SimonD

Good luck! I used to sit on a local planning committee. That was fun 🙄 Actually, I should caveat that. It was very interesting dealing with all the applications and considering them in light of the local plan. What drove me crazy was the committee itself. Our chair was a barister who used to constantly have to remind committee members that its role was about planning law, not aesthetics, personal opinions and amateur architectural design.

crikey, I didn't think there were many constraints on this one anyway - can you imagine lower quality newbuilds going up?

Lol, yes indeed. With you reading the boiler manuals and @JohnMo reading heat pump manuals, we're pretty much covered on the BH library of knowledge side. I only really read the manuals when I'm on training or on site!

Not necessarily. If the boiler puts a slug of heat into the system and it senses it gets too hot in the heat exchanger because heat isn't dissipated quickly enough into the system, then reducing output can resolve this and enhance the heat output. It's behaviour I've seen when boilers can't modulate down enough for the heat load of the property so it fires, tries to modulate, then shuts of and not enough heat gets delivered out to the system. But in these circumstances you often get poor distribution of heat to the radiators, so they fail to even reach set flow temp due to poor circulation. I'm just suggesting this as a process of elimination. Maybe there's a slight problem with flow rates, but then as you say you'd end up with either a very wide DT or a very narrow one. So my suspicion is more leaning more towards the flow temp not being the same as the temp displayed.

😉

You just beat me to it...

In my thinking, the flow is there. The flow right outside the boiler would equalise with the hex/internal temp shown at 80C, if it was indeed at that temp. Like @marshian says, this would easily happen within 10min of running. So the question to my mind is why don't we seen that? The laws of thermodynamics suggests that it isn't actually there. The WB engineer would have plugged the WB diagnostics in which would have confirmed sensor readings etc. including water temp., flow rate, surely.

So it's a bit slow to heat everything up. That's not necessarily something to be concerned about as it really depends on system volume etc. But this is a curious issue. I'm sure the WB engineer would have checked the flow rate volumes and was satisfied the boiler was getting the right flow rate? Your radiators are getting warm to the same temperature as the flow out of the boiler, I presume throughout the whole house. And you're seeing approximately the correct delta T between flow and return. If the flow was very restricted your radiators would unlikely get to this temperature at all - from cold 20mins is not unusual, nor is 40 for 3 column radiators, especially if they aren't baffled. When you adjust the target temperature on the boiler the resultant measure flow temperature increases by exactly the same amount? E.g. 70C give flow of 51ish, 80C gives 60Cish flow. If you reduce the target temperature, does the same thing happen? One of the things I would check is to log the pattern of flow and return temperatures - i.e. do they fluctuate or remain stable over time despite the boiler ramping output up and down? And while this is happening do your radiators maintain temperature? I would probably also go into the installers menu and rate the max output of the boiler - IIRC the 8000s can still do this? Maybe drop the max output down to 22kW or even less and see what happens? Other than check your mag filter isn't clogged up, I'm inclined to suggest this is an issue with the boiler and perhaps in his suggestion, the WB engineer knows something we don't. The reason is that somewhere in the back of my mind from a couple of years ago was an experience I had with a WB8000 where the onboard system control did very funny things that are reminiscent of this problem here, it's just my memory is too vague, but it's something that didn't get satisfactorily resolved even after being passed to WB technical and warranty (not my original installation mind you, just a problem I was asked to look at).

So next question, which you may have already said, but I can't recall, is if your radiators get warm at all? And does the towel rad serving as bypass get warm too?

Nah, I've had these 8000s go over 100C when there's insufficient flow and call for heat. The bangs are really quite scary! Yes, and it may be that if the boiler is able to modulate down in time, that's why we haven't heard to op tell us it sounds like the boiler is blowing up and switching off.

Yes, possibly, but it's intriguing that the Delta t across flow and return is about 16C.

Op says target temp of 70C. With the flow temp on pipes ony reaching 53-54, I wonder what the return temp is on the pipes as that would tell up a lot. @EinTopaz?

Ah well, done. You read the manual properly to find the info not in the tech specs but in the "Before installing/commissioning the unit, make sure to check the following points :" section! Who would have known...😉 It would be interesting to know more about the flow switch installation and requirements. On the ABV side, I've just started a discussion elsewhere about the suitability of ABVs on modulating systems or where variable speed pumps are being used. There seems to be some contradiction around this. What's your view?

I've just re-read your op where you say the the flow pipe out of the boiler doesn't rise in temperature. Again, you've surmised correctly. This can often happen when the heat load in the circuit is too low for the output of the boiler - the boiler fires up and before it gets to modulate down the water in the heat exchanger gets too hot. Are you experiencing short cycling? As it does the boiler get really hot, like 80C plus and then turn itself off for a while before re-starting?

I'm guessing there is an auto bypass fitted somewhere on the system near the boiler? IIRC the minimum length of pipe for this according to the MIs is about 3m? You're pretty much overpiped, so on a gas boiler upping the 22mm pipes to 28mm would make no difference. As you've surmised, you have a flow problem, but it may not be low flow, but flow returning back to the boiler through a portion of your pipework acting as an uncontrolled bypass. Last time I had this was with 2 37kW Worcesters piped into a low loss header.