Running combi boiler in "condensing" mode

rob7475

We currently have a worcester greenstar 30si combi which was installed in 2010 (think it's a mk4). We use Tado to control it and everything works fine.With the ever increasing gas prices, I'm looking at how I can run the boiler more efficiently. 

Our house is a 1920's semit with a cavit wall that's not insulated (the house is quite exposed so CWI isn't recommended. Double glazed throughout and 270mm loft insulation. I think the heat loss for the house is quite high but have never calculated it.

We have the heating flow temp set to full on the boiler and the radiators get hot throughout the house and the house feels warm reasonably quickly. To try and run things more efficiently, I've turned the heating dial down to just below 4 so hopefully the boiler runs in condensing mode more. I've compared heat up times over the last two days when external temperatures etc have been the same and have found that:

1. with the boiler on full, the house heats from 19 to 21.5 in 1.5 hours.
2. With the boiler set to just under 4, the house heats from 19 to 21.5 in 2.5 hours

The external temperature was around 9 degrees on both days so not overly cold.

So, although the boiler may be running in condensing mode, it's taking much longer to heat the house.

So, realistically, am I actually saving any gas by running things at a lower temperature?

BUFF

If it is actually condensing v non-condensing, yes. & the lower that you can run it the more efficient it will become.
It's also better/easier for the boiler & therefore likely to result in an extended life.

4 is 68C flow so should be condensing
Full is ~80C & highly unlikely to be condensing.

It is possible that your radiators are undersize for the lower flow temp. This will be true if they were properly sized for a 75 or 80C system design but  it's not unknown for rads with higher outputs than necessary to be fitted (same with boilers).

chris_n

If you have turned your flow temp down the boiler will modulate more to keep the water temp lower. If you want to see any differences you would have to monitor gas usage on identical days over an extended time rather than measuring heat up time. You could leave the flow temp lower while the weather is milder and turn it up if you have to (and be less efficient) when it gets colder. If that is what you have to do then as BUFF says you need bigger radiators.

rob7475

Thanks for your reply.

I'll have to take some meter readings and try and figure out the consumption but I can't see how it saves money to have the boiler running for so much longer? The way I understand it, you save around 10% gas when condensing. However, if the boiler is running for 50-60% longer to reach the desired room termperature, surely those savings are wiped out?

Most of my radiators are oversized (i fitted most myself so oversized them to try and future proof a bit). I think the main issue is that my house probably has quite high heat loss due to the lack of any wall insulation

shiraz99

This is the big issue with turning boilers down to run at 55C or lower. Yes, they will run more efficiently in condensing mode but the rest of your CH system, eg, your pipes and radiators may not be sized correctly to take full advantage. The lower the flow temperature the bigger your radiators need to be to compensate. Up until now most installations would've been designed for boilers running at 70C or more, having said that a lot of heating engineers have been oversizing radiators for years so you might be alright. I'd go around have a look at what radiator types you have, if you have type 11 rads in a lot of places then you might want to consider upgrading to the same size in type 21 or 22 as they'll output so much more heat.

Also, if you really want to save money you might consider also turning the room thermostat down a degree or two. If my house was already 19C there's no way I'd have the heating on at the moment.

victor2

rob7475 said:

Thanks for your reply.

I'll have to take some meter readings and try and figure out the consumption but I can't see how it saves money to have the boiler running for so much longer? The way I understand it, you save around 10% gas when condensing. However, if the boiler is running for 50-60% longer to reach the desired room termperature, surely those savings are wiped out?

Most of my radiators are oversized (i fitted most myself so oversized them to try and future proof a bit). I think the main issue is that my house probably has quite high heat loss due to the lack of any wall insulation

It's not just the time it takes to get the house up to temperature, it's also the cost of keeping it there. Compare the same conditions for days with the boiler at max and a lower flow temperature. Comfort may be compromised a little at lower flow temperatures as you wait longer for the house to warm up, but MAYBE you could start the boiler sooner at the lower flow temperature and still reduce cost.

Only experimentation with your actual circumstances will tell you, and even then you can't control the weather!

shiraz99

victor2 said:

rob7475 said:

Thanks for your reply.

I'll have to take some meter readings and try and figure out the consumption but I can't see how it saves money to have the boiler running for so much longer? The way I understand it, you save around 10% gas when condensing. However, if the boiler is running for 50-60% longer to reach the desired room termperature, surely those savings are wiped out?

Most of my radiators are oversized (i fitted most myself so oversized them to try and future proof a bit). I think the main issue is that my house probably has quite high heat loss due to the lack of any wall insulation

It's not just the time it takes to get the house up to temperature, it's also the cost of keeping it there. Compare the same conditions for days with the boiler at max and a lower flow temperature. Comfort may be compromised a little at lower flow temperatures as you wait longer for the house to warm up, but MAYBE you could start the boiler sooner at the lower flow temperature and still reduce cost.

Only experimentation with your actual circumstances will tell you, and even then you can't control the weather!

But if you had a system with weather compensation you could take full advantage of it 😁

BUFF

rob7475 said:

Thanks for your reply.

I'll have to take some meter readings and try and figure out the consumption but I can't see how it saves money to have the boiler running for so much longer? The way I understand it, you save around 10% gas when condensing. However, if the boiler is running for 50-60% longer to reach the desired room termperature, surely those savings are wiped out?

Most of my radiators are oversized (i fitted most myself so oversized them to try and future proof a bit). I think the main issue is that my house probably has quite high heat loss due to the lack of any wall insulation

The boiler won't be running flat out all the time in either case. Usually what happens is it will start off at a high burn & then as it approaches running temp. it throttles back (modulates) so you might expect the 68C running boiler to start modulating sooner. Similarly, at the other end the 80C running boiler is more likely to overshoot the set temp.
Obviously individual circumstances can vary but in general the evidence is that it will save energy (& hence money).

.How did you calculate their sizes? It doesn't sound like they were sized properly (output) for the building/lower flow temp conditions.
& the larger the differential between outside & internal temps the higher your heat loss will be so as shiraz99 suggested consider if you really need over 19C?

markin

2.5c in 1.5 hours seems far to long for this time of year, Are they all bled and trv's open? Set it as low as you can stand, And bump it up in cold weeks.


If you drive at 40mph rather than 70mph it takes longer but should use less 'Gas' up to 15% less.

sandy700

markin said:

If you drive at 40mph rather than 70mph it takes longer but should use less 'Gas' up to 15% less.

Yet driving locally and never exceeding 30 mph I get 30 mpg but on the motorway at 70 -75 mph I get 39 mpg.

markin

sandy700 said:

markin said:

If you drive at 40mph rather than 70mph it takes longer but should use less 'Gas' up to 15% less.

Yet driving locally and never exceeding 30 mph I get 30 mpg but on the motorway at 70 -75 mph I get 39 mpg.

30 zones are all stop and go traffic, if you drive 40-50 on the motorway you should save fuel.