Lowering return temperatures

[Hollinside]

Evening all. Enthusiastic have a go guy here! I did get into the whole calculation of heat capacity of flow and calculated resistance after we had a few terrifying quotes when I bought the house - Quite a large 3 floor 7 bed Edwardian house. I fitted a 300 litre unvented cylinder and full central heating (25 years ago!). Worry not- a proper engineer checked it over and although the boiler room was deemed ‘ugly as xxxx’ it was actually all good, safe and commissioned. We had a standard 60 kilowatt boiler and grundfos 25/80 pump- changed 11 years ago to a condensing boiler range rated at 25 kilowatts and 7m wilo erp pump. No problems whatsoever other than one dhw expansion vessel ruptured .

Radiators were calculated to heat rooms at 55 degrees c flow ( yeah some rads are huge and we do use a few huge fan convectors on low flow/ low fan ). No problems still and only a couple of tiny random pressure drops on all those years . And we get a LOT of condensate - so I’m happy. Obviously as it gets colder we manually turn up the flow temp (next replacement boiler will probably be opentherm or similar with compensation ( once someone pulls together a decent and comprehensive offering)

As we creep toward 65 degrees c flow temp we do get a lot less condensing- and this bothers me! Has anyone ever installed a radiator with no valves in line with the return line just to lower the return temp and make this condense more?

I’d be interested in this or if there is a domestic version of a temp differential valve for domestic use. I like to try and plan ahead for future developments - even thought about a small heat bank and a higher output 1:10 modulating boiler - but at the moment lowering flow return temp is my pressing question- every radiator balanced/ throttled to an inch of their lives including the DHW coil.

any thought

 

[Scott_D]

The more heat that you ‘dump’ the harder the boiler will have to work to re-heat so efficiency will be lost.

 

[Hollinside]

The more heat that you ‘dump’ the harder the boiler will have to work to re-heat so efficiency will be lost.

We do have a cold spot we can use the extra heat in. Boiler Would be working harder but condensing more so additional saving can still likely be made with a larger feed/ return differential on the primary circuit

 

[siricosm]

Assuming everything is balanced properly, the most obvious way to lower the return temperature is to increase the size (or add more) rads. In most systems the rads are undersized to give condensing on the coldest days. Also it is tricky to exactly size every rad for each room, so assuming everything has been adjusted to give you correct heat in each room, you will find some rads get hotter than others, and these will typically have more flow. You might consider increasing the size of the hottest ones first, you might only need to enlarge a few to get the result you desire. You can test this idea by putting a small fan on them, which will increase their effective size temporarily.

In the end, if the rads are large enough to heat the house with a return temperature low enough to keep the boiler condensing, you should not need to turn up the flow temperature on the boiler on cold days. Another way to find the (undersized) rads to target would be to keep the temperature at 55 (or as high as possible with full condensing) and look which rooms are too cold.

Installing an extra rad inline might drop the return temperature to condensing range if it was large enough, but where would the heat from that rad go? And would you want such a rad on all the time? if it were inline you would not be able to turn it off, and it would be running when it is not so cold and boiler would be condensing properly without it. There is no point in overheating part of the house just to make the boiler condense, as that kind of defeats the purpose. A rad with no control is almost always a bad thing, with the possible exception of a heat dump for solid fuel, and even then that is just a safety/control issue.

Another possibility could be to try increase the duty cycle of the rads, it could be that those small rads are big enough even at 55C if they were on all the time in cold waether.

 

[Hollinside]

Assuming everything is balanced properly, the most obvious way to lower the return temperature is to increase the size (or add more) rads. In most systems the rads are undersized to give condensing on the coldest days. Also it is tricky to exactly size every rad for each room, so assuming everything has been adjusted to give you correct heat in each room, you will find some rads get hotter than others, and these will typically have more flow. You might consider increasing the size of the hottest ones first, you might only need to enlarge a few to get the result you desire. You can test this idea by putting a small fan on them, which will increase their effective size temporarily.

