Range Rating: Just Add up the Radiators’ Heat Output?

[ndfred]

I have a Vaillant boiler and want to set d.0 to something sensible, right now it is set to auto and can range from 6 kW to 37 kW. Adding up my radiators' heat output from the Stelrad website gives me 25.7 kW and my hot water cylinder coil rating it 23.9 kW.

The Heat Geek Range Rating Method is really difficult to perform so I have been looking for something more straightforward. I try and schedule my hot water cylinder heating time away from my radiator heating time, and settled on 26 kW as the max power output for my boiler, basically just above the sum of my radiator's heat output.

Is that a reasonable approach? My current system is an S-plan layout with a Vaillant ecoTEC plus 637 (flow temperature set at 70 degrees) and a megaflo Eco 250i Unvented Indirect Cylinder (factory thermostat set at 60 degrees it looks like), with a Nest doing on / off controls for both heating and hot water. I am setting d.0 "Heating partial load" on my boiler.

I can't really change the flow temperature since I need to heat up the cylinder, and can't really have modulating controls since the Nest / Vaillant won't support that, so capping the boiler's power output seems to be the best way to improve efficiency without making changes to the wiring or controls.

Thanks!

 

[SimonG]

Do a whole house heat loss.

 

[Last Plumber]

You could install the Vaillant senSocomfort controls and have true weather comp with hot water priority.
Until then, yes, set it to 26 Kw.

 

[Chuck]

In my opinion, which is based on measurements taken with a data-logger, the sweet spot for the flow temperature in the OPs type of system is 65°C. This is high enough for the megaflo to reheat nicely (without too much boiler-cycling as it approaches the 60°C set point) and low enough for the the radiator circuits to keep the return temperature at or below 50°C when it matters.

 

[ndfred]

Super helpful advice, thanks!

• Whole house heat loss: going from the Heat Geek Heat Loss Calculation page, I get about 16.1 kW and 17.8 kW with on/off controls, I might give that a try and see if the house heats up well, and I will try and dig up an energy assessment of the property when we first moved in which might have that calculated already
• Vaillant senSocomfort / sensoHOME is what I was looking at, but I am renting so I might have to stay with my current controls
• I will try 65 degrees flow temperature as well, and see how that affects hot water / heating

 

[John.g]

If you are really interested in your rad outputs a few simple tests with your vaillant will show you.
You rads rated output are based on a "50 deg" basis which is the mean radtemp - the room temp, 20C roomtemp is generally used so to get the rated output of 26kw requires flow/return temps of 75/65, mean of 70C, minus 20C = "50deg" rad.
You are choosing a 65C flowtemp, with all rads on and up to temperature, note d.40 (which should be 65C) and d.41 the return temp. These will be your total rad outputs from the following....d.40/d.41/dT/output%/kw
(75/65/10/100%/26kw) "50deg"

65/55/10/62.9%/19.5kw
65/50/15/68.8%/17.9kw
65/45/20/62.9%/16.4kw.

If you are further bothered to see if your original 26kw is accurate, just reduce d.0 a few kw below the nearest of the three numbers above (or post the flow/return temps) and the flow temp should then start falling.

 

[ndfred]

I caught the boiler in a heating cycle (Target Flow Temperature 70 degrees, Heating partial load 26 kW) and wrote down d.40 / d.41 values, outside is 12 degrees:

• 16h23: Nest heating starts
• 16h28: 33 / 27
• 16h29: 52 / 34
• 16h30: 59 / 39
• 16h31: 60 / 40
• 16h32: 60 / 41
• 16h33: 61 / 41
• 16h34: 62 / 42
• 16h35: 63 / 43
• 16h36: 64 / 44
• 16h37: 65 / 45
• 16h38: 67 / 47
• 16h39: 60 / 59, Nest heating stops
• 16h40: 59 / 58
• 16h41: 59 / 58
• 16h42: 58 / 58
• 16h43: 58 / 58, boiler stops making noise (fan or pump off?)

We didn't reach the target flow temperature, but got close after 15 minutes with all radiators loaded which seems really quick. With a 45 degree return temperature it looks like we are condensing which is good news (apparently 45 ~ 54 return flow temperature means condensing mode). This was only 16 minutes of heating though, when we come back from set back temperature in the morning this would be much longer.

I guess the jump from ~45 degrees to ~58 degrees is because the TRVs kick in and bypass a few of the radiators, reducing the heat loss as the water gets through the circuit and back to the boiler? Or maybe the fact that the boiler no longer tries to heat the water has some effect on that (though in that case, I would expect flow temperature to lower to return flow temperature, not the other way around)?

I just set the target flow temperature to 65 degrees and will check both how quickly my water gets hot, and how much longer it takes to heat the house.

From your suggestions @John.g we would be looking at 65/45/20/62.9%/16.4kW so I may be able to reduce boiler power output even further. I will try and change one thing at a time though, and see if 65 degrees flow temperature suits our home before I play with the boiler's power output again.

 

[ndfred]

Just checked while the hot water cylinder was heating up, and 65 degrees flow temperature is not enough: the boiler stays at 65 flow / 60 return forever. Bumped flow temperature back up to 70 degrees and it reached 70 / 67 after which the Megaflo's thermostat cut off. That makes me think the Megaflo thermostat is set at ~65 degrees.

Anyone know how to measure water temperature in the Megaflo? The thermostat on the cylinder is also a really cheap knob you turn with no temperature markings. I would like to make sure it is set to 60, but can't really tell.

So back to 70 flow temperature until I can fix this without setting the Megaflo's temperature too low.

 

[Chuck]

Just checked while the hot water cylinder was heating up, and 65 degrees flow temperature is not enough: the boiler stays at 65 flow / 60 return forever. Bumped flow temperature back up to 70 degrees and it reached 70 / 67 after which the Megaflo's thermostat cut off. That makes me think the Megaflo thermostat is set at ~65 degrees.

Sorry, I should have said to reduce the Megaflop thermostat to 60°C. Megaflos have a pocket (a small hole though the insulation) that you can put a thermocouple probe into. Alternatively just run the hot tap nearest to the tank continuously into a cup and stick a thermometer into the water in the cup once it is hot.

 

[ndfred]

Gas inspection happened, and the technician was not particularly helpful as to how to set this all up. I will try this setup next, based on Ultra efficient gas boiler design & installation from Urban Plumbers on YouTube:

• 45-50 degrees hot water
• 55 degrees flow temperature
• 60 degrees legionella cycles every 10 days (manually)
• 16 kW to start, probably won't need as much eventually but will need to measure flow - return to refine that

Any thermocouple you would recommend? Temperature at the tap might be easiest.