sharpener

bedroom rads. 6 of them.

+1 from me. Have had "pumping over" probs before, this was the solution IIRC.

Buffer? I'm thinking of a layout which while charging the tank albeit slowly on E7 and topping up in the afternoon will also charge a buffer tank to Tmax (?70) in order to feed the bedroom rads am and late evening while the HP is simultaneously running the UFH at a lower temp with better CoP. This will minimise the operating time at high temp whilst also keeping a good load on the HP to reduce cycling. A 200 litre store will fall at 10 deg/hour while putting out 2.5kW which sounds about right for this application. The TRVs start to close after about half an hour anyway, we lie in bed listening to the gears whizzing round.

I like the dye idea. If vented tank then much easier, and unless your header is very high no risk of overpressuring the HP, go for it! Or you could adopt a wait-and-see approach, if the existing tank is large for your usage and/or recovery time is unimportant (both true in my case) then you might get away with charging the tank albeit slowly on E7 and topping up in the afternoon when heating demand is low or non-existent. Retrofitting the HX sounds like a free-standing mini-project anyway. Something related that I have considered doing if I have to have a new cylinder for the HP is re-purposing the old one as the buffer tank to offset some of the cost.

Presumably you are thinking of the Daikin-specific Sunamp this one. I think they tweak the phase-change mix to suit the attainable flow temp of the HP so you need to be sure your particular Daikin is compatible and find out what DHW temp you would get. AFAIR they all have 22mm tappings so concurrent showers would be no worse that with a cylinder. The water circuit in it is not large so would not serve as a buffer for the HP ir you need one. I looked at these 2 yrs ago when Sunamp were also offering their phase-change products as a thermal store for the heating side. They discontinued them for this application and were unhelpful when I enquired if the DHW products could also be used in this way. There have been some horror stories about bursting units and subsequent warranty claims. I would avoid unless you are desperate for the space saving.

Yes to all the above. Even with a HX rated at 44kW(!) (with Tin = 80C, don't know what else is in BS12897) deltaT is 5deg, I would go to some lengths to avoid that in my system. You can tap into the HW flow anywhere there is a hot water tap, but the feed is difficult because for an unvented tank you have to get to the tank side of the pressure regulator. Is this G3 work? If the HX fails you might contaminate the DHW with antifreeze, don't know if propylene glycol is classified as toxic but ethylene glycol certainly is. Or conversely over-pressurise the HP loop, dumping it to waste or conceivably causing damage. I have not heard of big scaling problems with combi boilers in Cambridge (v. hard water) so perhaps that is not an issue ?because of the flow rates ?=> noise?.

Out of interest had a look at Mixergy web site. They claim Mixergy tanks are proven to deliver up to 30% more usable hot water when compared to a conventional cylinder* with a reference to the NPL. The energy savings seem to stem from stratification and heating variable amounts of water according to a demand profile - which also enables you to get away with a smaller tank. Nowhere can I see an explanation of how it actually works. Compatibility with heat pumps is achieved with a control box and and external plate heat exchanger(!). If you are going to need that I would say @JamesPa's idea is at least as good. See you have cross-posted, will read yours and then maybe edit this.

Without knowing more about the iBoost and its 3kW dimmer module I don't know whether a) applying 240V AC to the output when the dimmer is telling it to be OFF would cause any internal damage b) the iBoost control circuitry seeing 240V on the output when it is expecting to see 0V would register as a faulty dimmer module and it would cause an error condition or shut down until reset. Ditto putting the chopped waveform from the dimmer back onto the HPs immersion heater terminals, do we know what the internal circuitry is? The wiring to the HP will also act as an aerial for any interference from the iBoost. OTOH it might all be nothing to worry about. Personally I would spend a tenner on the relay as 0.1% of the system cost and sleep more soundly.

I'm not sure how an iBoost would react to something else in parallel with its output. I built my own diverter but I can't say if that would like it either. Perhaps better to have the HP operate a changeover relay for this too as suggested upthread.

