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pdf27

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pdf27 last won the day on April 13 2021

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  1. If it is you have the same problem: if you control everything on roomstats (TRV or conventional roomstat) then because they'll never quite agree the system will spent a lot of it's time heating the house only using one or two radiators (room-to-room heat transfer typically being much stronger than room-to-outside). This is masked with a boiler where you can easily oversize and accept a modest penalty in efficiency and cost, but this is much more of a problem with heat pumps. Correct, in so far as the mean internal temperature is potentially slightly reduced by running some rooms a bit cooler than otherwise. This will reduce the heat loss through the fabric implying slightly reduced total heat demand. However, with a heat pump you also have to cope with the effect that it does this by greatly reducing the surface area of radiators. This means that for the same heating power delivered to the house the water needs to be warmer in the circuit, reducing the COP (boilers also see this effect, but it is much weaker). Unless you have very big temperature differences between rooms with all the TRVs wide open (in which case they are needed for comfort), the COP effect from cooler water will outweigh the total heat demand effect from cooler rooms. Indeed. The problem is that you've got two control loops if you have both a TRV and weather compensation active: the weather compensation curve is based on the assumption that the radiator area is predictable (i.e. for a given outside temperature, the TRV valve position and hence total radiator area is known). In this situation one of three things can happen: The weather compensation is set correctly, so the correct room temperature is only achieved with the heat pump running steadily and all the room stats open (i.e. the water is at the lowest temperature at which it an achieve the desired comfort level). In this case both the thermostat and TRVs are essentially redundant. The weather compensation is set too low, so the thermostat and/or TRVs are continually calling for heat and the house is always too cold. The weather compensation is set too high, so first the TRVs will close off sections of the loop and then the thermostat will stop calling for heat. Because the volume of water in the loop will be quite low as the TRVs start to close, a buffer tank will be required to prevent short cycling. It is also worth noting here that for very well insulated houses (such as those many people on here have built), an additional effect comes into play. Heat dissipation varies as a power law with the temperature difference between the floor and room. This has a stabilising effect - if the flow temperature is 25°C and the emitter is sized to keep the room at 21°C on a design cold day, even with no control system whatsoever then the room must be between 21 and 25°C. Underfloor heating will happily do this (from memory @jack runs his flow temperature at 25°C), radiators will struggle.
  2. One thing to note here: thermostatic radiator valves are probably not helpful with a heat pump: As already noted, water volume is reduced when they close increasing short-cycling problems. Closing a valve reduces the available radiator area, meaning the water needs to be run hotter to heat the rest of the house. Unheated rooms will mean that the radiant temperature of the walls from those rooms is lower, so the rest of the house needs to be slightly warmer for comfort - again increasing the flow temperature. Ideally a heat pump would use weather-compensation to adjust the flow temperature to the minimum possible - TRVs will make this much harder. Essentially they work by allowing you to run some rooms cooler, reducing total heat demand. However, this is not the same as reduced **energy** - heat pumps benefit very strongly from reduced flow temperatures, and it is probable that they'll be better off running cooler with the TRVs removed or fully open.
  3. I'm wondering if they've assumed the heat pump is a 23 kVA one (i.e. 100A draw) through not reading your email correctly. Have they given you a written copy of exactly what they're quoting for? Also, it's possible the network locally would support 60A or 80A, but not 100A - if so you might have to look at your demand calculations more carefully.
  4. We've had an architect so far to take us through to applying for planning permission. I thought I had a very good idea of what I wanted beforehand, but they've given us something both much better and significantly cheaper to build than what I had in mind - with my estimate of the reduction in build costs being 4-5x what we've paid them so far. As a more general note, your specification so far seems very hurried and inconsistent. For example, you're talking about using a flat roof to reduce cost, and then a few lines lower about using a GSHP with borehole for heating. Boreholes are seriously expensive, while pitched roofs aren't all that much more expensive than flat - so from a value engineering point of view you don't seem to have given it a huge amount of thought which might give you problems later if you try to get someone just to draw up what you want.
  5. Slightly random thoughts: Are you allowing for the avoided cost of tiles/slates from in-roof PV? For this and assuming a reasonable export price (5p/kWh is available from Octopus, and Agile export is averaging about 20p/kWh at the moment) I got a payback of 5-6 years. A 7 year payback is actually really good - where else are you going to get a 10% return on an investment? Note that this is reliant on the cash being available however - PV prices are only going down, so it isn't something you really need to do now. Battery prices are coming down really fast: if you have the opportunity then leaving a space allocation for a battery makes a lot of sense, but the economics of buying one at the moment are really questionable unless you treat it as a toy with any cost savings being a bonus.
  6. It's all in the emissions of the sheep, not the processing, and that depends heavily on how you count it - they're ruminants (lots of methane) and the land use changes from what would have been largely temperate rainforest in prehistory are pretty severe too.