In the end, if the rads are large enough to heat the house with a return temperature low enough to keep the boiler condensing, you should not need to turn up the flow temperature on the boiler on cold days. Another way to find the (undersized) rads to target would be to keep the temperature at 55 (or as high as possible with full condensing) and look which rooms are too cold.

Installing an extra rad inline might drop the return temperature to condensing range if it was large enough, but where would the heat from that rad go? And would you want such a rad on all the time? if it were inline you would not be able to turn it off, and it would be running when it is not so cold and boiler would be condensing properly without it. There is no point in overheating part of the house just to make the boiler condense, as that kind of defeats the purpose. A rad with no control is almost always a bad thing, with the possible exception of a heat dump for solid fuel, and even then that is just a safety/control issue.

Another possibility could be to try increase the duty cycle of the rads, it could be that those small rads are big enough even at 55C if they were on all the time in cold waether.

Absolute the best advice. I too am concerned about an in-line return only fed radiator and the control of the temperature in that area. I do like to run the boiler as low as possible all the time as most of the year we see a very low need for space heat - just a few degrees extra needed in the house. At the moment we are running at 65 degrees and the return temp is just above 50. Bigger rads are definitely the ‘by the book’ way of doing things but there are 26 big beasts and I’d say five could do with being larger although they are the largest we can get at the time ( i have looked at triple panel rads but quite pricey) - I think I’ll do things properly- it would be the better albeit more costly route. I love our huge fan convectors run slow and low but they were tremendously expensive- necessary for the big rooms (8x 11 meters almost 5 meter ceilings). I didn’t want umpteen radiators in those rooms! Thanks for snapping me out of my dream state 👍🏻

 

[OffshoreGas]

If you reduce the flow rate through the rads the heating medium had more resonance time and more heat energy will be transferred to the room.

 

[Stigster]

If you can reduce the pump speed and therefore water velocity that will lower the return temperature. The problem there is if the pump speed is too low for the system you have it may not heat the house adequately. It's all a real balancing act!

 

[John.g]

Also installing TRVs and raising the flow temperature will result in lower return temp, for example, a 80c flow and 40c return gives the same rad output as 70c flow and 50c return.

 

[Chuck]

As we creep toward 65 degrees c flow temp we do get a lot less condensing- and this bothers me!

As @Stigster said, you're pumping water around the system too fast. Get hold of a couple of suitable thermometers and go round and adjust/check the temperature difference between flow and return connection for each radiator to be 10 degC (or a touch higher) with the TRV (if fitted) fully open. For a system with TRV's fitted I prefer a smart pump in 'constant pressure' mode.

My advice is not to obsess about this too much. In practice, you'll only make a few percent difference to your gas consumption and you'll discover that TRVs + condensing boiler with finite turn-down ratio = short cycling in spring and autumn. The only cure for this is to increase the thermal mass of the CH water by installing a thermal buffer store, which is over-kill for most domestic systems.

It's a sad fact that although condensing boilers have been standard for many years, many of them almost never run in condensing mode because they are attached to poorly balanced emitter systems with insufficient thermal mass. Walk along a residential street on a cold day and look at the plumes from flues to see what I mean. If more than one in ten is not billowing uncondensed water vapour I'll be surprised.

 

[Hollinside]

As @Stigster said, you're pumping water around the system too fast. Get hold of a couple of suitable thermometers and go round and adjust/check the temperature difference between flow and return connection for each radiator to be 10 degC (or a touch higher) with the TRV (if fitted) fully open. For a system with TRV's fitted I prefer a smart pump in 'constant pressure' mode.

My advice is not to obsess about this too much. In practice, you'll only make a few percent difference to your gas consumption and you'll discover that TRVs + condensing boiler with finite turn-down ratio = short cycling in spring and autumn. The only cure for this is to increase the thermal mass of the CH water by installing a thermal buffer store, which is over-kill for most domestic systems.

