Ed Davies

Indeed, if it's just heating a 90 litre buffer tank (and that's not losing heat anywhere) then the return temperature ought to be ramping up quite quickly, in less than an hour. My return temperature question was on the assumption that it was heating the slab.

What's the return temperature and flow rate? The heat pump won't raise the temperature of the water that much in each cycle round. Picking one arbitrary (but reasonable sounding number) out of thin air, one circulation pump says it does 0.33 m³/h. That's 330 litres/hour = 5.5 litres (or 5.5 kg)/minute = 0.092 kg/s. The specific heat capacity of water is just under 4200 J/kg·K so it's capable of carrying away 386.4 J/s (= watts) for each kelvin (°C) of temperature rise. If your heat pump is putting in 5 kW then the temperature rise will be 5000/386.4 = 13 °C. If your flow rate is higher (might well be as that 0.33 m³/h is against the maximum allowed head, I think) then the temperature rise will be smaller.

I understand the conventional expression is “well hard”. Particularly applicable to boreholes in chalk.

Indeed, that's the solution if @Onoff does have that problem. I assume @Nickfromwales is trying determine, first, if he does have the problem or not.

Isn't another way to tell if it could do overrun to look at the wiring for the pump? If it comes off the boiler control board then it could be an overrun one so you have to do the experiment @Nickfromwales suggests. If the boiler controller doesn't have a pump output, so the pump's wired straight off the wiring centre, then I think it can't be an overrun one? The pump for the non-system (vented) oil boiler here is wired direct from the wiring centre (i.e., in series with the switched outputs of the zone valves and in parallel with the boiler itself) even though it's actually inside the boiler case.

This. People are so used to products which are only small incremental changes relative to existing technology and are sold to huge markets that they're largely shielded from how much of engineering is “suck-it-and-see” learning. It's difficult for small firms making products in relatively small quantities to sell into a consumer market to get the right balance between presenting it as for early-adopters and a mass-market product. Just to ramp up to decent production levels which have any hope of making money they're pressured to move to mass-market quickly but it's easy for that to backfire if there's any glitch along the way. I think there's a need to be a bit more nuanced about the continuum from science-fair project to high-street product both on the part of the sellers and in the minds of the buyers.

Pump keeps running for a few minutes after the boiler stops to make use of the heat remaining in the boiler. Otherwise it just goes to waste (e.g., for an outdoor boiler) or waits and gets used when the boiler next runs. E.g., for the oil boiler in the house I'm renting when the thermostat calls for heat the S-plan wiring sends power to the pump and boiler but the boiler is already hot enough from the last run so doesn't fire up for a minute or two. The thermostat called for heat at just before 10:53. The power consumption increased as the pump started. and about 1.5 minutes later the water got to the radiator in my study and its temperature started to increase and the temperature sensor there began to notice. At 10:55 enough hot water in the boiler had been replaced by cold that the boiler started up further increasing the power consumption. It'd be more efficient for this extra pump run to happen at the end of the cycle but that'd be a bit more complicated with simple S-plan wiring where just the valve-motor current goes through the thermostat. Problem is, if the boiler controller is controlling the pump and does the overrun but only gets a signal that the call for heat has ceased because the zone valves have closed it results in the pump pushing against closed valves.

The broadcast forecasts are OK for getting an overview of the weather pattern, or at least they were until they started dumbing them down leaving out the synoptic charts (it's a few years since I've seen a TV weather forecast so maybe they're better now 😀). To see an interpretation for your own area it's best to look at a couple of web sites. I don't bother with the BBC one as it's still too broad-brush; AFAICS they only have one forecast for Highland for example when the west and east coasts can be dramatically different. My favourite for quite a few years has been XCWeather (AIUI, set up by a bunch of hang-glider pilots so very interested in “microclimates”, etc) though I've recently taken to using the Met Office site as a cross check. The advantage is that, though they're based on the same input data, they use completely different models (XCWeather uses the US GFS model) so they're to some extent independent. Each of the models is pretty good most of the time but have weaknesses in some circumstances so if they concur then it's quite likely to be right, if they differ it's a bit more difficult.

Yes, but there are some which are marketed as completely waterproof (e.g., Protect A1) which it would be a bad move to use and it was worth clarifying as the OP didn't specify which sort of felt they had in mind. The polythene over the top is mostly for airtightness though as a VCL it'll be a small help rather than a hindrance. Remember that the bottom surface may not be that well ventilated.

Quite a few people have done it using roofing felt as you suggest. Just to be clear, though, it needs to be a vapour permeable membrane type rather than fully watertight stuff used on some roofs. Obviously you need to make sure that you don't block any ventilation. The usual thing is to put a taped polythene sheet over the top of the joists to make the whole thing airtight and further reduce the water vapour migrating downwards before putting the floor deck back.

Yet another reason to use semicolons instead of hyphens.

That's a wee bit generous, isn't it?

Quite mild on my site today - 10 °C wouldn't surprise me. Metars from Wick were bouncing between 8 and 9 °C all day. OTOH, 4 °C at Inverness now (well, an hour ago at 15:20Z) so obviously some break in the weather between the two and likely the line's just the wrong side of you vs the forecast. Also, AIUI, you're in frost hollow, whose effect presumably depends on cloud cover, wind direction, etc, and there's a limit to how much they can take local conditions into account. I find that up to 48 hours out they're usually pretty good, after that it's just indicative. I really hope they're wrong for this week as I want to get my east gable closed off but stiff breezes between east and south won't let that happen. Getting pretty depressed about it, it's been months of only being able to do the odd day's work.

My opinionated opinion: airtightness testing should be done before the insulation, let alone the plasterboard, is installed.

Seems sensible. Have just had a dig through my box of network cables and they seem to all be 568Bs, somewhat to my surprise as there was a mixture as I remembered it. (Well, apart from the coax network cable at the bottom 🙄 ).