  7. That's fall arrest only. If you're going for fall restraint (i.e. a rope that stops them reaching the edge), it isn't going to be required. Yep. Fall arrest needs a rescue plan in place, etc. - bad news all round and a lot of extra work, because being left hanging there is lethally dangerous. Fall restraint is much simpler though and should be adequate for this roof. I think we may be talking cross-purposes here. I've been responding to the question "what would be required for safe access on this roof", not "what do building regulations require for such a roof". The simplest answer is either that you propose not to clean the skylights or that you propose to employ a company who will clean it according to their own safe system of work. That doesn't require you to provide anything at all since neither requires fall arrest or restraint equipment. Not 100% sure on whether you're covered - I'm an engineer not a lawyer but would be nervous about where the liability lies if you provide equipment that later proves to be defective or is misused because you didn't also provide training.
  8. Easiest option is to go for something compliant to EN 795 - that's suitable for both fall arrest and fall restraint, and is pretty cheap provided you've got something suitable to bolt it on to. Something like https://www.equip4work.co.uk/tractel-pa-fixed-anchor-point.html?keywords= should be fine.
  9. Might be worth getting a cheap induction hob on eBay or something to try out first if it might be controversial. I got mine (middle-aged NEFF) for £40 from a builder who had ripped it out during a kitchen upgrade, and it can be safely run off a 13A plug so it would actually be really easy to set it up next to an existing gas hob if I had the space. Even with a gas boiler it may be worth going for underfloor heating - no obstructions on the walls for furniture, etc. and higher efficiency since the boiler will be condensing more efficiently. Also provides better future-proofing against an eventual gas boiler phase out or ban - heat pumps will always work better at lower flow temperatures.
  10. Only works if it's a genuine backup though - a gas combi won't work without electricity for instance. That means a hot water cylinder is a backup to either gas or electricity to give you hot water during a power cut, but not an extended outage - for which only PV + battery or a solid fuel stove would provide sufficient backup.
  11. Depends how deep you go. At the surface the ground temperature matches the air temperature and you've basically got an ASHP. As you start to go deeper, the thermal mass of the ground means that the temperature is the average of the last few days - and the deeper you go the longer this average gets until with a borehole it's pretty much constant at the average temperature over a year (~10°C). Correct, heat is coming downwards from the sun. Water flowing through is effectively just widening the area of the collector. Unless you're in monsoon conditions this isn't likely to have an enormous effect - essentially it makes the coil act as if it's buried less deeply. Minimal impact - it really tracks air temperatures over a period of time, sun helps but really isn't a huge deal. Correct. It's better thought of as a low-temperature heat sink which you can reject heat to very efficiently (possibly without even a heat pump) in summer for cheap cooling. Not really - it's broadly comparable with fan power for an ASHP because the heat capacity of liquid is so much higher. COP values include pumping losses as I understand it. Care does need to be taken with the ground loop design though - if everything is in series the pumping losses can stack up, which is why manifolds are often used to run loops in parallel. Pretty much, although in exchange you get a GSHP whirring inside - no fan, but you still have the compressor. This is probably the main selling point for me, and why I'm willing to pay a small premium for a GSHP over an ASHP. Not sure this is a pro - I'd say large area of ground required, so neutral if you have the ground and major negative if you don't. This is the averaging effect - cold snaps usually don't last more than a week or so, meaning you're taking advantage of the mild weather a few weeks (or months, depending on depth) ago when heating during a cold snap. You lose out when making hot water in summer though (the ASHP gets lovely warm air for this), so overall the effect isn't as big as it's cracked up to me. Again, it really comes down to ground area - particular soil types need more or less ground array area for the same heat load. Unless you're drilling a borehole, the effect of this is modest in the grand scheme of things.
  12. Oh yeah, and the proposed way of calculating pump flow is wrong too. What you should instead be doing is plotting the pump pressure versus flow curve (typically called a P-Q curve) on the same axis as the graph shown above. Your operating point is where the two curves intersect, and the flow rate at that point needs to be greater than the minimum ground-slide loop flow rate. Many pumps have more than one operating curve (essentially there is one curve for each speed setting) and this needs to be considered as well.
  13. That's the minimum flow rate of coolant (glycol) passing through the heat pump. This will be set to ensure that in the worst case (lowest ground loop temperature + maximum heat demand) the coolant does not freeze up. Why would they run an inverter-driven pump on the ground loop side? To benefit from it you'd have to run a calibration routine for the ground loop and program it into the heat pump, and the savings would be trivial (tens of watts).
  14. As an aside from the boiler .vs. ASHP question, why do you want a gas oven and hob? Ovens are almost 100% electric now and this seems to include all the good ones. Having switched over to an induction hob a few years ago there is no way I'd voluntarily go back - just as controllable, vastly easier to clean and when not in use I can use it as an extra area of worktop.
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