Willis heater UFH. Heaters in parallel or series?

[SteamyTea]

2 minutes ago, JohnMo said:

W represents Watts (as in a surname)

Written as watt(s) when referring to power, Watt when referring to the man.

Same with joule (J) and Joule.

Kelvin(K) and Kelvin, can get away with that as the temperature is at the beginning of the sentence.

 

Edited Monday at 18:09 by SteamyTea

[Russdl]

Thanks for that explanation @John Carroll much appreciated. 👍🏻

[Nickfromwales]

3 hours ago, SteamyTea said:

It should, units are important for clarity.

kW

kWh.

You’re easy to wind up, lol. ☺️

[Mike]

8 hours ago, Russdl said:

On 19/01/2025 at 23:17, Mike said:

It has no trouble reaching it's maximum of 38°C within a short space of time

That's interesting, I've never seen temperatures even close to that after 6 hours of running... Out of interest, what size is your UFH system? Mine is 824 meters of pipe (no idea how many litres that equates too).

I've 5 circuits totalling 273m of pipe, 16mm diameter. If that has a wall thickness of 2mm that would be a capacity of about 31 litres, so call it 40 litres with the manifold, heaters & associated pipes. Pumping at between 100 & 200 litres / hour and assuming the manifold thermostat does no blending, that would circulate the water through the whole system every 12 to 24 minutes. The pump only tells me in 100 litre chunks, so not sure exactly where it is on that scale.

 

On the same basis your system would have a capacity around 102 (93+9) litres, or 2.6 x the capacity of mine. So, at the same pump setting as mine, it would take between 30 and 60-ish minutes for one complete circulation.

 

8 hours ago, Russdl said:

I've turned the pumps down as well to slow down the rate of flow, perhaps I need to turn them down further? I do have a 7°C temperature drop between flow and return which I believe is optimum.

I've seen various figures suggested as optimum in the 5 - 10°C range. I don't have any calculations to justify my current setting - it just seemed like a good starting point to adjust the flow so the heater was running near the maximum I set it to (nominally 40°C) without it hitting the limit and switching itself off.

 

Edited Monday at 23:27 by Mike

[John Carroll]

On 19/01/2025 at 22:17, Mike said:

I'll qualify this by saying that mine is still running via a (suitably rated) extension lead, so I only have the temperature gauge on the manifold & finger on pipe to work with.

 

So, based on my limited experience (the end of last winter and this), I suspect that you're just running the pump too fast. After experimenting manually with different settings, the pump tells currently me that it's running at 2W, pushing between 100 & 200 litres / hour, which seems to work well; It has no trouble reaching it's maximum of 38°C within a short space of time and without the heater cutting out.

 

 

What make/model of pump do you have MIke and what mode/setting is it on, where are you reading the 2W?, I really can't see it pumping anything at that minuscule power, any A rated pump I've come across will require at least a closed valve 4W at the lowest setting, if you can get a accurate reading then the pump curves will give very accurate indication of the circulation rate, most A rated pumps now not only display the pump power in Watts  but also the flowrate in M3/hr, very useful.

Edited yesterday at 00:51 by John Carroll

[Mike]

10 hours ago, John Carroll said:

What make/model of pump do you have MIke and what mode/setting is it on, where are you reading the 2W?

It's the latest generation of the Wilo Yonos Pico (type 25/1-4), running in UFCH (constant differential pressure) mode. It will go right down to 1W; there's a digital display that shows the power consumption & flow volume.

[John Carroll]

52 minutes ago, Mike said:

It's the latest generation of the Wilo Yonos Pico (type 25/1-4), running in UFCH (constant differential pressure) mode. It will go right down to 1W; there's a digital display that shows the power consumption & flow volume.

A excellent choice and a lovely pump, IMO!!, I have a 5 year old 6M version (no flow display) in a oil fired conventional rad sytstem, these pumps, as you are no doubt aware, can have the head incrementally changed in 0.1M steps in both CP and PP modes, to a min 0.5M minimum (on mine anyhow) and yes, just tried mine, it does display 2W at 0.5M in CP (and) PP mode.

Even so, you must be circulating something through your system, would expect at least 0.5Ms/hr, say 8.0LPM, ~ 5W or more.

