sharpener

Sounds worrying! As @JohnMo says upthread, the issue is a balance between eroding the film in service and leaving time for it to re-form in between, 24/7 operation doesn't allow that. I am hoping there is plenty of time in my case, the model shows a safety factor of > 4x, I don't know how authoritative it is but that is a big margin. I will be sure to have them fill the circuit from the mains. Despite treatment my harvested rainwater is a bit acidic but I know the mains is slightly alkaline.

No, please see the explanation upthread, the HP will turn down to 6kW, this will result in a flow velocity of 2 m/s which is just OK.

Massive difficulty in the route (see other thread for the full horror of it).

Doesn't have to because it will be split 7.5 kW to the TS and only 4.5 kW to the coil, so in winter 1.5m/s won't be exceeded. The 12kW Aro + will modulate down to 6 kW so in summer when not using the heating it might get up to 2 m/s, even at this velocity I could run for 6 hrs/day before getting a problem with erosion, see this discussion. . 210 l, this will give me a heatup time from tepid of 75 - 80 mins which in the small hours on E7 will be fine. Even with visitors we do not need more HW than that. There is also the immersion on a solar diverter to get us out of trouble.

Bit of an update. Electrician now happy to connect to the spare ways in the CU. Plumber did come as well, wants to change pipework to cyl coil from 15mm to 22. I thought we had knocked that one on the head back in August using yr cyl heating simulation @JamesPa so it is a bit of a backward move to have to re-visit it. As you say, I have the "wrong kind of house". Have already explained that heating the TS at the same time as the cyl will share the HP's output between them, so neither will have to accept the full 12kW. Will propose to define this by using a flow setter to restrict the flow to the TS (otherwise having negligible resistance it will hog all the flow and starve the coil anyway). Will proportion the flow 40/60% to suit expected daily heat cycle - 9kWh to TS and 6 kWh to cylinder - then they will take the same time to heat up and so continue sharing the o/p right to the end of the run. Lower heat transfer rate to cyl will mean smaller delta T so lower HP flow temp and better CoP, win-win if I can get them to accept it.

How do you change this parameter? I have played with the VR720 simulator but don't remember seeing it in the settings. IIRC where there is a finite proportional gain you classically need an integral term as well to stop there being a long-term error as a result. Not sure why it would need to be adjustable.

AFAIK the Vaillant Arotherm Plus does modulate the flow along with the output in order to maintain the 5C delta T. They have approved the calcs which show the heat transfer reducing as the tank heats up which imply this. I can also use the noise reduction mode to turn the whole thing right down in summer, I am positive that turns down the pump too. The data sheet describes the 550 mbar as "Remaining Feed Pressure" so I am assuming this is what there is left to drive the external building loop at the full flow rate of 2065l/hr. Even if it means just the static head at zero flow, it is in any event so large in comparison with the projected 195mbar drop across the coil circuit that I am not worried assuming the usual sort of pump curve that is convex towards the top rh corner. The 195 mbar result from this simulation was for the 20% glycol case, so was on the pessimistic side; we don't need as much as that because we are in the SW, the installers have yet to tell me exactly what they think is appropriate. 10% would give 4016 J/kg K or a 4% drop which is not exactly a disaster in the context.

That would equate to 1.8 m/s. Maybe the difference is a correction for the density of glycol solution? My formula is 1kWh = 861 deg - litres, which is 3600 seconds in an hour / 4.184 specific heat of water. Agrees exactly with Vaillant data sheet. Yes, I see the 1.5 m/s is widely quoted and comes from the Copper Development Assn (it agrees with the erosion energy method here on the assumption it is running 10h 40 mins a day). My worst case scenario is in summer where 6kW (which is the min output of the HP) is reheating the cylinder on its own from stone cold, which will take 90 mins. According to the same method 90 mins/day at 2m/s equates to 360 (mins x velocity^2) which is only 25% of the permissible 6 hours (1440 mins x velocity^2). Because of the square law 1.6 m/s is much much better and 90 mins of that corresponds to 16% so it is in fact allowable for 9h 22.5 mins in every 24. John Hearfield states that anything up to 2.0 m/s is OK from the POV of noise. I will see if I can find the performance curves of the Grundfos pump in the existing boiler and try it on max, that might give me a feeling for it. Currently it is on min and inaudible everywhere. Wouldn't be my first choice but access to change it is massively difficult, it would need precision drilling up through a multilayer acoustic ceiling with metal laths in it, then the chipboard flooring 8in above that and finally up a duct where the boiler flue is. I might just tackle it with my long drill extensions but I would not want the installers to try as the acoustic tiles are easily damaged and can't be painted over. Thanks for the help guys, looks like it's OK from the erosion perspective, and we can assess the noise before putting the expensive gylcol in.

