therefore lower overall gas use? Is the newer boiler more efficient than my current one?

I’m also exploring the option of installing smart TRVs, which might cost around half of what a new boiler would.

Any advice or suggestions would be greatly appreciated!

Thanks in advance.

km1500

"The Potterton Promax SL 30 has a SAP seasonal efficiency rating of 88.8%. The Promax SL 30 is a Condensing Conventional boiler with a maximum power output of 30.18kw that uses Gas as its fuel source. This boiler will provide efficient heating and subsequently reduce fuel bills and carbon dioxide emissions."

88.8% efficiency is pretty good imho.

dharm999

Are you sure there isn’t something else going on, e.g. a valve or pump stuck open/not shutting fully?  I’m in bigger house than yours, granted it’s much more modern, and our gas usage is less than half yours, and we have underfloor heating in over half the house which is supposed to use more gas than a standard set up.

Over what period of time does the 100kwh relate to?

On-the-coast

I've a similar size and age house but boiler is only 9 years old. 

Across the 6 months of the year that heating is on I average 100 kWh per day.  

But… it’s relatively warm now   So Nov and April I might be averaging 50kwh per day, whereas if it’s below freezing all day it can be 200kwh. 

100kwh in 3hrs (plus hot water) sounds like your boiler is operating on max power even in October which does not seem right. 

Albermarle

It is not a very scientific comparison, but for info I live in semi detached about 60% of the size of yours. It is also a 100 years old and mainly solid brick. Some newer and older double glazing. 13 radiators
Over the last 4 weeks ( variable weather) we have probably had the heating on about 3 hours a day on average and hot water one hour. Gas usage is around 50 Kwh a day.
We have a newer condensing boiler which are normally around 15% more efficient than a non condensing one.
So if you take into account the older boiler and the large size of the house ( and detached) then maybe 100Kwh a day is not so surprising. Especially if there are a few of you living there.
I guess with colder weather, the usage could double.
Normally it is better to let old boilers die naturally as it takes so long to get a return from the savings on gas.
However with a large house and a high gas usage that could change the calculations a bit.
You could get some small savings from better modulation from the boiler but the condensing is the main saver. Also  probably you do not need a 30KW one, if you are looking at boiler costs.

housebuyer143

I just replaced my 22 year old boiler with a new one and the savings are about 30%, but that's not huge money and it's probably going to take 10 years to recoup the cost of the installation based on savings alone. Your house is bigger and so maybe you recoup it sooner but I don't think replacing your boiler is suddenly going to save you a fortune or make it affordable to run. 

My house is about 1800sqf and we have 21 rads and I am using 9-10kw an hour to run the heating/HW. We are in a fairly modern house, new DG, insulation 400mm in loft, cavity wall etc. 

On my old boiler we were doing as you were, 2 hours heating here and then another 2 hours and in the winter we were easily clocking up 80kw a day. It appears to be more efficient to leave it on longer because they seem to use more kw in the first half hour before reducing down. 

Edit - too add, you don't need such a large boiler if you do replace. System boilers don't need as much power as a combi. I have range rated my boiler to 16kw from 21kw and it does 21 rads with no issue. If you do replace, speak to a few people about it because BG tried to tell me I needed 24kw replacement when the one being taken out was 18kw. 

dharm999

Out of curiosity, is it more efficient to have the HW come on 30 minutes before the CH?

housebuyer143

dharm999 said:

Out of curiosity, is it more efficient to have the HW come on 30 minutes before the CH?

I find that if HW and CH are on at the same time they both work sub par, so CH isn't as hot and HW takes longer to heat. I always ensure they are not on at the same time. 

ThisIsWeird

PBD113 said:

My Question:

Would replacing the boiler help reduce gas consumption? Although the new boiler has a higher gas consumption on startup, it should modulate down—could this lead to quicker heating and therefore lower overall gas use? Is the newer boiler more efficient than my current one?

I’m also exploring the option of installing smart TRVs, which might cost around half of what a new boiler would.

Hi PBD.

I have to say, your daily kWh figures don't mean much to me - I don't know if it's 'high' for a largish detached house (from what others say, it is a bit?) - but can only suggest that replacing your boiler with a (slightly) more efficiency condensing type is unlikely to transform your situation. A saving? Yes. But significant? Unlikely.

