marshian

Link me up to an example of a similar rad to what you have on the net please - I'm assuming they have convector flutes on the back and they aren't just flat surfaces both sides and single panel?

Sounds like a recipe for harmony! Oh it isn't

I do get that but if you are fitting longer rads you can always raise the rads slightly and run the pipes from the original exit along the back of the rad hidden from view Issue for me with the K33 was wall space was at a premium - french doors at one end - fireplace in the middle on one side furniture on the other side - leave one wall with a doorway to fit a rad that just meets the requirement... It's thickness wasn't a problem as nothing else is there.

Ahh my error 800 wide 600 tall K3 Steelrad is 1911 w at T50 T50 to T30 = 1911 x 0.515 = 948 w so even two of them won't crack 2 kW heat loss although you could run them a little warmer 800 wide by 700 tall K3 Steelrad is 2170 at T50 so 1117 w at T30 OT My K33 1400 x 700 is rated at just over 4kW at T50 - it'll happily keep the room at 19 deg C running WC flow temps I'm running it at DT11................... I need 660 w in that room Add two squidgy people in the mix and the temp climbs to 20.5 (which is about the limit of my comfort and the bottom of Mrs Alien's accept zone)

Ouch if your heat loss in a room is well over 2kW and you current spec rads are 1100w at DT50 the the rad spec was bloody terrible for a condensing boiler - looks like the builders choose fancy over functionality and cheaped out on the size to minimise costs You'd need rads rated for 5500w in total at T50 to run them at around 50 deg flow temp I'm not even sure you can get K33's in 600 width that will hit that output.

I'd have to agree with that - boiler total kW oversized with a relatively high lowest modulation and rads that need higher flow temps to meet the heating needs of the property with a very low delta between flow and return https://www.heatgeek.com/do-we-really-need-dt20 I think you'd have a huge change in house/room comfort and system performance going to conventional T22 rads sized for a less than 50 deg flow temp Is it a possibility to swap the rads you have around to increase the sizes in some rooms and reduce the expense of purchasing more rads for all rooms? I'd be very concerned about a 50% increase in gas usage for moving from scheduled to 24/7 (ish) I reckon mine is a good deal less than 10% increase for a move from scheduled to 24/7 (and that's with a boiler that miss-behaves occasionally)

If it's on and not clicking off then the cycling is down to the boiler

Old boiler couldn't support Open Therm - so yes I was managing room temps via TRV's however I had one room that needed much longer than all the others to hit target temp - so when other rooms reach temp and the Wiser Hub stopped the boiler it buggered up any chance of getting the one room to actually reach the temp required

No I mean the CH light on the Hub - flame on the app is just saying room is not at target temp - flame out says at target temp

Assuming here you have Wiser Hub and One room stat (no smart TRV's) Watch the hub - if the CH light goes out it's killing the boiler because it's trying to stop temperature overshoot I agree this is an annoying characteristic If the CH light is not going out then it's not interveening My solution to the issue was to set the room stat or a smart TRV to a higher target temp that cannot be achieved - that way the Wiser Hub doesn't get to screw around making the boiler cycle or stopping it mid burn (which is what it did with my old boiler when I had it set up with a really long anticycle timer to give the rads chance to dump the heat)

Whats the delta like if you measure top left corner and bottom right corner of that rad?

How are you measuring your rad flow and return temps? Have you a picture of the rads (as an example) - even my old 1980's single panel, single convector rads at 60 deg C with the old boiler had a half decent temp drop Are they piped up BBOE or TBOE?

Looking at the installation manual it needs 21.5 Litres/Min at Max output pretty much in line with my ball park numbers. I'm guessing as the boiler has control of the pump it's not possible to slow the flow rate down and with 9 smallish rads it's going to have a narrow DT unless the boiler sees that and slows the pump down as well as modulating down to min.

Do that for all the rads and then sum up the total flow rate and you'll have a better idea of the total flow rate. Rule of thumb I've used over the years is for every kW of boiler output (up to 15kW) you need a min of 1 litre/min for each kW to keep the boiler happy (this is probably out of date for modern boilers with good turndown ratio's.) After 15 kW provided the boiler modulates down you can get away with an extra 1 litre/min for every 2 additional kW increase Example - current boiler is 16kW but range rated down to min - most it hits on start up is 58% of max output so 60% of 16 is 9.6 kW Current Flow rate is 0.6 m3/hr = 10 litres / min is more than enough to keep the boiler happy under start up conditions and in normal running Manual Says 633 Litres per hour is the expectation which is 10.5 litres/min

I think you can "back calculate" flow rate thro each rad if you know the flow temp and return temp??

Looks to me like the only way to widen the DT at the boiler and the rads is increase the size of the rads What boiler is it and what pump (If not in the boiler)

Your home is losing heat at OAT 5C - if you need flow temps of 60 deg C to maintain it at target temp then you don't really have a choice Well you do - have a colder house and get condensing efficiency - great or probably not 🙂 Boiler flow rate is really bloody crucial - read the installation manual it should tell you what it's min flow rate needs to be for it to be happy Screwing the flow rate down thro the rads and ending up with a really unhappy boiler isn't a great way to minimise energy usage MWT in the rads is where you need to look Hypothetical Example Rad Flow 60 Return 50 Mean water temp 55 but this is achieved at a flow rate of 104 l/hr and the heat output is 1214W Same rad Flow 62,5 Return 52.5 Mean Water temp still 55 but flow rate would be 114 l/hr Multiply that round all the rads and you quickly add all the increases of flow to make the boiler happier - it stops doing the overheat short cycle and just does equal burns every cycle

Why do you want a high DT?

My advice is simple Keeping the boiler happy with a decent flow rate thro it is far more important than squeezing the DT out of individual rads to drop the return temp Squeeze too much out of the rads and you drop the flow rate thro the boiler Drop the flow rate thro the boiler and it'll go out on temperature much faster Every time the boiler fires you'll send a load of energy out the flue before it modulates down this is a bigger waste than running with a narrower delta at the boiler with a good flow rate

Bingo - so the heat is going somewhere So if the boiler is firing and running for a short period of time you aren't going to get warm water back to the boiler very quickly the rads will take that without giving a higher return temp If the boiler hits temp limit and shuts down then it's going to send "unheated" water round the circuit What does surprise me is that the boiler doesn't shut down because the flow temp exceeds the return by a large margin - many boilers don't like to see difference between flow and return that exceeds 20 deg after a certain period of running. Good example of this was my my old glow worm that faulted out if flow and return went above 20 deg after 5 mins - I had to run the downstairs toilet rad at a high flow to feed back return water to the boiler - the room quickly warmed up and the TRV shut down but by that time the boiler was getting a warmed return from all the other rads

Resulting in a lot hotter water returning to the boiler if the bypass has opened due to system pressure being higher than the bypass is set to Either way until we get some "firmer" temps it's probably not much point. It looks like the boiler running just UFH is cycling a lot more than is desirable - chucking heat out the flue on every start and nit running long enough to stabilise

I get above but if the boiler is throwing out 60 deg C I find it really hard to understand a return temp of 15 deg - in fact I'd struggle with a return less than 40 if the UFH is only taking a little heat out of the circuit via a mixer unit. Unless there is heat being removed elsewhere??

That doesn't seem right - no circulation pump is going to like being almost dead headed What confuses is me is why it appears the return temp is so low - Avg human skin temp is 33 to 36 deg C so anything cooler than that is going to feel colder. I can only "just" feel the difference between my 30 deg flow and 24 deg return on my rads but they both feel cool.

And here

Some discussion here