Water Meter system boiler vs combi

FreeBear

Steveotwo said:

JohnSwift10 said:

When I had a hot water tank I used 3kWh a day for the pilot light and 7 kWh a day to heat the water.
After I got a combi boiler my daily use (in the summer without the heating being on) is about 11 kWh a day so no difference in gas usage overall  but I am using more gas to heat the water water.

As for water use I have no idea as I live in Scotland and we don't get the option to have water meters.and I am paying £667.81 a year for water used by 2 OAPs

So the combi is less efficient?

With a combi, you only heat the water as you use it (plus a bit of waste in the boiler & pipes). With a system boiler, you have to heat the whole tank, plus water in the boiler & pipes. On top of that, the boiler will fire up on a regular basis to maintain the water temperature in the tank unless you have it on a timer.

Unless you are a big user of hot water, a combi may well work out a lot cheaper & efficient to run.

JohnSwift10

ThisIsWeird said:

JohnSwift10 said:

When I had a hot water tank I used 3kWh a day for the pilot light and 7 kWh a day to heat the water.
After I got a combi boiler my daily use (in the summer without the heating being on) is about 11 kWh a day so no difference in gas usage overall  but I am using more gas to heat the water water.

Not sure these figures add up.

With your old hot tank system, if you'd had a boiler that didn't have a pilot light (like all boilers these days), then you would only have used around 7kWh per day to heat your water? And your combi - which doesn't have a pilot light - uses 11kWh for essentially the same job? That can't be right.

If your figures are correct, I can only guess you are using a lot more hot water.

All other things being equal, a combi should theoretically use a little less energy than a stored water system, simply because a stored hot water tank will lose some heat through its skin over the day.

The old system was part of a warm air system and the hot water tank did not have a thermostat.

I was surprised too when I saw how much gas the combi was using.

The shower I had then was a power shower, it used a pump to feed the hot and cold water.

My shower now is a mixer and it probably does use more water than the old one.

Kiran

Other things that might be worth considering. A system boiler gives you the additional options of being able to take advantage of solar thermal amd solar PV to heat water, it allows you to benefit from agile pricing if you use cheap rate electricity to heat your tank. I'd go as far as to say the cylinder and controls are on par with respect to the amount they can save you over just changing a functional boiler for a new one

ThisIsWeird

JohnSwift10 said:

ThisIsWeird said:

JohnSwift10 said:

When I had a hot water tank I used 3kWh a day for the pilot light and 7 kWh a day to heat the water.
After I got a combi boiler my daily use (in the summer without the heating being on) is about 11 kWh a day so no difference in gas usage overall  but I am using more gas to heat the water water.

Not sure these figures add up.

With your old hot tank system, if you'd had a boiler that didn't have a pilot light (like all boilers these days), then you would only have used around 7kWh per day to heat your water? And your combi - which doesn't have a pilot light - uses 11kWh for essentially the same job? That can't be right.

If your figures are correct, I can only guess you are using a lot more hot water.

All other things being equal, a combi should theoretically use a little less energy than a stored water system, simply because a stored hot water tank will lose some heat through its skin over the day.

The old system was part of a warm air system and the hot water tank did not have a thermostat.

I was surprised too when I saw how much gas the combi was using.

The shower I had then was a power shower, it used a pump to feed the hot and cold water.

My shower now is a mixer and it probably does use more water than the old one.

Blimey.
Still surprises me - I just cannot see a combi using more gas that any other gas-fired alternative. 
The only explanation I can think of is additional water use, but even then it's such a difference.
I cannae explain it.

Steveotwo

Kiran said:

Other things that might be worth considering. A system boiler gives you the additional options of being able to take advantage of solar thermal amd solar PV to heat water, it allows you to benefit from agile pricing if you use cheap rate electricity to heat your tank. I'd go as far as to say the cylinder and controls are on par with respect to the amount they can save you over just changing a functional boiler for a new one

How would I go about adding these solar options? I always thought these where very expensive?

Steveotwo

Another question then, lets say it breaks today and cannot be repaired. A heat pump is unaffordable to me right now, would replacing with just a modern system boiler be more cost effective given its cheaper than upgrading to a combi system? Are modern system boilers much more efficient? @Kiran @FreeBear

BarelySentientAI

Steveotwo said:

Another question then, lets say it breaks today and cannot be repaired. A heat pump is unaffordable to me right now, would replacing with just a modern system boiler be more cost effective given its cheaper than upgrading to a combi system? Are modern system boilers much more efficient?

Modern system boilers and modern combi boilers are essentially the same efficiency.

Steveotwo

BarelySentientAI said:

Steveotwo said:

Another question then, lets say it breaks today and cannot be repaired. A heat pump is unaffordable to me right now, would replacing with just a modern system boiler be more cost effective given its cheaper than upgrading to a combi system? Are modern system boilers much more efficient?

Modern system boilers and modern combi boilers are essentially the same efficiency.

Ohhh, so theres no reason for me to spend an extra £800ish on going combi? are they abit less efficent because they have a pilot light on all the time or am I wrong here?

BarelySentientAI

Steveotwo said:

BarelySentientAI said:

Steveotwo said:

Another question then, lets say it breaks today and cannot be repaired. A heat pump is unaffordable to me right now, would replacing with just a modern system boiler be more cost effective given its cheaper than upgrading to a combi system? Are modern system boilers much more efficient?

Modern system boilers and modern combi boilers are essentially the same efficiency.

Ohhh, so theres no reason for me to spend an extra £800ish on going combi? are they abit less efficent because they have a pilot light on all the time or am I wrong here?

Pilot lights are old tech.  Modern boilers don't have them.

The difference was explained before:

Combi heats water as you need it.
System heats a tank of water for you to use as you need it.

They both do these at basically the same efficiency.

When you have a tank, it will cool down a little bit, so the system boiler has to turn back on sometimes to heat it back up.  That might mean you use a little bit more gas overall, but that all depends how good the tank insulation is, how much hot water you normally use, and various other things. - system is "worse" than combi for that

A water tank would probably also have an immersion heater so you could use electricity to heat up the water if you needed/wanted - system is "better" than combi for that

If you change all your plumbing and remove the water tank, adding a heat pump in the future (or deciding you want solar and can heat water with the 'spare' electricity, or you get a clever electricity tariff and someone will pay you to use electricity, or...) will need you to add a water tank back again - system is "better" than combi for that

There is no right answer.  There are reasons for both.

FreeBear

Steveotwo said:

Another question then, lets say it breaks today and cannot be repaired. A heat pump is unaffordable to me right now, would replacing with just a modern system boiler be more cost effective given its cheaper than upgrading to a combi system? Are modern system boilers much more efficient?

Indeed. Modern system & combi boilers can achieve efficiencies as high as 98% for central heating. To get to this sort of level requires a low flow temperature (typically, 50°C or lower) which in turn means larger radiators. Depending on how old the original boiler is, the radiators may have been sized with a flow temperature of 80°C. More modern installs will use 70°C, but to use 50°C, you need to double the radiator size to get the same effective heat output.

When it comes to heating domestic hot water, the boiler needs to run at 5-10°C hotter. Depending on the DHW temperature, you would probably be running the boiler outside of condensing mode, and efficiency will drop to 86% or less - I aim for a DHW temperature of @45°C in part, for better efficiency, and also because I dislike scalding hot water.

DHW is a relatively small part of your total energy consumption in a year - For most people, heating is the major part, so it makes sense to target conservation measures there. Increased insulation, reduction of cold draughts, lower flow temperatures, and smarter control (programmable thermostat).