Increased Combi KW size vs running cost

Torque79

I currently have an old conventional boiler 14kw with cylinder which is starting to fail.  

I have been suggested the following Combi boilers
Main        30kw 12lt HW 
Baxi 830 30kw  12lt HW 
Baxi 836 36kw. 15lt HW 

Annual estimated electricity usage 1232 kWh          - average usage in postal area
Annual estimated gas usage          14980 kWh        - below average usage in postal area

I am realising my electric bill is probably lower than the average in my area due to still currently using an old conventional boiler 14kw with cylinder.

I understand that Combi require greater kw as it heats the water instantly and directly.
is a combi going to significantly increase my electric bill ?

Can a 36kw Combi be run more efficiently mid settings to be comparable with running a 30kw Combi with settings higher?

thank you.

 

grumbler

AFAIK, the only 'powerful' electric part is a pump that is less than 100W and works only when CH is on. And even then it doesn't work permanently. IMO, excessive gas power has very little effect on electricity usage, if any. And even pumps have several setting for the speed/power.

In terms of the gas usage 36kW is the maximum power. AFAIK, modern boilers have adaptive combustion control and don't burn more gas than needed. 

FreeBear

You need to look at the flow rate of your cold water - This will dictate the power required to give you hot water at a reasonable temperature. You then need to add up the total power of all the radiators. Make sure that the boiler can modulate down to well below your maximum radiator requirement.

For example, The WB Greenstar 4000 combi (30KW) will give you 11l per minute with a 40°C temperature rise (so with cold water temp of 10°C, that mean hot water at 50°C) - At lower flow rates, the temperature will be higher. For heating, the boiler can modulate down to ~3KW from a maximum of ~24KW. So if you only have 8KW worth of radiators, the boiler will be ticking over quite nicely and can drop even lower to keep the property at a comfortable temperature before it starts cycling on/off.

From the ErP datasheet, average electricity consumption would be around 38KWh per year (depending on hot water & heating demand). As for gas consumption, it is quite likely to be 20-30% less than what you are currently using.

plumb1_2

The size of the boiler depends on several factors, how many bathrooms, how much demand on the hot water supply. Number of rooms/size of rooms. What controls you have fitted to save energy.
Heat only and Combi boilers are condensing. Upto 20% saving, Upto is variable each installation is slightly. different.
Get a couple of heating guys in to quote and offer options and opinions.

markin

Getting rid of a tank is a mistake if you ever want solar and to heat it with free electric or on an E7 or other time of use tariff.

And one day you will likely need a space for a tank anyway if you have to go with a heatpump in 15+ years on the next replacement.

ThisIsWeird

Hi Torque (gettit? Lol...)

cough

As Markin says, consider the pros and cons of going combi, and make sure the decision isn't just based on the GasSafe's advice.

To answer your Q, electricity consumption by gas boilers is pretty minimal, and I certainly cannot see any significant difference between boiler types. A combi will only use leccy when actually producing hot water (and CH), whereas a heat-only boiler will be running for longer in order to heat a cylinder full over a longer period of time. But, little electricity consumption either way. So, I can't see any reason to consider leccy consumption in your boiler decision.

If you have been used to the hot flow from a cylinder, good chance you may be disappointed by that from a combi boiler; it'll be slower to fill a bath, for example, and will be strongly affected by a second tap being turned on. Having said that, you will enjoy the powerful 'mains' flow when a normal tap is used, and they should also give cracking showers. Swings and 'bouts.

A 35kW jobbie will give a noticeably better flow, but may also require a gas pipe upgrade (so might the 30kW boilers, of course) but if only the 36er needs this, then factor in a few extra £undred for this.

Both types - depending on make - should modulate down to around 5 or 6kW when CH demand is low, and then simply 'cycle' (go on and off) when demand falls below this - eg, when most rad TRVs are closed and the house's heating demand is satisfied.

If you fit a combi, it should be good for, ooh, ~15 years. At that stage there will likely be other alternatives available - a transition to hydrogen, maybe? Or, HeatPumps will be cheaper, commonplace, almost certainly improved, but as Markin says, this will need a cylinder - Thermal Store. A 'Store will also be required to take advantage of PV power.

Your call whether to fit one now, or wait. Personally, I'd wait until the tech has developed further, and come down in price.

So, keep your existing boiler type, or go combi?!

Is your existing hot cylinder also kaput? If 'yes', then that would make the decision easier for me - I think I'd also go 'combi' for the time being, and then see what transpires over the coming decade. You could replace it, of course, but with what - a similar type of cylinder which will be no use for a HeatPump? Or go more costly Heat Store now, when it isn't really needed? I don't know.

If the existing cylinder is actually working fine, and perhaps just needs a new thermostat, immersion element, motorised valve, and a smart new jacket, then I think I'd just go for a new Heat-Only boiler for the current setup - it's the cheapest option, and also the devil you know. In which case, I'd also seriously consider making it a 'system' boiler - 'seal' the whole system - and lose the wee F&E tank in the loft. Just a better setup all 'round.

How much of the above makes sense?

macman

The electricity usage on any gas boiler is trivial.

