marshian

My thoughts exactly - with an open system every rad getting the correct flow rate to meet the heat loss of the rooms I wasn't sure why a higher DT was going to drive any more efficiency.

At a DT of 6 Deg at the boiler for the period before the change usage was 2.0 kWh (OAT was 10 Deg) Boiler burning once per hour - Boiler at start fires at 58% and then over the the 90 secs it slowly modulates down before settling on the min output of 10.8% for the rest of the cycle. At a DT of 11 Deg at the boiler for the period after the change usage was the 2.2 kWh (OAT was still 10 Deg) But the boiler was cycling 3 times in an hour (I think the additional gas used was as a result of the more frequent initial fires when for the first 90 sec it fires at 58% and then over the the 90 secs it slowly modulates down with the higher target temp the modulation stayed at a slightly higher rate for longer before settling on the min output of 10.8% for the rest of the cycle. I actually learnt something from the exercise (or rather remembered previous boiler behaviour) sometimes it helps to write down my thoughts. So my heating circuit is pretty close to 130 Litres with all rads open with a flow rate of 9 litres per min it takes 15 mins for the return water to start to rise and the boiler flow temp starts to increase until the over temp point is reached At WC Flow temps the boiler allows an overshoot of the target temp of 4 Deg C Target flow temp is 27 Deg C but the boiler runs until the flow temp hits 31 deg C (Basically extending the burn length) When the burn is finished the circuit cools down to around 4 deg below the target so 23 Deg C (this takes around 20 mins ish) With a higher flow temp and the same return (so a wider DT) and a slower flow rate 3 lires/min the return water stays fairly low and as the boiler is targeting a higher flow temp it runs a little harder for longer. The time it takes for the return temp to drop to the restart point is much quicker - I think possibly because the warmer water never gets back to the boiler during the burn period (Typically the restart temp point was reached in 10 mins) I wouldn't call it short cycling and at 3 cycles per hour it's still much lower than 6 cycles per hour limit

Where do you sit on this table?

Brings back memories of the first time I went under my suspended ground floor.......... I ended up gathering up all the brick rubble mortar chunks, empty drinks cans and sandwich wrappers and then section by section sweeping up all the small stuff When I eventually got round to insulating between the floor joists it was a much nicer place to be

Mean yes 29 Flow temp 32 - return 26 but a burn time of 25 mins and coast of 25 mins = 50 mins for a cycle Increased flow temp to 36 - return stayed at 26 so yes mean increased to 31 However no house over heat because burns were 3 x 7 mins and coasts between burns of 10 mins (3 x 7 plus 3 x 10 = 51 mins for 3 cycles but crucially only 21 mins of actual heating time in the same period) I see no benefit to a higher DT at the expense of increase cycling but I maybe missing something

I'd probably agree based on his content However today I've tried to get a wider DT (I manged to get it to just over 10 by reducing pump speed and lowering the flow rates thro the rads) and whilst the boiler managed the situation it did it by increasing the cycling and shortening the burn times (Flow temp 36 - return temp 26) The house didn't overheat but I can't see the point of increasing the level of cycling for a higher DT

Had a discussion with Andrew Millward on a Viessmann Boilers Owners Facebook group (Andrew is a heating engineer who posts stuff on Youtube - they tend to be shorts and often of a technical nature - not so polished as "Urban Plumbers" or "Heat Geek" but good content all the same) He thinks I'm running the boiler with a far too narrow DT with 3 times the flow rate I could use if I increase the DT and suggested I target a DT at the boiler of 12 Deg rather than 6 Deg. He believes it will be more efficient with a larger DT. So I did a repeat of my earlier condensate collection and efficiency calculation - 97.23% was the result pretty much identical to previous. So I'll adjust all the flow rates thro the rads, tweak the Weather Compensation curve to target a higher flow temp (maybe 40 Deg C which should give me a return of 28 Deg C so not far off my current 32 F 26 R) and see if that makes any difference to the boiler behaviour. Fundamentally the return temp drives the condensing efficiency improvement so I don't think it will make much difference. I think it will possibly result in fewer cycles a day (currently 1 fire per hour plus a HW cycle a day) so 22 to 25 a day so boiler runs longer which may get a small benefit in terms of eff but my biggest concern is upping the flow temp target might result in a bit more creaking pipes - I remember how much they creaked every time the boiler fired on scheduled heating with higher flow temps and I like how little they do now.

That to me is normal - after a period of time the HEX, Flow and Return are all going to equalise temp wise until the boiler fires up again when the HEX and flow pipe are going to rise rapidly. The return pipe will only start to rise when it's recieving warmer return water from the heating circuit if the emitters can't transfer all of the heat into the rooms

Agreed

Cold pipe or Hot Pipe? Bathrooms are normally warmer that other rooms - if it's a cold pipe it could be condensation forming on the pipe rather than an actual leak when the bathroom has a high humidity level (ie when having a bath)

If that was my first time tiling - I'd look at that and say 1. Well I learnt a lot 2. It's not "trade quality" but it was my first time and I saved a few quid (which I spent on tools) 3. Next tiling I do will be much better I did - next time was much better (because not first time) and I learnt some more and got better tools.......... Then work got busy and I paid a tiler to do a floor - it was only down for 2 years before I pulled it up and re did it - that was the last time I paid a professional (been tiling 20 years etc etc) to tile anything. Think about it the first floor tiling I ever did 29 years ago is actually still there........... The second floor I tiled is also still there......... No pictures

