SimonD

You need to use the manufacturer's flue for the relevant boiler. There's no getting around this. The flues nowadays are mostly 100mm with 125mm options in some cases where necessary. It is no major issue to install a vertical flue through a pitched roof so you'll either need to decide on the boiler now, or just get the roof done and then fit flue later when more appropriate.

No daft question, but yes, it pretty much is. The internal pump does both duties using an internal diverter that you'd see on a combi. I reckon it's the same hydraulic unit as used on a combi minus the plate heat exchanger and uprated pipe size for the DHW flow and return. The normal intake of fresh water becomes the return from the cylinder. All using the system water.

So just to go back to my original conclusion that you're in the Tado sphere and may as well just stay there without changing anything and you specifically asked me to suggest a different controller for your hot water. I'm not trying to steer you down that road due to the upheaval required to do so. My statements were more to explain that your system is not doing what you think it is doing, nor probably what you meant for your system to do when you employed your installer. You thought you had priority hot water and you thought your boiler/controls combo would modulate the boiler that you'd chosen for its ability to modulate and the installation you have does not appear to do either of those. You currently have a fairly traditional system which is either a Y or S plan depending on which motorised valve you have installed, with the additional of a low loss header. This means that the flow temperature of the boiler going into your heating system has to run at a minimum temperature required to heat your DHW to sufficient set temperature and it will do so at all times regardless of the heat demand of the house. So if your hot water is set to 50C the flow needs to be at least about 8C higher, ideally more. At present, your boiler will only modulate in order to keep the flow temperature at that fixed temp. You can obviously get your system running as efficiently as you can by balancing it to mostly be in condensing mode. It's difficult to find reliable data to provide specific figures on the savings, but what we do know is that with either load compensation or weather compensation based modulation provides savings in the region of about 10-16%. With the use of PDHW it's going to be a bit more because you can benefit from the efficiency advantages of lower flow temp & the boiler condensing at lower temps for space heating. But then this also depends on usage patterns and correct system setup. If you are going to undertake further work, then to get the best out of the system you should also install modulating controls, that are either load compensating, weather compensating, or both. I'd probably also ditch the low loss header at the same time. I'm also wondering whether you have some comeback on your installer depending on the nature of the contract and what this specified, bearing in mind that doing all the above mentioned stuff is not really a complex high cost design process.

Yes, it's a 2 pipe install. For PDHW the two pipes between the outer flow and returns and gas pipe would be used to supply the DHW. So it is possible to modify your existing installation, but not without some work. I can't quite make out all the pipework in your photos, but you'd need to run pipes from these boiler connections straight to your hot water cylinder and this can bypass your low loss header pipework completely. You'd then modify the existing pipework that currently goes through the motorised valves to your water cylinder and just cap it off. Wiring wise, you could still use your existing cylinder stat, but wire in a Viessmann cylinder demand terminal box which converts the 230v call from the cylinder to low voltage to go into your boiler to run the 4 pipe PDHW. That way you can still control the schedule from your Tado app. Otherwise, you could rewire to hot water control using the low voltage DHW cylinder temperature sensor. That should, in theory, work...

Do you know if your boiler has been installed with the 4 pipe system or 2 pipe system? Take a pic of boiler and pipework below maybe?

It is totally relevant because it reduces the load on the boiler as well as bringing operating (flow) temperatures down to improve running efficiency. It's also likely to extend running times of the boiler.

I'm as completely sure about this as I can be, this is an excerpt from a complaint I made to Tado about receiving incorrect guidance from their technical team before buying the controller from Tado for a customer - all of which worked out to be somewhat embarassing for me, but no apology from Tado: The situation is different for the combi option and there are rumours there's a european version that does support modulation, but I've been unable to source one and feels like a waste of time trying any further.... That's a interesting point. Tado have already come up for criticism for removing modulation from the controls they sell in the UK without telling anybody, particularly the combi version. But also, if you read more into the paper they use to sell the efficiency gains, you'll find that it does not appear to be based upon the modulation of the boiler. Here's a bit that I find quite laughable: The bold highlights are mine, but to me it seems fairly obvious that if you're not at home needing heating and the heating is turned off, you're going to save energy! Link to the report for full reading: https://www.ibp.fraunhofer.de/content/dam/ibp/ibp-neu/en/documents/ibp-report/579.pdf You might also note that Viessmann Direct don't appear to sell Tado, but only Viessmann controls... As you're already in the Tado ecosystem, there's no point trying to swap out anything for just part of the system. Live with what you've got and only when it's time to switch the whole thing do you reconsider what you have as a whole.