 

What is the pump flowrate  showing??.

 

[Mike]

42 minutes ago, John Carroll said:

Even so, you must be circulating something through your system, would expect at least 0.5Ms/hr, say 8.0LPM, ~ 5W or more.

It's at 0.7m and reports 2W & 0.1m³/hr. But just below 0.2m³/hr, so somewhere in the 100 - 200 litres/hr range as it only displays to one decimal place.

 

The latest generation (launched 2018/2019?) did improve the efficiency, so that will one reason the power is low. As it's UFCH there are plenty of smooth bends, but hardly any elbows or tees to hamper the flow and, being an apartment, no second floor to pump to.

 

Edited 14 hours ago by Mike

[John Carroll]

1 hour ago, Mike said:

It's at 0.7m and reports 2W & 0.1m³/hr. But just below 0.2m³/hr, so somewhere in the 100 - 200 litres/hr range as it only displays to one decimal place.

 

The latest generation (launched 2018/2019?) did improve the efficiency, so that will one reason the power is low. As it's UFCH there are plenty of smooth bends, but hardly any elbows or tees to hamper the flow and, being an apartment, no second floor to pump to.

 

 

Doctor Willis will be delighted if his heaters are working as designed, ie convection only!.

 

12mm ID piping allowing ~ 60M/loop will have a friction loss of around 0.19M @ 1.0LPM, 0.4M @ 1.5LPM, and 0.68M @ 2.0LPM.

 

Maybe increase the pump head temporarily to say 2.5M or even 3.0m (as long as you don't run out of power), note the flowrate, the % error will then be less, and the flowrate at 0.7M will be the sqroot of (0.7/2.5) or sqroot of (0.7/3.0) X the indicated flowrate.

[Nickfromwales]

May be of interest, that I fit Willis heaters in parallel in an infinite loop, with flow and return to the emitters taken off that loop a-la close-coupled tee arrangements.

 

Much less stress on the pump when the TMV on the UFH manifold is close to or at the chosen set point.

[John Carroll]

6 hours ago, Mike said:

It's at 0.7m and reports 2W & 0.1m³/hr. But just below 0.2m³/hr, so somewhere in the 100 - 200 litres/hr range as it only displays to one decimal place.

 

The latest generation (launched 2018/2019?) did improve the efficiency, so that will one reason the power is low. As it's UFCH there are plenty of smooth bends, but hardly any elbows or tees to hamper the flow and, being an apartment, no second floor to pump to.

 

 

 The flowrate is somewhere between 0.1M3/hr & 0.2M3/hr, assume 0.15M3/hr,

One heater ON, split flow, the dT is, 3.0*860/(0.15/2*16.66*60), 34.4C, assuming heater output temp at 38C (stat not tripping at ~ 40C) then the inlet  temp must be a impossible 3.6C, or else stat not tripping and outlet temp ~ 59.4C (from inlet temp of 25C), mixed with the other heater outlet at 25C gives a outlet temp of 42.2C. If the flow, for one reason or other is all through one heater then the dT is 17.2C to give the same 42.2C outlet temp and stat probably not reaching cut out.

 

I gave some advice a few years ago in the installation of a Willis (gravity circulation) heater to a HW cylinder with Iboost from solar PV and the only way it could be got to work was to use the Willis stat in a supervisory role and use a pipestat to do the switching because the Willis stat would not switch until around 75C, probably due to very slow circulation despite using 22mm  piping, its still working fine.

 

Edited 7 hours ago by John Carroll

[Mike]

  

6 hours ago, John Carroll said:

Maybe increase the pump head temporarily

Good idea. I'll report back...

 

1 hour ago, John Carroll said:

 or else stat not tripping and outlet temp ~ 59.4C (from inlet temp of 25C)

That's possible. As above, it's nominally set to 40°C, but the dial on an an immersion stat isn't a precision instrument and the only temperature gauge is the one on the manifold (the one reading 38°C), so I've no way of knowing how close the Willis is to 40°C in reality. It will be getting a set of sensors & a Pi control system once the rest of the refurb is done - in time for next winter, I hope!

 

Edited 6 hours ago by Mike