sharpener

What do ppl think of the Heat Engineer s/w? £10 for a one-off survey seems almost too good to miss and the HelpGuide a lot of useful information. But if there are negative views here I might not spend any more time on it.

+1, This also stops it sucking in air through the shaft gland. It is a puzzle isn't it! Helpful perhaps to draw a diagram with the elements positioned to show the correct relative pressures. As @JohnMo implies upthread, the baseline is the static head at the bottom of the tank i.e. suction side of the pump. From there the pressure rises across the pump and falls again across the HX, leaving a residual delta P as the motive force for the flow through the tank, and this last term will also raise the level in the overflow pipe. If it is less than the height from the water level in the header tank to the top of the overflow bend the pump will push the water through the DHW tank, if it is more then it will pump over into the header tank. In one house that had a vented system I remember increasing the height of the overflow bend by a foot or so to cope with turning the pump up a notch. In your setup however I expect most of the resistance and hence most of the pressure drop will be across the HX and not much needed for the tank. As a further puzzle, does it help if instead of abandoning the coil you put it in series with the HP feed to the HX (or possibly the return)? After scratching my head I can't be sure which if either will give an advantage. But from first principles increasing the total heat exchange surface area can only be a good thing. I guess its actual usefulness will depend on the relative areas of the two. Also, from my previous work on MVHR air-to-air HXs I recall that the dwell time in the system is some kind of figure of merit, and this would increase as well.

I never said anything about the bathroom rads - because there aren't any. It was only meant as a sanity check. But do you have any explanation for how these figures above come to be so radically different? The city plumbing calculator looks more or less identical with the one on several other plumbers' merchants' web site (cosmetics apart) but I do not know where it originally came from. It seems to take account of quite a number of underlying variables but I have no idea of its accuracy. I do know that the result is not very sensitive to the exact window sizes for example, however I don't know if 50cm thick traditional stone walls are very different from 9 in brick with no cavity, or how much of the total heat loss this represents. Certainly the thermal mass will be a lot greater, but in the steady state this will not matter and I would expect the thermal conductivity to be less. The whole lot might anyway be swamped by errors in the ventilation rate assumptions for all I know. As a relative latecomer to all this I would appreciate a link to a better online calculator if there is one e.g. the mythical Jeremy H, for rough sizing only so I am not looking wholly in the wrong ballpark for HPs.

Good point from @JohnMo, I think he is probaby meaning a pump constructed (expensively) of s/steel and bronze, which does not rust even with no inhibitor. They are used for e.g. secondary loops to ensure a hot DHW supply to distant taps.

What is the construction? Thanks to Heat Geek a useful cheat sheet below. For our barn conversion I took a figure from this of 75 W/m^2 to represent stone walls with double glazing and thick loft insulation. Result of 15kW agrees closely with the more detailed rads total from cityplumbing web site of 14.86kW, though that did not include the kitchen area as it has the Aga.

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?