Neither does my Viessmann which I found really bloody odd - instead it just seems to modulate based on flow temp rise If below target ramp up until target achieved If at target ramp down till target maintained If above target stay ramped down but keep running until a max of 4 Deg overshoot point then shut down I thought it was weird not to monitor return temp but the 4 deg overshoot is a nice trick to reduce cycling and extend the run times

Bloody love your maths based approach to everything - really makes me glad I joined up to this place

Small point not that it matters in this case but WB 35 kW boiler is 5.1 kW min according to the install manual Was very surprised to see the 30 kW had a 3.0 kW min (Bet that it's impossible to achieve in reality

I can see how you ended up with it - many gas engineers oversize boilers "To be on the safe size" it's how I ended up with a 24kW boiler for a house heat loss at the time of 6kW but with T11 rads and a flow temp of 60 to 80 deg (no weather comp so flow temps were manually adjusted) the boiler cycling was manageable. My issues with that boiler started when I changed the majority of rads from T11's to T22's then dropped the flow temp to 50 and it was a bit sh1t but I lived with it. (I subsequently installed 75 mm PIR under the suspended ground floor (horrible job) - increased the depth of loft insulation and resolved a few uncontrolled ventilation issues - so was then at ~4 kWh heat loss and the boiler was unable to manage the house temps without cycling excessively

Ahhh that makes perfect sense although two boilers mean two service bill/maintenance costs and I'd be fretting about which one was driving the bills but that's me all over

It really is amazing how much detail is in the install manuals if you dig into the detail - first manual I ever read was for my glow worm - I only read it because it was driving me nuts and I thought I could dial out it's problems - I got to understand the valiant/glow worm anti cycle settings which was a complete revelation when trying to manage a boiler with a minimum output 2.5 times the heat loss of the house. Despite the boiler being a cheap end of the market the manual at least laid out the critical basics like min flow rate required which is often absent in a lot of manuals I read

Fair point - but as I said I quite like reading boiler install manuals and it never hurts to check

That's the thing about boilers they don't always respond in the way you think they will - range rating my old glow worm boiler to 10 kW consistently reduced the warm up time by about 20 mins - yet it was a 24kW boiler Right now it's modulating up and down like a whores drawers - get it in a more steady state and it might actually improve the house warm up and reduce consumption

My favourite pastime is reading "Boiler Installation" manuals Item 3 - Flow temp sensor at the heating block.............. Seems the boiler can be range rated although I'm not sure how low it can be rated - looks like it can't be range rated to min of 5.1 kW but it can be throttled back to around 15 kWh as a max Settings under "heating" menu it seems Also seems like the pump curves can be adjusted too but I think I'd start with capping it's enthusiasm to ramp up to 90%

A few years ago when I was trying to make a glow worm boiler with 24kW rating and 10 kW min heat a house with a 4 kWh heat loss I had an issue with the boiler hanging or faulting out on warm up - the boiler did not like a DT at the boiler greater than 20 Deg C. If it faulted out it required a manual reset when I got up which also meant a cold house - I wasn't popular when this happened. The situation was always worse the colder the outside temps got - it wasn't unusual for the rad/room temps overnight to drop below 16 deg C when we were on scheduled heating slots (one of the reasons I wanted to go 24/7 with weather comp) Boiler flow temp was set to 55 deg C so on start up the flow would quickly get to 55 but with 130 litres of water in the rad circuit the initial return would be 16 Deg C (55 -16 = 39 Deg C delta so you can see why the boiler might not be happy) If it was just over 20 deg C it would modulate down to 10kW min until the DT at the boiler dropped below 20 Deg C at which point it would ramp up to max driving the DT to greater than 20 Deg C and at this point it would hang again dropping down to min 10 kW and then wait while the DT dropped below 20 again where it would rinse and repeat until all the rad were feeding back water lest than 20 deg below the boiler set point. By this point the boiler would then short cycle as the circuit couldn't cope with 10 kWh being fired into it to replace a loss of 4 kWh I did a number of things to limit the impact Range rated the boiler to 10 kW (unfortunately the range rating is ignored on start up so once the house was warm it frequently short cycled) Set a towel rail rad to be fully open on the lockshield - it was in a toilet (so small room and had a TRV so it provided a useful warming of the overall return until the TRV shut it down after 30 mins) by which point the overall circuit was at a temp level that meant the DT at the boiler was less than 20 Deg C Reason for the Glow worm story is I wonder if this differential is driving the modulation behaviour I'd be inclined to actually go the other way - drop the target flow temp and run the heating period for longer and if you can range rate the boiler down I'd do that for sure especially if you know your house heat loss?? PS how is the HW (Hot Water) heated if not by gas boiler?

Good call on that as a potential reason but the system and boiler are relatively new I wouldn't expect it to be clogged up like mine was once

@EinTopaz probably needs to explain this better - right now it reads that the boiler is running for 10 mins and reaching it's target before overshooting I fail to understand how a flow pipe leaving the boiler can be 30 deg lower than the water temp reached inside the boiler - it's likely to be 30-40 cm of pipe max so unless it's going via a blast freezer there is heat going somewhere

10 mins to overheat that's not exactly a short cycle scenario IMO.............. My old glow worm 24kW with a min of 10kW could heat up, overshoot and shut down in less than 4 mins if the house was up to temp and so not a lot of heat transfer going on from the rads. The water leaving the boiler would be same temp as the display said it was!!! Here it seems to be a big disconnect that doesn't make sense

If the boiler is producing 70 deg heat (according to the display) then unless the display is miles out or it's indicating a set point rather than an actual temp out - I would expect a similar temp at the flow pipe - so I'd wonder where the heat is going!!!