RonRamsgate

The gas boilers mentioned just use minimal electric to power a fan, pump and a few indicator neons. Maybe less than your existing set up as the new combination gas boiler will ave an energy saving pump-not much but better than less.

Converting from a non combination boiler system to combination boiler system has so many advantages and the opposite. Its already been mentioned at how many bathrooms do you have which is super important.
Some gas combination boilers are not so efficient when drawing off hot water but offer better efficiency when heating radiators. 
Hopefully the existing boiler and its output has been professionally selected and not guessed at. Maybe the heating load (radiators) is just 12.5Kw ish. The new combination boiler can be onsite adjusted down to suit the load.

Baxi or Main would be my first choice, same boilers    just different initial cost and warranty period. 

Torque79

Thank you for taking your time to respond.  

1 adult (he says!) in a semi-detached, 2 bed, 1 bath (no shower but shower mixer on taps), 2 sink (kitchen / bathroom)
Glow worm F55 conventional boiler with cylinder and loft tank,
9 radiators (2 very small, 1 very small double, 4 small, 1 long, 1 long double)
Only gas appliances are the boiler and the cooker. 

Currently CH upstairs however no CH downstairs (somtimes one rad has a little warmth) and HW is intermittent!  


From the engineers that have been suggestions are: 
"replace air vent on cylinder + new pump and may need new valves won't know till start OR Main 30kw or baxi 30kw" 

"change pump (I believe re-balancing as well) OR Main 30kw or Baxi 30KW"

"Main 30kw or Baxi 36kw" 

The flow rate has not been checked by an engineer.  

plumb1_2

Torque79 said:

change vent on cylinder 

do you have a small header tank in your loft?

If not and there’s a small vent on the cylinder, you’ll have a primatic cylinder.
If your gonna upgrade then fit a indirect cylinder if using heat only boiler 

Torque79

ThisIsWeird said:

Hi Torque (gettit? Lol...)

cough

As Markin says, consider the pros and cons of going combi, and make sure the decision isn't just based on the GasSafe's advice.

To answer your Q, electricity consumption by gas boilers is pretty minimal, and I certainly cannot see any significant difference between boiler types. A combi will only use leccy when actually producing hot water (and CH), whereas a heat-only boiler will be running for longer in order to heat a cylinder full over a longer period of time. But, little electricity consumption either way. So, I can't see any reason to consider leccy consumption in your boiler decision.

If you have been used to the hot flow from a cylinder, good chance you may be disappointed by that from a combi boiler; it'll be slower to fill a bath, for example, and will be strongly affected by a second tap being turned on. Having said that, you will enjoy the powerful 'mains' flow when a normal tap is used, and they should also give cracking showers. Swings and 'bouts.

A 35kW jobbie will give a noticeably better flow, but may also require a gas pipe upgrade (so might the 30kW boilers, of course) but if only the 36er needs this, then factor in a few extra £undred for this.

Both types - depending on make - should modulate down to around 5 or 6kW when CH demand is low, and then simply 'cycle' (go on and off) when demand falls below this - eg, when most rad TRVs are closed and the house's heating demand is satisfied.

If you fit a combi, it should be good for, ooh, ~15 years. At that stage there will likely be other alternatives available - a transition to hydrogen, maybe? Or, HeatPumps will be cheaper, commonplace, almost certainly improved, but as Markin says, this will need a cylinder - Thermal Store. A 'Store will also be required to take advantage of PV power.

Your call whether to fit one now, or wait. Personally, I'd wait until the tech has developed further, and come down in price.

So, keep your existing boiler type, or go combi?!

Is your existing hot cylinder also kaput? If 'yes', then that would make the decision easier for me - I think I'd also go 'combi' for the time being, and then see what transpires over the coming decade. You could replace it, of course, but with what - a similar type of cylinder which will be no use for a HeatPump? Or go more costly Heat Store now, when it isn't really needed? I don't know.

If the existing cylinder is actually working fine, and perhaps just needs a new thermostat, immersion element, motorised valve, and a smart new jacket, then I think I'd just go for a new Heat-Only boiler for the current setup - it's the cheapest option, and also the devil you know. In which case, I'd also seriously consider making it a 'system' boiler - 'seal' the whole system - and lose the wee F&E tank in the loft. Just a better setup all 'round.

How much of the above makes sense?

I was told a like for like system would cost too much and the combi was suggested.  
I had originally though it was the 3-way-valve but the most recent engineer ruled that out as an issue. 

The cylinder does seem to work as HW is produce just intermittently and I suspect that is the pump. 
It seems to be the pump and perhaps there could even be sludge or air or the need for rebalancing. 
I have the cash to cover for a new Combi, what I am trying to avoid is spending on repairs and subsequently find more problems e.g. it is boiler performance. 

Heat Pump - I have just been reading / watching a video about them:
Ground HP: seems to be very expensive and compromises land as you can't then build on-top, there is a bore-hole option which is even more expensive.
Air HP: seems to be the more financially viable option but then your home needs to be efficient re no draughts. 
Conclusion: HP are for new build homes where they are designed with HP in mind and not for old draughty homes.