Most people don't. When I quote for installs, I take time to try to explain to customers what it takes to install and configure a new system that is as energy efficient as possible. Often they're surprised because none of the other companies quoting even mention anything like this. Just like the heat pump market though, the boiler companies and controller companies don't make things easy. It takes a long time to understanding what's on offer for which make of boiler and how to configure it to work properly and then you get the big names like Tado and Hive, for example, marketing their controllers that apparently produce massive savings but neither do any modulation control in the UK, just relay.

Are you aware that this product does not do load compensation with the boiler, i.e. manage flow temperatures to get full benefit of your modulating boiler? It only does relay switching for both hot water and central heating? I'm a bit of a critic of Tado given that in the UK and also from their professional installer's shop that ships from Germany, you can't get a controller that does digital control and modulation for system/heat only boilers (S & Y plan), only for the combi. And if you want modulation via Opentherm E-bus or Km-bus for a combi you pay a hefty premium, £200 versus about £89. I now always spec other controllers because of this. For single zones, as a contoller, the Honeywell T6R-HW is far better because Opentherm is supplied as standard, as is the process of setup compared to the Tado (IMHO). If you want full efficiencies from your boiler and heating setup, I'd recommend you change your controllers. Your heating engineer really should have told you this given the importance you put on modulation.

With your tado, where did you get it? And when you bought it did you ensure that it was definitely a version with digital control possible of the boiler?

Generally speaking with 25kW you'll get to see about 9-11l/min with an instantaneous temperature lift of 30-35 degrees C, sometimes you can be lucky to see a bit more.

No, it should actually improve the efficiencies and reduce wear and tear on the boiler. One of the reasons for this is that if you're heating up your cylinder for a longer period of time from empty, the flow rate throught the cylinder and boiler is high enough for the return temperature to go above 54C. Therefore, there is likely to be a long period of time where the boiler is not condensing. It's also more likely you'll see higher return temps causing boiler cycling. If you run PDHW and allow the boiler to top up the cylinder frequently, the time period where the return is above 54 is reduced, therefore efficiencies are improved. This is partly the case because with modern cylinders manufacturer's don't tend to advise that flow is balance using a gate valve so you have full boiler/pump flow rates without much restriction. You can further improve your efficiencies by reducing cylinder temp and thus required related dhw flow temp to get the cylinder to required temperature IYSWIM.

This is where I'd like to know the set up of the system, the controls being used and how it's wired up because on some systems, like the Evohome system I have installed, I have full control of this so I can set a differential, pump over-run time, minimum run time etc., and why using something like the Viessmann cylinder stat wired directly to the boiler on a 4 pipe system is designed to make this approach as efficient as it should be. Hence why I'm confused about how the PDHW has been approached in this installation, but also why the system isn't being used as it is suggested it's been installed. @Adsibob?

But Mixergy won't solve this problem because it learns your regular patterns of use and therefore you'll still need to manually top it up following unusual hot water usage.

As @ProDave really. This is not really priority hot water, you keep the hot water on all the time to top it up automatically, but you can obviously turn it off over night. And also look at your hot water recirc. and the timing of the pump. This is where it's important to understanding usage patterns, particularly short draw offs as that's where the energy losses can be greatest with long runs and poorly insulated pipework. It connects straight into the boiler and is a low voltage connection - it tells the boiler by how much the cylinder is below set point and then by how much it is above it to manage firing up and switching off. It also manages boiler dhw output according to set temp of the cylinder to reduce re-heat time: You don't get this if you have a normal cylinder stat connected via relay connection within a wiring centre. All that then happens is you have to manually set to DHW output temp. at the boiler and then it fires up according to the cylinder stat. If you have sufficient cylinder coil output, then it is better for the boiler to manage the temperature differential between boiler output and target cylinder temp. This is definitely not PDHW. With PDHW you want a higher boiler output temp, which is above the desired DHW temp, and with PDHW the system should close off all flow to the central heating system during a cylinder heat cycle to permit the great flow temp. You don't want to boiler modulating for CH during cylinder heating. Then when hot water demand is satisfied, the system is able to reduce flow temps according to either weather comp or load compensating controls to your central heating circuit. If you are mixing these two functions, you're undermining the whole pdhw approach and you may end up providing flow to the DHW cylinder for very long wasteful periods and it will never fully get to desired temperature, which may be why you're experiencing times when the cylinder doesn't reach desired temp. I think we need to know exactly how your system has been configured with the motorized valves and wiring because something doesn't ring right here.

