SimonD

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.

Here's the manual from Samsung - the serial number doesn't completely tie up, but it seems close enough altough Samsung's website suggests it's a split unit but the manual says mono. The unit and phe resistance curves are on page 25. I can't find a minimum flow rate as the specs just have a - in the tables for this. Not entirely helpful. But that's generally the feedback I've heard about Samsung and their heat pumps in the UK. EHS_MONO_ODU_IM_EN_DB68-05387A-07_181129.pdf

The 4kW Grant unit modulates down to 1.25kW at outdoor temp of 12C and water temp of 30. My own 6.5kW unit has shown itself to tick along quite happily at 1.2kW even though the spec sheet says 2.4kW. As @JohnMo says add a 50l volumiser and this could work. The other option if you decide not to go down the BUS grant route is an exhaust air heat pump like this: https://www.nibe.eu/en-gb/products/heat-pumps/exhaust-air-heat-pumps/s735

Better to plumb the flow and returns from the heat pump straight into the ufh manifold and remove the pump and tmv from the ufh, then run the heat pump at a realy nice low temp. I think this was the plan that was too much for the plumber you had in? Which is a worry..

I think it depends on the type of plywood. If it's a decent quality birch ply at 18mm I'd personally have no problems compared to a generic 25mm far eastern hardwood plywood as it will hold the screws. The last time I got some 25mm hardwood stuff, it was terrible, full of voids etc. Just a very non scientific observation was that I left some interior grade 15mm birch plywood outside for over a year and it just went grey. I also left some generic far eastern exterior grade hardwood 18mm outside which delaminated in a few months over winter.

That is the pump on the ufh manifold. The pump you have on the primary circuit which is key for flow rates through your heat pump is a Wilo Pico - that's the one we need to know more about as that is the one causing the flow rate error.

This is quite important. At 15l/m or 0.9m3/h the smallest Pico can only deal with just below 3m head on pump setting 3. Your heat pump at the required 17l/min has a pressure drop of 0.5m head leaving you with a residual of less than 2.5m head on the pump.

Try https://www.national-broadband.co.uk/ Works for us. Has access to all networks and aerials etc. Router supplied by them.

What is the model of the Wilo Pico?

What is the exact model? Ideally we need to know if you need to increase flow rate or if it's a pressure drop issue.

This is a problem from my perspective as it's too prescriptive. The English system at least gives provision for multiple ventilation strategies. Natural ventilation really has to be a choice even in very airtight buildings - and despite what some many in the industry say, it with good design works in those building too. Making MVHR mandatory just gives the industry a golden invitation to cream as much as they can out of it.

Your approach to the schedule will increase consumption as the boiler needs to run harder when it is turned on at the beginning of each heating cycle. But as long as the system is kept in the condensing zone - return below 54C - at all times, you will see a saving eventually. It's always worth experimenting with different heating approaches, like keeping it on tick over for the whole day with a night time setback and then see what happens, both to your costs and your comfort. Each house behaves differently so needs some testing.

Not quite - but we might be talking cross purposes due to different building regulations. I'm referring to England and Wales as I'm not familiar with the Scottish ones. So in England, Part F provides a minimum rate per m2 of floor area, or alternatively a minium rate per number of bedrooms. I'm sure with some properties, depending on the height of rooms, this will be approx 0.5, but in my case it comes out at 0.34 ACH for continuous whole house mechanical ventilation, so the ventilation is going to vary by property. And, of course then Part L says you need to design not to over ventilate. Part of the problem is that people see the 0.5 ACH given in category C of Table 3.8 of the CIBSE Domestic Heating Design Guide, for example, and assume this is a specified/required amount. It isn't, as it says clearly in the guide that houses that have been designed and built with high levels of airtightness require specialised advice to avoid over ventilation. So in these cases, reduced ventilation rates can be used with sufficient justification, usually in my experience to just do the basic calcs using the figures given in the approved documents.

We had quotes for Rationel windows. Interestingly the Declaration of Performance gives a specific window size for this best performance value. I was also a bit intrigued that the declaration of performance gives the same value for different types of window and openings at that same size. Our triple glazed windows from a different manufacturer all have a different whole window U-value that was given within the quote and design stages. Whole window value average out at something like 1.03 across the whole house in the end with the detail not differing much from the above image although our frames are a bit more chunky. No, definitely don't do that and rely on it. I've been playing around with using AI for heat loss calcs and system design using several AI models and each of them have got some fundamentals wrong. There's no doubt that Jeremy's heat loss sheet is great value tool, but heat loss calculation methodologies have moved on a bit since that sheet was built. With and MBC passive frame and foundation, whoever is doing the heat loss modelling should be provided with the thermal bridging psi values which are used to input into a full room by room heat loss calculation. Also the heat loss nowadays includes a whole house fabric ventilation loss together with a room ventilation loss which is derived from blower tests because it now considers losses due to wind loads (thus pressure differences across the building). Essentially, the previous way to do heat loss calcs would lump ventilation and infiltration losses into one figure, nowadays these are separated in the calculation, to hopefully reach a more accurate heat loss. It does. As I mention above, the old way of using ACH is heat loss calcs is to lump infiltration and ventilation losses together. So the 0.45 in the spreadsheet must be assumed to be a combination of the 2. The 0.45 for a very airtight house with MVHR I'd suggest could be a bit high with a well detailed build. Mine where I have designed a natural ventilation system (yes @Gus Potter I'm with you on natural ventilation 😉 ) comes out at 0.38 ACH even when using intermittent extract ventilation in bathrooms, for example.