It's a sad fact that although condensing boilers have been standard for many years, many of them almost never run in condensing mode because they are attached to poorly balanced emitter systems with insufficient thermal mass. Walk along a residential street on a cold day and look at the plumes from flues to see what I mean. If more than one in ten is not billowing uncondensed water vapour I'll be surprised.

Ooh. Don’t set me off! My brother had an old back boiler set at maximum on tiny old radiators- he now pumps water around that at 80c and the return isn’t far off 80. He’s a dumb sod, he wonders why it always breaks down. I actually do think I’m obsessing a bit about squeezing every drop of energy out of it- To be fair I’m doing a decent job. One neighbour has £600 a month avg gas bill , ours is £130. It’s a big Edwardian house in conservation area so we’re not allowed to have double glazing etc My brothers is a fifty year old small 3 bed with every mod con and he still pays more! I was told I was obsessing off the other half- 25 litres of condensate in summer. 11 in winter mode. So thanks for the affirmation

we run the pump in auto mode as most rads have thermostats and we have one “problem section” which is 15 mm pipework behind some very decorative plasterwork serving the top floor serving about 8kw of radiatora- it’s far from ideal but god only knows how that was ever fitted! pump currently runs at 32 watts (it’s a wilo yonos pico 25 1-8 ). I’m probablynot going to get much better than updating to some larger rads/ fan coils and it’s not broken down in 11+ years.

Hopefully by the time this needs replacing someone will have a comprehensive opentherm/ other protocol 9+ zone offering- All in one place. Viessmann were incredibly helpful but my god- it was mortgage territory

 

[garygvl]

Also installing TRVs and raising the flow temperature will result in lower return temp, for example, a 80c flow and 40c return gives the same rad output as 70c flow and 50c return.

yes but it screws the heat exchanger in the boiler

 

[John.g]

Yes, realistically I suppose 65c should be used as the minimum flow temp to cater for DHW (legionella) and if one allows a hx deltaT of max 24/25C then if original rad deltaT was 10C then raising the flow temp to 72C will give a return of 48C but I would agree that there is a bit of obsessing with condensing as even a return of 48C will only give a few % gain in boiler efficiency, a condensing boiler even if never in condensing mode will be ~ 7% more efficient than a SE boiler under the same conditions, my recollection of my 14 year old SE Firebird 90S is of a 230C flue gas temp whereas it should be quite normal to have a flue gas temp of ~ 80C? or even less with a
condensing unit.
Edit.
Of course maybe modern boilers (not combi) can or should be able to be configured to give a very high flow temperature on DHW only and as low as desired on CH only which would give far more flexibility in attaining very low return temperatures provided that the rads have been suitably oversized in the first place.

 

[John.g]

Ooh. Don’t set me off! My brother had an old back boiler set at maximum on tiny old radiators- he now pumps water around that at 80c and the return isn’t far off 80. He’s a dumb sod, he wonders why it always breaks down. I actually do think I’m obsessing a bit about squeezing every drop of energy out of it- To be fair I’m doing a decent job. One neighbour has £600 a month avg gas bill , ours is £130. It’s a big Edwardian house in conservation area so we’re not allowed to have double glazing etc My brothers is a fifty year old small 3 bed with every mod con and he still pays more! I was told I was obsessing off the other half- 25 litres of condensate in summer. 11 in winter mode. So thanks for the affirmation

we run the pump in auto mode as most rads have thermostats and we have one “problem section” which is 15 mm pipework behind some very decorative plasterwork serving the top floor serving about 8kw of radiatora- it’s far from ideal but god only knows how that was ever fitted! pump currently runs at 32 watts (it’s a wilo yonos pico 25 1-8 ). I’m probablynot going to get much better than updating to some larger rads/ fan coils and it’s not broken down in 11+ years.

Hopefully by the time this needs replacing someone will have a comprehensive opentherm/ other protocol 9+ zone offering- All in one place. Viessmann were incredibly helpful but my god- it was mortgage territory

These posts might help you in your quest for maximum condensing effect but you will need very large rads or fan coils.

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