If you go for the heat only 100-W you lose the built in PDHW capability that comes with the system version which supports a 4 pipe installation and weather comp., plus it has better connected functionality out of the box. That way you also get the full benefit of the modulating pump working with the boiler where you wouldn't get that with the heat only. A small bit of extra pipework is a small price to pay for the long-term benefits.

I don't think you need anything nerdy. I reckon you're best off simply setting up your system to be priority domestic hot water and keep the cylinder topped up. By modern accounts that's more efficient than letting the cylinder drop to cold and doing a full re-heat cycle, especially if there are times the timer isn't set long enough. Just make sure the cylinder temp sensors are installed on the cylinder correctly and adjust to the right temp. I can't remember exactly which Viessmann boiler you have but it will have a 4 pipe layout capability for PDHW using the Viessmann low voltage cylinder stat. I'm surprised this wasn't the installed setup, or is it?

Thanks, yes, it's on the reveal and I don't have 150mm. Pics below. Also come to think of it, I've got a very thin length of ewi at the top of the doorset and sidelights. I'm wondering whether this has contributed the issue as it's a big heavy door, it's on a timber frame and the ewi is thin along the top so not much area to reinforce with the mesh. There's probably a lot of potential movement, and it seems it's only appeard following the very cold snap immediately followed by warm wet weather. (and yes, before anyone says anything, I do need to tide up the render on the door frame 😁)

I clearly must have had a moment when I did my rendering on the EWI as I've just noticed cracks in two corners around my front door frame. Clearly, forgot to detail the mesh correctly, or even put it in place. Can someone advise on the best method of fixing this? How much of an area do I need to take off to add mesh and how do I blend the repair in to the existing?

For PDHW you can also use a normally open and normally closed 2-port valve if you're modifying an existing s-plan, but the boiler should ideally support two temps of course.

It's in the photo

Yes, that's more helpful. From the looks of it you have one pump on the flow side - this pumps your hot water into the heating system and hot water cylinder - and one pump on the return side - pumping the cooler heating water back to the heat pump. The flow appears to be on the left, the return on the right, just after the system filter (the black bubble thing). I'm not familiar with the Mitsubishi Zubadan and can't find any tech specs indicating a need to install two pumps in series, nor can I find the heat ex resistance values other than that the pipework needs to be designed for flows that can exceed 1.5m/s. It's not unheard of for this to be recommended in some circumstance, or when one pump sits before a buffer and another after a buffer, but the question still remains as to why on your system? And then we've got the question as to why you're not getting sufficient flow rates even with the two pumps. I would be calling up Mitsubishi technical department to ask questions about the installation recommendations to determine the need to 2 pumps but it does seem like you need to get someone in to take a closer look at the installation. Was the installer MCS certified? And do you know which Certification body they're a member of (e.g. NICEIC, OFTEC etc.). It is possible to raise a complaint through these organisations for FWIW, but at least if it starts costing you money getting someone else in, you've got a better case to claim back the costs. If you complain through these bodies, the installer does at least have a time limit to act. Because you did manage to get heat to your cold radiators when you closed some trvs, I still think it's worth trying to balance the radiators while you're trying to sort this all out. It's also worth getting yourself set up with melcloud so you can see the functioning of the heat pump and energy use.

It is unvented.

In most domestic properties, you don't need a commercial circulating pump capable of that kind of head and flow volume, so you choose a pump more suitable, like a domestic circulating pump - for example: Grundfos UPS3 or a Wilo Pico, each of which also have proportional pressure