Yes it does, it assumes the nominal 12kW will be divided as 7.2 kW to the TS and 4.8kW to the cylinder in proportion to the flow rates. 5 deg delta T multiplied by the max flow rate of 2065l/hr from the Vaillant manual equates exactly to the 12kW output. I am not concerned about the heat transfer, that has been dealt with in other calcs (and a long discussion involving a simulation thanks to @JamesPa), only whether I can get by with the existing 15mm pipework (even if it is less than ideal). The pressure drop across the TS will be negligible in comparison so I will need to restrict its flow with a flow setter to get the right apportionment otherwise the coil will be starved. Can you provide a source or link for this please? If I tweak the proportions to 7.5 and 4.5 kW it will come out at 1.5 exactly which sounds OK then.

Is 1.6m/s reasonable for intermittent use in a domestic setting? My existing S-plan installation has bizarrely only got a 15mm feed and return to the coil in the OSO 210l tank. The performance is fine on my Kidd oil boiler. Perhaps the original installer in 1995 thought that using 22mm pipe would steal all the flow from the radiator circuit? Am now planning to replace the boiler with a Vaillant 12kW heat pump. The existing cylinder is in an inacessible void above the airing cupboard and V have eventually agreed I do not need to replace it*. I intend to heat up a 260l thermal store in parallel with the cylinder, so plan to divide the flow 60%/40% in proportion to the typical usage using a flow setter for the TS. This will result in a flow through the coil of 2065/60 * 0.4 = 13.8 l/min so from this calculator the velocity would be 1.6m/s. And the pressure drop over a 6 m run using 20% glycol of 190mbar, which is well within the HP's capacity of 550mbar. * Provided I fit a secondary circulation pump to improve the heat transfer from coil to tank. Not germane to this question.

Yes so it would seem from comments on various websites. Disappointing for a major brand name. And expensive for what they are, the cheapest I found from a reputable-looking source was £27-99 IIRC. I recently wired in a miniature 240V mains relay which has an even smaller coil. And over the years I have had quite a few dealings with specialist coil winders over microphone transformers, mains toroids, voice-coil type actuators. Don't recall any problems with them going open-circuit intermittently or in response to thermal stress. But funnily enough I was going to put in a plug for St Ives Windings and then I found this so may be it is not that uncommon. Must dig some old Neve mic transformers out of my junk box, no idea they might be worth £150 apiece. But I digress.

There was, fortunately, and the solenoid from it seems to have fixed the problem. Boiler has been cycling on and off on its thermostat ever since. Beats me how a coil of wire on a former can be quite so unreliable, it's not exactly new technology. Might cut the old one open when I have a spare moment. Two of the four installer people who came for a further site visit this morning are expecting additions to their families, so now it has been put back until April. Funny this was not known about when the dates were put in the diary in December.

Can't view flame but I can hear the burner stop firing a few seconds before it locks out so my suspicions are on the solenoid. Startup and shut-down timings are OK so I don't suspect photocell or control box. Don't think it is meant to try re-lighting on flame failure outside of the startup sequence. No problems with voltage dips and there is no hydraulic bypass. Will see if there is an old pump in the garage.

