marshian

Just a comment....... I like solvent weld pipe far more than I like gasket fittings but when using solvent weld I always consider how I'm going to be able to either remove the section or rod / hose out the pipework in the case of blockages - washing machine powder can build up in pipe work if "someone" over doses the machine on a regular basis (I think this is amplified by cooler temperature washes). In fact the think about future access / maintenance applies to a lot of plumbing in homes.

In a DHWP set up the flow to rads or UFH stop - the heat source re-charges the HW cyl and then the system reverts to heating House doesn't or shouldn't lose much temperature in the time it takes to do HW unless it's a massive tank

It seems that way initially but like most things but you do get you head around it eventually Unheated rooms will steal heat from wherever they can - let me help with an example My house (stupid T shape) has front bedroom above front hallway both having 3 external walls and with the added bonus of north facing - being a "lets not be wasteful with energy sort of chap" (Mrs Alien says "tight b'stard") I didn't heat the front hall for most of the time just bringing it up to temp for the few hours we might use it (arriving and leaving). The Rad in the bedroom above had to have much higher flow rates as well as a higher set point than the other bedrooms (one because of the 3 external walls and two because some of the heat was being stolen by the hall below) Now you might say but heat rises and yes it does but a better statement is that heat always moves to cold and it doesn't care if down is a direction it can move in. So now I heat the front hall to 17 deg - it's cooler than the rest of the house but it does have an internal door to separate it - as a result the bedroom above needs a lower flow rate to the rad because the difference between the two rooms is much narrower So by all means run rooms at lower temps but consider the impact on rooms above or below is all I would say - if you size rads or UFH for low flow temps to provide the heat needed based on a heat loss calculation you could find that the room won't reach target temp if it's having it's heat stolen by a room above or below or even beside it.

Blimey 34.5 litres a min is 2070 litres per hour.... that feels like a heck of a lot of flow.

Modern pumps are a real head fug - they are a bit to damn clever sometimes My limited understanding is the faster the pump speed the smaller the delta between flow and return so you give the emitter more time to get rid of the heat (That's the impact in my house) but in your case where you are trying to get the emitter to give off more heat it may well prove effective - guess testing and monitoring are required Having said that constant curve is what I settled on (mainly because the flow on proportional settings changed from when I first set the pump up to maybe a week or so later - some learning logic I think)

Just to add to @JohnMo's comments - Covered very well below with a bit of the maths Hell yes - Been there done that - "Smart TRV's" that can call for heat from the boiler even if only one room needs it will result in massive energy wastage as the boiler fires up and then potentially short cycles until demand increases

You might not need to split the circuits - Heat Geek did a video on pump distortion not that long ago

Every house I appreciate is different and responds accordingly but I was running 2 deg setback in most of the rooms for 4 hours at night and 5 hours in the day and in a week of relatively stable OAT's I used more energy on the setback days than I did on the 24/7 (if anything the setback days were slightly warmer although I was only looking at the average temp between coldest and warmest temp across 24 hrs (it could have been colder for longer on the warmer days) It's one of those experiments that you do for fun

I know that on my house set up (all rads) setback temps (2 deg setback) just don’t save me any energy - the reheat from setback uses more energy than maintaining a stable temp 24/7 perhaps if I used either a smaller setback or a larger setback it might be different but if I used larger setback I might as well just go back to a schedule and a smaller one seems pointless

I'm running pure WC mode but I still have a hallway thermostat - it's acting as a master off switch if the house ever gets to a temp that heating is not required because the hallway has the only rad that is undersized for the area and heat loss (Includes full height stairwell and unheated upstairs landing)

Oh yes sizing for 35 v 45 results in a significant increase in rad size as I found out I specified DHWP and WC on my boiler install and got S plan............ To give the installer credit he recitified the error and we got there eventually but fundamentally it was what he knew and he thought it would be fine.......... He also couldn't read a WC curve properly until I explained it... Trouble is installers talk a different language to customers - when you talk their language they don't always think you actually know what you want.