I looked into the Mixergy a while ago and while I can't remember exactly how it was supposed to work yes I thought it was gimmicky and not worth the money. If the idea is to recirculate the stored water through an external HX then that should work and be relatively cheap. It would help in my case where the tank is boxed in as part of the airing cupboard construction so would be a major undertaking to replace though am hoping to avoid the need. AIUI a plate-to-plate HX has two separate circuits so four connections hence I don't understand it either (have designed various all-plastic air-to-air HXs for heat recovery).

1. Yes 2. Yes. You could use an ordinary timeswitch to pull in a 16A changeover contactor during your off-peak hours but have the n/c contacts going to the PV at other times. This would also meet the requirement for Legionnaires' protection with a longer period once a week. I envisage it all going in a 4-module DIN enclosure which any competent electrician should be able to wire up for you. 3. Some HPs have/need built in boost heaters (see elsethread re Daikin), some don't. The buffer tank is a convenient place to put it if not, CoolEnergy fit them as standard. They also can provide extra heating power in extreme cold weather if the spec is marginal. Technically you could do it but the PV is probably more valuable to firstly run the HP and then top up the DHW cylinder temp after the HP has had a go at it.

That's always been the idea. Actually since I can do my own plumbing and wiring I intend to do most of it myself under a Building Notice, with the possible exception of G3 and F-Gas work (if any). @JamesPa has a cautionary tale regarding the planning implications of DIY though My reading is that the "single control system" would have to control the Aga and WB, which although techically possible would require remotely-operated air/fuel controls on both appliances. There is a Toby controller made for the Aga but it is a bang-bang servo and not well regarded amongst Aga owners; there was a better one, now discontinued. I haven't looked into the WB but it would need to be wireless in addition, as it is at the other end of the house. Both are IMO a complication too far even if there were an MCS contractor willing to integrate them.

It's a bit more sophisticated than that, and it seems to have a wireless OAT sensor as well No it wouldn't. The limiting thermal resistance in the path from HP to DHW will still be the degK/kW of the existing coil, the thermal resistances are all in series just like an electrical circuit. And you have interposed the HX which will introduce an extra temperature drop. It's all a balance between doing nothing and hoping the existing boiler keeps going, replacing it with a new one and continuing to use fossil fuel, or switching to an HP. I suspect that in a DCF calculation the capital cost of replacing the cylinder and rads might well counter the additional running costs from the poorer CoP but I do not yet have all the figures (and it will of course depend on the discount rate you choose anyway).

Yes to all of that. The oil boiler installed by our predecessors is now 28 years old and Kidd have gone out of business so I want to replace it at a time of my own choosing i.e. this summer not Christmas Eve. No mains gas here but you will doubtless understand why I also do not want to dispense with the Aga and WB stove. However the MCS rules only allow account to be taken if they can be "fully and correctly integrated into a single control system" (which I cannot achieve with any known HA product) hence leading to MCS over-specifying the HP.

Their web site is big on flexibility Each project has it's own specific heat requirements, room layouts and end user requirements, whilst each system component has different flow rate and temperature requirements in order to run efficiently. We take all of these variables and more into account when designing our systems to deliver you a comprehensive design that is simpler to install and guaranteed to perform. but in the end they have cancelled the pre-arranged visit to their offices and declined to bid. I think they are basically MCS designers and my wish to combine an HP system with the other heat sources I already have put them off, though they cite the difficulty of insulating the house to a higher standard and the need to replace radiators. As posted elsethread I am now looking into using the Daikin Altherma HT range as a way of achieving more of what I want without wholesale replacement of rads and HW cylinder. They are split systems but the overall capital cost might be much the same with less disruption. They also have a bi-zone add-on which allows rads and UFH to run at different temps which sounds promising. The CoP will not be wonderful but with free PV available might just be enough.

I am struggling with the running cost comparison at the moment. Our oil costs 66p/litre plus VAT which is around 8.5 p/kWh. So even with a CoP of 3.6 the HP would need an average electricity price of 30p/unit to break even. On E7 we pay 15 and 45p/unit and there is quite a lot of surplus PV as well, however we will also need to run at a higher temp some of the time for water heating and rads so it doesn't look promising. (We already have MVHR upstairs and there is not much more scope for insulation.) PS I am surprised the OP has cavity walls in Victorian terrace, they are common enough where we live but AFAIK all have solid walls. However one advantage of a terraced house can be that "central heating" means you are central and they provide the heating.