Kidd Very High Efficiency 60,000 BTU boiler mfd 1995, working fine until this morning. Has Inter model 2011 burner with Danfoss control box. Regularly serviced, last time in 2022, routine parts only (nozzle, gasket). Starts and fires up without hesitation but stops after approx 15mins and goes to lock out. Will re-start immediately when I push the reset button. Usual service engineer (ex Kidd) suggests fuel starvation. Tank 1/3 full. No recent oil deliveries to cloud the issue. Have checked the filter at the tank, no debris, changed it anyway. Disconnected flexible pipe at boiler union to copper, steady stream of oil comes out. Fire valve not tripped. AGA teed off the same pipe in house runs good and hot and even survived the filter change. No obvious point for air to be getting in to the oil feed. Motor, pump, control box etc must be OK or it would not run at all. What do I try next? Am loath to spend much money as I hope to get ASHP installed next month!

Yes, then see if the house is still warm enough.

It will cost a small fortune to run it when it is not absolutely required to maintain your house at the target temperature, as resistive heating has a CoP of 1.

Thanks. Not sure whether it is suitable, there are pix here and here which I don't really understand. Nor why it would be described as enabling the use of two heat sources. Seems expensive even before you add an actuator. I may have found an alternative way of doing it now. Two people have replied to say the Vaillant HP de-energises the heating valves when it is calling for DHW, so actually the S-plan valve setup I already have should work fine even though Vaillant technical have advised against it.

To switch the feed to my rads between heat pump and thermal store I need a 22mm motorised 3-port valve. Ideally it would have a centre off position so I do not need a separate 2-port valve as well. Does anyone make such a thing?

Not sure what the purpose of your thermal store is, unlike electricity the oil is the same price day or night. However assuming you can get reasonable stratification I would think the way would be to allow the rads to deplete the store until it is all at 45C assuming they will provide useful heat at that temp. Then run the boiler at 70 to recharge the store until the return to the boiler rises to 50 or 55 and stop at that point which will ensure a good long condensing run.

Conflicting requirements. Low enough to condense but not so low as to get corrosion. I don't suppose many installations achieve a low enough flow to reliably get 20C drop across all radiators. So with a flow set to 70 it will probably not condense at all. There used to be a Danfoss aftermarket Boiler Energy Manager which would control a boiler to maintain the correct return temp for WC. Same principle, worked well on an Ideal Mexico, cut gas bill by 1/3.

And most places you wish it was actually hot, most the time it's just cold. If the expectation is that it will be cold then the correct form of words is "Danger, hot water might be hot."

Even if you don't have mixer taps TMV still only required for vulnerable people e.g. in retirement homes. One more thing to go wrong. We managed perfectly well without them for decades. Now the fashion in public places is to put up notices saying Danger, hot water may be hot. FGS!

The usual Vaillant configuration for DHW shows a diverter valve (9g below) which is energised for the DHW position. There are also separate two-port valves 9b for the heating circuits: But I already have a two-port valve for the HW coil in an S-plan setup so I am wondering if I can keep this instead. It would seem to depend on whether the controller de-energises the heating valves (9b) when there is a demand for HW, it seems likely but can anyone confirm this? TIA

Am also planning a Thermal Store - though not as big as the OP's 500 to 1000 litres. It will be charged during the night at cheap rate so the CoP will not matter too much, and initially used to heat bedroom rads during the evening. I expect some experimentation before I find out how much effective heat storage there is. If it is more than adequate to warm up the bedrooms either I will extend it to serve the living room rads during the early evening or dial the temp back using weather comp to get a better CoP. For @adrian82 I would think the requirement for night-time heat is so large that it would be worth using a high storage temp and then blending it down with either a standard UFH mechanical thermostatic valve or an ?Esbe electronic mixing valve, some HPs can control them directly. Don't think a heat exchanger will help, there is no need to keep the circuits separate and it will introduce a temp drop across it. 1kWh is 861 litre degrees. So 25kWh (thermal) is 21,500 litre-degrees and a 1000 litre tank charged up to 51.5 deg will be able to give up all the heat into a UFH circuit running at 30deg. What is not clear is if the quoted 25kW (h) is electrical energy in or thermal energy out. If its electrical input then you will need 3x as much storage assuming a CoP of 3 which would be a massive great tank. But still very much cheaper than batteries.

Pre-dates widespread use of R290 refrigerant so I would have thought badly out of date now. A bit like Council Tax bands.