Boiler flow and return temperatures.

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I have my boiler (Vaillant Ecotec 18kw) set on 14Kw and max flow temp of 62°. I also have my pump set on the slowest setting in the belief that if the water is flowing through a radiator slowly, there’s more chance of extracting the maximum heat from it before it returns to the boiler. The system works fine and keeps us warm and supplied with hot water even on the coldest day.

While I was poking around up in the loft today (where the boiler is situated) I noticed the temperatures of the flow and return as I have a clip on analogue thermometer on the flow and return pipes. The boiler was sitting at 62° but the thermometer on the flow was saying 40° and the return 24°. The flow pipe was too hot to touch for anything more than a few seconds and the return was lukewarm, as you would expect. The boiler had been running for some time.

Two questions just out of curiosity - I don’t plan to change anything but should the flow thermometer have shown 62° and if not, is a 16° drop between the flow and return enough to make the boiler condense?
 
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40 degrees is bathwater temperature so I know which one I'd be believing. My boiler is also a similar Vaillant, if you go into the menus it can show you internally measured flow and return temperatures.
 
Most boilers show the flow temperature on the main display, but some show the set temperature. I think Vaillants do show the flow temperature. If you can only touch the flow pipe for a few seconds then for most people that would indicate it is about 60C. So it sounds like at least one of your clip on thermometers is reading wrong. On Vaillant boilers you can get flow and return readings from the main menu. 24C would feel cold not lukewarm.

A lower flow speed will mean you get a lower return temperature. (But it won't mean the radiators give out more heat overall.)

Strictly speaking it's the actual return temperature that's important for condensing, not the drop between flow and return, but they are of course linked. The lower the return is below about 54C then the more the boiler will condense. If both your thermometers are equally wrong ;) then you might have a return of 46C, in which case it should be condensing moderately.

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20 degrees between flow and return on condensing boilers is what it should be, in order for it to be most efficient. If you know how to access the D codes, then D40 and D41 will show you flow and return temperatures respectively. 60 degrees on the display will be the temperature inside the boiler heat exchanger.
 
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Thanks all. I’ll have another look tomorrow with those D codes.
Most boilers show the flow temperature on the main display, but some show the set temperature. I think Vaillants do show the flow temperature.
I’m assuming it was the flow temperature as I watched it go up from 58 to 62 and stay at 62. I set it at 62 ages ago because I read that it has to be 60° to stop legionella.
 
Thanks all. I’ll have another look tomorrow with those D codes.

I’m assuming it was the flow temperature as I watched it go up from 58 to 62 and stay at 62. I set it at 62 ages ago because I read that it has to be 60° to stop legionella.
I believe the advice is to set the boiler temperature to 5 degrees c above the hot water cylinder temperature.
 
I believe the advice is to set the boiler temperature to 5 degrees c above the hot water cylinder temperature.
I can never get my head around cylinder temperatures. IIRC my cylinder is set at about 45 degrees as the thermostat is at the bottom of the cylinder. If I tried to set it at 5 degrees below boiler temperature it'd run forever or explode!
 
I can never get my head around cylinder temperatures. IIRC my cylinder is set at about 45 degrees as the thermostat is at the bottom of the cylinder. If I tried to set it at 5 degrees below boiler temperature it'd run forever or explode!
From what I have read cylinder thermostats are hugely inaccurate and placing them so low on the cylinder doesn't help. With all these new fangled heating controls you would think someone could come up with something that accurately measures the temperature.
 
20 degrees between flow and return on condensing boilers is what it should be, in order for it to be most efficient. If you know how to access the D codes, then D40 and D41 will show you flow and return temperatures respectively. 60 degrees on the display will be the temperature inside the boiler heat exchanger.
Right. This morning I raised the flow temperature from 62 to 65. I set the room stat at 25° and ran the system for 30 minutes with just the rads on that we have on in the evening - Lounge (no TRV), hall, downstairs WC and bathroom towel rail which do have TRV's set around halfway. Flow was 65 and return was 49°. I then opened fully all 10 rads in the house, set all TRV's on max and after 15 minutes with a flow of 65°, I had a return of 42°. Not a 20° drop with just four rads but I read the return had to be under 55° for it to condense. Does that seem all okay?
 
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While I was poking around up in the loft today (where the boiler is situated) I noticed the temperatures of the flow and return as I have a clip on analogue thermometer on the flow and return pipes. The boiler was sitting at 62° but the thermometer on the flow was saying 40° and the return 24°. The flow pipe was too hot to touch for anything more than a few seconds and the return was lukewarm, as you would expect. The boiler had been running for some time.

I don't rate clip on analogue thermometers at all, too many sources of inaccuracy. They make contact with the pipe, but not very good contact, and heat transmission is slow and subject to cooling between pipe and bi-metal of the gauge.

These are what I use - https://www.ebay.co.uk/itm/225084166392

Much cheaper than analogue and absolutely accurate, easier to read. My two agree perfectly with my Vaillant boiler's temperature display. I just clip the sensor onto the pipe using a bit of pipe insulation.
 
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I think you will find the sensor is in the actual flow, rather than the heat exchanger.
If you’re going to be pedantic Harry then it’ll be attached to the boiler flow, rather than in it. I didn’t say the sensor would be inside the heat exchanger. The temperature relates to the circulated water. I know exactly where the sensors are on a Vaillant EcoTEC.
 
Right. This morning I raised the flow temperature from 62 to 65. I set the room stat at 25° and ran the system for 30 minutes with just the rads on that we have on in the evening - Lounge (no TRV), hall, downstairs WC and bathroom towel rail which do have TRV's set around halfway. Flow was 65 and return was 49°. I then opened fully all 10 rads in the house, set all TRV's on max and after 15 minutes with a flow of 65°, I had a return of 42°. Not a 20° drop with just four rads but I read the return had to be under 55° for it to condense. Does that seem all okay?
Seems ok, but it’ll just condense less. What about leaving the others on the minimum?
 
I’m assuming it was the flow temperature as I watched it go up from 58 to 62 and stay at 62. I set it at 62 ages ago because I read that it has to be 60° to stop legionella.

The boiler output/flow temperature has to be set higher than your cylinder thermostat, otherwise your boiler would run forever, trying to heat the cylinder. Some suggest at least 5C higher, I would suggest 10C higher, especially with a mechanical stat, but it is the water in the cylinder which has to at least once per week be raised to >55C to kill of Legionella.

My fancy Vaillant controls, have a built-in system to do this once per week, but I don't use it - I like my HW hot, so I have set to maximum 60C HW. My controls allow separate flow temperature for CH versus HW, so when called to heat the cylinder it runs at 75C. It uses a sensor for cylinder temperature, rather than a mechanical switch, so it is very accurate.

When called upon to serve the CH, the flow temperature is decided by the control system, for best efficiency.

It would be well worth you're installing the Vaillant control system, for your boiler, it does work rather well.
 
Seems ok, but it’ll just condense less. What about leaving the others on the minimum?
I suppose I could do but would that be more economical - heating rooms we are not in? During the day, both kitchen/diner rads are on from 6.00am until approx 7.00p.m. so I imagine the return temperature is in between the other two. Only have the four rads on from approx 7.00 - 10.30pm.
 

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