Replacing your boiler is likely to gain you a rough ballpark of, ooh, 10% greater efficiency? As KM points out, your current boiler is not far of 90% efficient already, and a modern condensing boiler will be roughly-mid-90's at best? That doesn't sound as tho' it'll transform your bills.

But it will cost you ~£2k upwards to have it done, and that's a lot of gas.

Almost always the consensus is to keep your old boiler running for as long as you can.

Whilst I don't know if your consumption is somehow 'excessive' for your size and type of house, there are some pointers;

1) You have - gulp - 21 rads? But you already turn off the ones not being required, so that's good. Q - do the ones that are on become 'hot'? And do they all heat their respective rooms ok? If so, then that suggests there's nothing really amiss with your current system.

2) You have a modern unvented DHW tank, so that will be well insulated, so no savings to be had there.

3) You have replaced all the TRVs, so they all work fine - they shut off reliably when the room temp is reached? That sounds fine too.

4) Your main house has a Thermostat and Programmer in the hall? Are they a combined unit - a Prog Stat? That may be a source of improvement, but again unlikely to be transformative.

5) Your extension has 4 rads and a Center EHE0200361. How does this work - what actually controls the temperatures in the extension rooms?

And, you've done sensible things like draught-proofing - a biggie, in my experience - and trying to control the heating more carefully.

(I don't understand your CH time settings - why does it turn off at 6pm?!)

Ok, you seemingly have a large house, and not anywhere near current insulation standards! Ultimately, 'insulation' is the key to reduced bills and comfy living, but it's also costly and disruptive. So, keep the 'insulation' goal constantly in mind, and try and improve it whenever the opportunity arises - eg, if you are reskimming the walls in a room, then add insulation to the insides of the external walls at the same time, for example. Double-check that you've kept draughts at a minimum, from under skirtings, through floorboards, that sort of stuff.

With luck, grants for insulating homes will only become more prevalent, so hopefully the opportunity to really improve this will come along at some point.

So, what to do now? Since, imv, replacing your boiler would be a waste of money, I would look at perhaps updating your controls, using a standard that will be compatible with a new boiler somewhere down the line - future-proof your choice. (Of course, the future could be HeatPump - who knows!)

Smart TRVs should provide you with optimum control over your heating; each rad will only demand what it needs, and when. Don't expect a transformation, tho', as you already seem to be quite careful, but my thinking is, when you do finally go for a new boiler, you will likely want this ultimate form of CH control too, so why not fit it now, and start to benefit right away?

You don't need to Smart every radiator - those in guest or spare rooms can remain 'manual' TRV, and be turned up only when actually needed, meanwhile keeping them on 'frost', doors shut, and windows cracked open to 'vent' - the rooms will be fine.

FreeBear

ThisIsWeird said: 4) Your main house has a Thermostat and Programmer in the hall? Are they a combined unit - a Prog Stat? That may be a source of improvement, but again unlikely to be transformative.

Some research done on energy savings to be had from a fully programmable thermostat (one that allows for different temperatures throughout the day/week) suggests that you could save as much as 15%. In reality, the saving is likely to be around 5%.  Even a few percentage points saved on 100kWh per day would pay for a Tado or Wiser fairly quickly. Although I'm not using anywhere near that much gas, I reckon I saved around 10% with smart controls on an old Baxi back boiler.

ThisIsWeird

FreeBear said:

ThisIsWeird said: 4) Your main house has a Thermostat and Programmer in the hall? Are they a combined unit - a Prog Stat? That may be a source of improvement, but again unlikely to be transformative.

Some research done on energy savings to be had from a fully programmable thermostat (one that allows for different temperatures throughout the day/week) suggests that you could save as much as 15%. In reality, the saving is likely to be around 5%.  Even a few percentage points saved on 100kWh per day would pay for a Tado or Wiser fairly quickly. Although I'm not using anywhere near that much gas, I reckon I saved around 10% with smart controls on an old Baxi back boiler.

Which system would you recommend? Is one better or more affordable than the other? Are they both Opentherm or similar? Could they cope with, say, 15 Smart TRVs, or is it best to split them - eg, extension certainly, but then upstairs-downstairs?
Ta.