Five degree differential btw CH outflow/return - condensing combi

[Kmac]

I recently had a Baxi Duotec 24 installed. The commissioning paperwork says that the central heating outflow is 70 degrees, return is 65 degrees (DHW at 55 degrees, if you're curious). It serves seven rads in six rooms, including one towel rail - all bar the towel rail have TRVs and all are fully open.

The loss seems really low to me and I'm concerned about efficiency - one of the drivers for getting our almost 20 year old Vokera non-condensing combi replaced was to improve the heating efficiency. I just want to make sure that I'm doing that.

Several things are confusing me:
1. The installation engineer told me very clearly not to put the central heating outflow temperature down below 70 degrees because "that's what the manufacturer recommends". I can't find this recommendation anywhere.
2. Don't condensing combi boilers rely on relatively low return temperatures (lower than 65 degrees, anyway) to condense?
3. I don't think the radiators have ever been balanced on the system - the boiler installer certainly didn't do this - could this be a (contributory) cause?

From the bits of information I have, it seems to me that what I want to do is have the outflow nice and hot but have a much greater loss across the system, returning at a much lower temperature, right? How might I achieve that?

For background, what's prompted my curiosity is that it's taking ages for room temperatures to get up to what the thermostat demands (19 degrees is all) and the rads seem much less hot to the touch than they did with the old boiler. They used to be scalding, but I have no idea what temp the central heating was set to because the temp gauge on the old Vokera was bust. We have our heating on evenings and weekends - off all day during the week and on low (15 degrees) at night.

Any ideas appreciated...

 

[newcastle phill]

Depends on the original radiators. If they were sized small, as many were years ago (make em small and burny hot) then you may never get them to put out enough meaningful heat while maintaining a low return temp.
With old boilers efficiency wasnt dependent on return temp as they didnt condense.

 

[jtsplumbing]

Have you tested the temperature of the flow (out pipe) & Return (In pipe) just below the boiler, (don't go off what they have wrote in the installation book) and see what the difference is

 

[Kmac]

Depends on the original radiators. If they were sized small, as many were years ago (make em small and burny hot) then you may never get them to put out enough meaningful heat while maintaining a low return temp.
With old boilers efficiency wasnt dependent on return temp as they didnt condense.

With the old boiler they were indeed burny hot - so, if I follow you, even if I do balance the rads, I may never get a decent differential anyway if they are undersized...

...reading between the lines of your reply though, I guess you're confirming that in an ideal world I'd have a much lower return temp with a much wider differential? For me, I'd actually like the rads to be hotter too, given how long it now takes the rooms to heat compared to the old system...

...hmmm.

Point taken on measuring the temperatures myself - I just don't have a thermometer (yet!). I take it this is best done when the heating has been on for a while?

 

[Chalked]

It all depends when you measure the temperature. At first you get a large difference, but after all the rads become hotter. The return temperature will go up near to the flow temp.

Oversized rads help, but still eventually the return temp will be similar to the flow.

 

[SimonG]

I have an idea, that you are over thinking it.

All rads have trv's bar the towel rail. Room stat in the bathroom?

 

[lame plumber]

reduce the pump speed, see what happens

 

[Kmac]

I have an idea, that you are over thinking it.

All rads have trv's bar the towel rail. Room stat in the bathroom?

Part of me wishes it was that simple. Room stat is wireless and not fixed to the wall - sits in the living room (well away from the rads!) during the day and the bedroom at night.

 

[Kmac]

It all depends when you measure the temperature. At first you get a large difference, but after all the rads become hotter. The return temperature will go up near to the flow temp.

Oversized rads help, but still eventually the return temp will be similar to the flow.

Ok, makes sense - so when should I measure the temp?

And what am I aiming for - if anything in particular?

 

[Swizz]

Get those rads properly balanced, if captain plumb put inhibitor into the system then it could have reacted and left you with air in the system so make sure the rads are bled. After this i'd take a look at your pump, turning it can help!

Also you're right about the condense.. you want it around 55 if you want to save some cash.
The 70 degrees sound a bit too high!

 

[Chalked]

Get those rads properly balanced, if captain plumb put inhibitor into the system then it could have reacted and left you with air in the system so make sure the rads are bled. After this i'd take a look at your pump, turning it can help!

Also you're right about the condense.. you want it around 55 if you want to save some cash.
The 70 degrees sound a bit too high!

The problem with condensing boilers is that the boiler, to run at its most efficient, likes to run at 50-60 degrees. But older systems were designed with a higher flow temperature. Therefore, need 70-80 degrees.
Best thing in an older sysem, is to have new oversized rads, but this costs and causes disturbance to the decoration.
So a compromise is to balance the system, ie. Restrict the lockshield to get about 15 degree difference between flow and return. Tune the pump setting to suit the system .
Then try running the boiler at 60. If the house becomes noticeably cooler turn up a few degrees at a time.