@Steve W can you tell us how it copes with holidays? Can you turn the HP right off or does it keep the water heated to 15C even then, and how long does it take to get going from cold? Also could you say which model number you have got as they have several different ranges. I have been looking at Daikin because they advertise their high temperature HPs as drop-in boiler replacements, but this min temp seems to be quite a limitation! I was also pleased to read about your Tado setup, in the light of experience is there anything you would do differently? It certainly supports my plan to keep our Honeywell wireless zone valves and existing UFH zone controls, with the rads calling for the higher temp via a volt-free contact similar to your regime.

Not me. OP's plumber, see OP above. As @JamesPa said he will not need it unless it is a split HP, hence my alternative suggestion.

Sounds as though you have a relationship to treasure with this plumber. Maybe if instead of buying the F-Gas kit you helped with the training e.g. pay the course fee or travel or accomodation he would be williing to put in his own time to take the chosen manufacturer's course, then in addition to all the above you get the extra warranty and he gets a new skillset. So win win win win.

The words of mine you have signally failed to understand are <the return to the buffer tank>. After reading extensively on the subject - and benefitting from others' advice here - my intention is to have a 210 litre buffer tank. This according to the Gledhill buffer store manual is suitable for HP outputs in the range 6kW - 17.5kW and is substantially larger than the minimum buffer size for the two HPs I am considering because it is also to act as a thermal store. To mention two sources of advice in particular, the system diagrams here show several 3-port buffer tank arrangements which Heat Geek says is a good compromise between a good level of engagement on the one hand and unwanted mixing on the other. No idea what you mean, why don't you proof read your postings?

I built a little circuit which is triggered by turning on any of the bathroom lights, it pulls in a relay which increases the MVHR fan speed for 8 mins. (SWMBO did not want 15 mins which is AFAIR the time stipulated in the Building Regs for WC extractor fans.) Only occasion I have designed an OR-gate using 240V AC logic levels.

This has puzzled me too, I guess the explanation that the refrigeration cycle requires it on efficiency grounds it the closest we will get. The other constraints are the available head from the pump and a velocity limit of ~ 1 m/s to avoid flow noise in the pipework. For instance here is a result I got yesterday when considering a Grant 10kW - which has a min flow rate of 10 l/s and a max pump head of 6m i.e 600 mbar - to see what happens when only one TRV is open. From this ready-reckoner the pump can push 10 l/s through 44 m of 15mm copper pipe (which is about the distance to my furthest rad) with a velocity of 1.2 m/s. But with 15mm plastic pipe a head of 600mbar will only cope with a 15m run and the velocity is 1.6 m/s which is too high. The cross-section area is only reduced to 3/4 but the resistance is 3x as much because it is proportional to a higher power of the velocity. So this HP should happily supply my furthest rad on its own, there is about 10m of plastic but OTOH at least some of the intervening pipework is 22mm. In any case I am planning to maintain the minimum flow rate by way of an automatic bypass to the return to the buffer tank.

Yes I was interested in them for the thermal store capability. As they are less than 50 miles away I will take advantage of their face-to-face consultation option since what I want to do is quite complicated.

Chelmer Heating seem to design and supply integrated systems for self build and renovation projects. I wonder if anyone has used them in the past and can give me any pointers as to their capability?

I have been wondering about this. ATM I am charging to ~50% at night rate and the sun does the rest most days and there is enough left over for the HW. I don't think when I install the HP I will put it downstream of the battery system, as in a power cut it would exhaust the battery very rapidly leaving nothing for lights or cooking. If I tee it in before the battery CT then the battery will have priority but the HP will still have the benefit of any residual PV before it is exported. Like @SteamyTea I have a timer so the immersion heater can come on to top up the HW temp after the HP has done most of the heavy lifting with the benefit of the CoP. Currently I do the reverse, the immersion stat is set higher than the tank stat so the oil heating only comes on at the end of the day if the PV was insufficient.