It didn't change a thing (well all that did happen is the room that needed the highest flow lost temp because it was being shared out to other rooms and it's the last in the circuit) So I reverted it to a dumb boiler and manually set the flow temp - still did the same thing ramping up randomly mid burn cycle - often causing + 5 deg trip. Then I needed to do HW cycle so manually increased the flow temp First thing boiler was only slowly increasing temp (because range rated to 4 kWh - ahh bugger need to undo that setting - Increased range rating to Max because it's one button press in the menu. Then boiler reached my previously set max flow temp cap of 45 deg. So that got changed for 70 Deg and then the boiler froze as I was trying to exit the menu - it was doing HW so I just walked away and left it to heating water (it's happened once before and the solution was to turn the boiler off at the main switch) So when it finished the HW I powered down the boiler (made a cuppa and then switched it back on) house needed a top up heat wise so I range rated it back to Min and waited for it to fire. Guess what happened next? It fired - ramped up to 58% and over the course of 2 mins modulated down to min 10.6% and sat there at min for 17 mins and 30 secs - used 0.103 m3 of gas which by my calcs is 3.98 kWh....... It didn't miss a beat for the rest of the day. This morning however it was back to it's old tricks - starting to really get on my wick now So I switched off the boiler waited 5 mins and then waited for it to fire for the next cycle and it respected the min range rating again. So now every time it starts to play up I have a solution to the issue - it's not pretty and I don't believe it's acceptable but I have some information now to go back to Viessmann with to point out that it still has the same problem it had before (and crucially what resets it back to how it should operate) It doesn't seem to matter what pump speed I use Constant Curve 1 or Constant Curve 2 the only difference that makes is the DeltaT at the boiler is 9 deg C with CC1 and 5 Deg C with CC2 Pump in CC1 5.5 w 1.1 m head 0.4 m3/hr Pump in CC2 16 w 3.1 m head 0.6 m3/hr

Well it hasn't 😞 so I'm going to put my thoughts down and maybe a proposal for consideration from the great minds of Build Hub The "niggle" now only occurs between 6 deg OAT and 12 Deg OAT So lets cover the data I can provide Pump Pump Speed 0.5 to 0.6 m3/hr set on Constant Curve Watts 15 Head 3.1 m Rads I have 13 Rads all with TRV's in the house table below explains what they are room sizes Heat loss at -2.5 Deg C OAT and how they are set up (Because there are issues with getting the flow rate low enough for some rads and solar gain with some rooms) I regard the circuit as well balanced both upstairs and downstairs I have a pressure relief by-pass but it's fully closed as the circuit never shrinks enough to need it. Boiler Viessmann 100-W "Heat Only" 16kW set up with Weather compensation and DHWP (Using an outside temp sensor and HWD box) It's piped up as X Plan It's running a delta between flow and return at the boiler of 6 to 7 deg C at Weather compensated flow temps Weather Compensation Curve When it's not in the OAT temp range where I see the niggle this is how the boiler behaves Good Days On CH the boiler fires up when the flow temp drops "X" deg below WC Target (Gut feel is X = -7 Deg C below target temp) It will do an initial start at 58% modulation and then over 90 secs drop down to the lowest modulation level that enables the boiler to maintain the target flow temp typically 10.8% It will then run happily at that level until the flow temp exceeds the target by "Y" Deg (Pretty sure "Y" is 5 Deg C above the target temp) Bad Days When it's in the "Niggle" window its behaviour is different during the burn Even if the Boiler is range rated to min (19 out of 100 in the menu = 10.8% or ~4.0 kWh) the boiler ignores the range rating Initial start up is identical to above but very quickly into the period after the start it starts to miss-behave - modulation swings between 10.8% and 30% This frequently results in the Target temp + "Y" being reached during the higher modulation and the boiler shutting down before it's fully refreshed the water in all the rads that were open for heat. My thoughts are that the return temp is changing as some rads have quite a large flow and a decent delta between flow and return and some other rads have much smaller deltas and the boiler is trying to maintain the target flow temp so modulating up and down to provide a stable output. If you read all that - thank you. Proposal So I had a thought what would happen if I set all the rads to an elevated flow rate said sod the balancing control - leave the TRV's to manage the room temps and flows and bring back into play the by-pass (just in case all the rooms hit target temp and TRV's shut down the flow). Part of me says I should just try it and see what happens Part of me says the TRV's will end up managing the flows as they close and the flow rate in the system and the return temps will be the same so it won't change a damn thing. Anyone think anything else will happen?

Interesting point - maybe there is something I can improve as our current SUDG windows and doors were fitted in 1997 so are coming up for 28 years old................Question is how much better because there are no drafts from around the closures (the window seals are good) they haven't gone yellow like some cheap windows do. I'm thinking its going to be mainly thermal improvements in the glass and gas between the panes and they aren't big windows (circa 1.7m2 in total apperture area for each one) I'm guessing the payback for that level of capital investment would be a bloody long time

Link to Urban Plumbers Heat Loss Calculation Video on YT Watch it - you have most of the dimensions you need and for building fabric there are plenty of resources on the net for W/m2 I started to build my own spreadsheet years ago which was actually fairly accurate on the rooms I did but then I found I could pay £12 for a single use of Heat Engineer software and plug all the info in there and get a very accurate result (Provided you understood the inputs) however they now charge £100 for single use so I can't recommend them anymore. In a nutshell it's just Math but be very careful with ACH (Air Changes per hour) many on line tools massively over egg this element and as a result heating systems are over sized.