Trying to get my head round return flow temp, delta t, BTUs... help!

[houseonahill]

I spent hours yesterday evening reading about this and I am amazed (and intrigued) at how complicated it is. I already respected plumbers but now, woah, this is serious stuff!

Anyway, could someone help me understand this:

I have a new gas combi boiler, 30kw
When it was installed the plumber did a benchmark and noted down CH outflow temperature of 70C and return flow of 67C

This return flow seems too high to me. I currently have the CH outflow set to 60C as the manufacturer (Worcester Bosch) suggests condensing will only occur at a return flow of 52C or below, so I'm trying to keep the rads as low as possible while still heating the house properly.

Question 1: I would like to measure the return flow myself to check on it - any tips on the most reliable way to do this? (The boiler itself doesn't appear to measure this.)

Question 2: What is the most likely reason for a high return flow temp? Rads that are too small?

All the piping and radiators on the system are new. Most are double panelled and double-finned Myson (or Myson-style) rads. There is a 1650x500 towel rail in the bathroom as well as an electric underfloor heating system.

Question 3: The towel rail manufacturer advertises its BTU for this model at Delta 70 (naughty I know). But at Delta 60 we were told it would offer 2,850 BTU's. I'm trying to get a sense of what BTU's we're actually getting from the rail given the way the system is operating.

My rough calculation = ((out+return/2)-room temp) = ((60+52/2)-19) = Delta 37. Yes or no? And if yes, does that mean the actual BTU is only 1750 or so?

 

[flameport]

Flow of 70 is far too high for a modern boiler.
For a combi it can go much lower, provided it still heats the property adequately. The tradeoff is a lower flow/return will be more efficient but it will take longer to heat the rooms, and if too low it will never heat the rooms.
Radiators should be sized appropriately for lower flow/return temperatures.

Questions:
1 - Pipe thermometer on the return pipe at the boiler, such as https://www.bes.co.uk/clip-on-pipe-thermometer-63mm-22147/
2 - Flow is too high, radiators undersized, flow rate is too high.
3 - 70 isn't normally used, it's generally 50, however towel rails are useless at heating a room anyway so it really doesn't matter for that.
For other radiators there are conversion factors such as those here: https://www.tradeplumbing.co.uk/blog/btu-calculator-delta-conversions , it's not a linear relationship as in half the delta isn't half of the heat output.

Radiators should be sized in watts rather than BTU, and in the very near future lower flow temperatures of 55 or less will be the normal thing (and actually required for new installations).

 

[houseonahill]

Thanks for your reply, @flameport ! I'll definitely get one of those thermometers.

On the towel rail, I have to say it's not working badly. We've had overnight temperatures of 4-5C here, well into the morning, and the room has been warm enough with the heat mat on for an hour or two and the towel rail on for about 1.5 hours (boiler flow temp was at 55C at that stage - I'm just testing it at 60C at the moment to see how it performs.). I would prefer the room SLIGHTLY warmer, maybe a degree or two, but we're planning to improve insulation around the bathroom in the future so hoping that does the trick. And will be interesting to see how it performs on a properly cold winter morning.

But would you recommend swapping the towel rail for a rad? Could do that, just needs to match the existing pipe centres...

 

[Mister Banks]

When my WB Greenstar 24i was installed back in 2015 the installer recorded the outflow at 72c and the return flow at 70c. I find that nearly all online sites recommend setting the boiler thermostat to 70c.

Having the boiler outflow at approx. 70c seems reasonable as the hot water tank needs to be heated to no less than 60c which is what the HW thermostat is set too and is the recommended minimum temperature.

 

[ianmcd]

When my WB Greenstar 24i was installed back in 2015 the installer recorded the outflow at 72c and the return flow at 70c. I find that nearly all online sites recommend setting the boiler thermostat to 70c.

Having the boiler outflow at approx. 70c seems reasonable as the hot water tank needs to be heated to no less than 60c which is what the HW thermostat is set too and is the recommended minimum temperature.

Not strictly true, the reason for the HW setting is to prevent Legionella in your hot water cylinder, many modern systems can send different temp water to the HW circuit and the heating circuit, but that is the problem reading online sites , not all systems are the same , 70C is what I recommend the heating max is set to, but I am not female, they are always freezing

 

[Mister Banks]

Not strictly true, the reason for the HW setting is to prevent Legionella in your hot water cylinder, many modern systems can send different temp water to the HW circuit and the heating circuit, but that is the problem reading online sites , not all systems are the same , 70C is what I recommend the heating max is set to, but I am not female, they are always freezing

I'm on an S plan so the water to heat the radiators and the HW circuit is the same.

 

[ianmcd]

I'm on an S plan so the water to heat the radiators and the HW circuit is the same.

yes it totally is, why I said "not strictly true" you may be surprised that more than you will read this and think that everyone has the same system as you

 

[Razor900]

Sounds very much like the installer never checked the flow and return temps.

This time of year it makes no difference but come heating season turn the boiler stat down to 60 degrees and see if you still get warm enough. If you do turn it down to 55 etc

That's for a combi. System boiler need, (as Ian says), to be set higher to reheat the cylinder although there are many ways around it depending on the boiler

 

[vulcancontinental]

Bit crude but throttle the lockshield valves on the radiators down. Start at off then open a quarter turn those nearest the boiler and no more that one turn for those far away. 3 degree Δdelta T is certainly not optimal, slow the circulation down to get wider ΔT as possible. Don't know your boiler but the pump may be adjustable, if it is try other settings (should be in the manual either on the side of the pump or boiler parameters).

 

[Johntheo5]

I spent hours yesterday evening reading about this and I am amazed (and intrigued) at how complicated it is. I already respected plumbers but now, woah, this is serious stuff!

Anyway, could someone help me understand this:

I have a new gas combi boiler, 30kw
When it was installed the plumber did a benchmark and noted down CH outflow temperature of 70C and return flow of 67C

This return flow seems too high to me. I currently have the CH outflow set to 60C as the manufacturer (Worcester Bosch) suggests condensing will only occur at a return flow of 52C or below, so I'm trying to keep the rads as low as possible while still heating the house properly.

Question 1: I would like to measure the return flow myself to check on it - any tips on the most reliable way to do this? (The boiler itself doesn't appear to measure this.)

Question 2: What is the most likely reason for a high return flow temp? Rads that are too small?

All the piping and radiators on the system are new. Most are double panelled and double-finned Myson (or Myson-style) rads. There is a 1650x500 towel rail in the bathroom as well as an electric underfloor heating system.

Question 3: The towel rail manufacturer advertises its BTU for this model at Delta 70 (naughty I know). But at Delta 60 we were told it would offer 2,850 BTU's. I'm trying to get a sense of what BTU's we're actually getting from the rail given the way the system is operating.

My rough calculation = ((out+return/2)-room temp) = ((60+52/2)-19) = Delta 37. Yes or no? And if yes, does that mean the actual BTU is only 1750 or so?

You are on the right track but a exponential is involved, generally 1.3, occasionally 1.31 to 1.33 is used but very rarely. Your output above is
((37/60)^1.3) X 2850 = 0.53333X2850 = 1520. (^1.3 is "to the power of 1.3")
Rads are now rated at 50 deg.