I think the whole 4 pipe/2pipe thing is a false distinction. There is only ever one flow pipe and one return pipe attached to the heat exchanger; so all systems are really 2 pipes. It is really just a matter of terminology. If the system is split into CH and DHW circuits within the boiler housing then it is called "4 pipe". If the system is split into CH and DHW circuits outside of the boiler housing then it is called "2 pipe". But there is no difference in how it works and both "4 pipe" and "2 pipe" can do PDHW and allow two different flow temperatures.
Wiring up combi for s-plan
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I think the whole 4 pipe/2pipe thing is a false distinction. There is only ever one flow pipe and one return pipe attached to the heat exchanger; so all systems are really 2 pipes. It is really just a matter of terminology. If the system is split into CH and DHW circuits within the boiler housing then it is called "4 pipe". If the system is split into CH and DHW circuits outside of the boiler housing then it is called "2 pipe". But there is no difference in how it works and both "4 pipe" and "2 pipe" can do PDHW and allow two different flow temperatures.
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With a gas boiler able to extract latent heat from flue gases, the whole idea is analogue control, the typical motorised valve,
can only switch on/off, if there are enough of them say 10, using EPH thermostats for example, it can be close enough to analogue, and where you have an area often not required, in my case the granny flat, to turn off an area for weeks at a time makes sense, but in most cases we want a motorised valve which can gradually open or close, there is a whole list of electronic TRV heads.
each one has plus and minus features, and prices for £15 to £70 each, the best have cleaver algorithms that work out how long it takes to heat the room and can connect to a hub which in turn tells the boiler when to run, the cheapest don't connect to anything, but can be set with a schedule so room only heated when required, often using an economy setting so the room can be heated up faster to the comfort setting.
The thing is digital (on/off) will work, but analogue will use less energy, as to if enough to be worth the expense can't say, a well set up gas system can be 90% efficient, and poorly set up could be 75% efficient, but would the owner really know if they could get that extra 15%, likely not.
I look at my own devices,
it gives me a lovely graph, and the saw tooth is due to being an oil boiler, so the boiler is turning on/off all the time, if the same valve was on a gas boiler you would see a smoother graph. But the main house is basic split into 9 zones.
There are three basic methods of piping central heating, one pipe system is used in the heritage railway where I volunteer, the steam is simply released to atmosphere on the last carriage, but we have a real boiler, which actually boils the water, but with most central heating systems there are two pipes, but the water can be in series or parallel, in domestic mainly in parallel, series would work well with fan assisted radiators, but the parallel system with by-pass valves has been the method used for years, I looked at the ivector fan assisted radiator but the building management system to control it is rather expensive, and this is the problem, move away from the standard system and costs go up, we can look at all these fancy systems, but they cost.
So the first point has to be to look at the boiler, with my oil boiler, there is no point me looking at OpenTherm or anything else, but if I had a gas boiler, likely there would be some option for ebus control, but some will only work with a single thermostat, so then one has to decide if in the home your looking at, can it be made to work with a single thermostat.
My house that does not work, tried single thermostat in hall, but hall cools down too slowly, one can adjust the heat up time with the lock shield valve, but not cool down time, so at moment I have three wall thermostats, two for main house, and one for the flat.
I had to work with what I have, no real hope to re-pipe, so I am trying to make a silk purse out of a sows ear, and I have to look will spending £X on a new thermostat or TRV head give me enough improvement to be worth the cost. The system I have can be improved, and I will likely change some eQ-3 TRV heads for Wiser TRV heads, in key rooms, but since adding the second room thermostat I am using Wiser and Nest in parallel the heating has been reasonable.
each one has plus and minus features, and prices for £15 to £70 each, the best have cleaver algorithms that work out how long it takes to heat the room and can connect to a hub which in turn tells the boiler when to run, the cheapest don't connect to anything, but can be set with a schedule so room only heated when required, often using an economy setting so the room can be heated up faster to the comfort setting.The thing is digital (on/off) will work, but analogue will use less energy, as to if enough to be worth the expense can't say, a well set up gas system can be 90% efficient, and poorly set up could be 75% efficient, but would the owner really know if they could get that extra 15%, likely not.
I look at my own devices,
it gives me a lovely graph, and the saw tooth is due to being an oil boiler, so the boiler is turning on/off all the time, if the same valve was on a gas boiler you would see a smoother graph. But the main house is basic split into 9 zones.There are three basic methods of piping central heating, one pipe system is used in the heritage railway where I volunteer, the steam is simply released to atmosphere on the last carriage, but we have a real boiler, which actually boils the water, but with most central heating systems there are two pipes, but the water can be in series or parallel, in domestic mainly in parallel, series would work well with fan assisted radiators, but the parallel system with by-pass valves has been the method used for years, I looked at the ivector fan assisted radiator but the building management system to control it is rather expensive, and this is the problem, move away from the standard system and costs go up, we can look at all these fancy systems, but they cost.
So the first point has to be to look at the boiler, with my oil boiler, there is no point me looking at OpenTherm or anything else, but if I had a gas boiler, likely there would be some option for ebus control, but some will only work with a single thermostat, so then one has to decide if in the home your looking at, can it be made to work with a single thermostat.
My house that does not work, tried single thermostat in hall, but hall cools down too slowly, one can adjust the heat up time with the lock shield valve, but not cool down time, so at moment I have three wall thermostats, two for main house, and one for the flat.
I had to work with what I have, no real hope to re-pipe, so I am trying to make a silk purse out of a sows ear, and I have to look will spending £X on a new thermostat or TRV head give me enough improvement to be worth the cost. The system I have can be improved, and I will likely change some eQ-3 TRV heads for Wiser TRV heads, in key rooms, but since adding the second room thermostat I am using Wiser and Nest in parallel the heating has been reasonable.
Thanks. How much does PDHW differ from S-Plan ?
In what terms?
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PDHW is done in several different ways, depending on the boiler manufacturer. Some use a standard S-Plan and then use their own controls to make it work. That is what Vaillant do. Others change the zone valves to use a normally open valve in place of a normally closed valve. Ideal do it that way. Below is a fantastic video showing how to do PDHW on an Ideal Vogue. It is worth watching the whole thing. But starting at 4 mins he demonstrates very clearly the difference between S-Plan and PDHW.
Great video, thanks!
Do all modern boiler manufacturers allow for PDHW though ? I;ve come across some Viessmann 050/100-W combis (2022 editions that are a which? best buy if I recall) but I can't seem to find much on converting them to PDHW.
I posted above that I don't know whether combis can do PDHW. It's usually for system boilers and sometimes also heat only boilers.
I've read quite a few posts on here previously and this is what I have gleaned.
Viessmann use a 4 pipe solution for PDHW on their system boiler:
https://viessmanndirect.co.uk/files/73d58c6e-4bb5-4f7a-bb39-a3590001c5f7/WB1B_C System Boiler Wiring Instructions (Mod).pdf?srsltid=AfmBOoqkzXsk7TaFKOPT1s4X5djvsLToc6Ih9Z5-VcyFIGNR_-MbafEw
Worcester Bosch do something similar using this internal diverter kit which is available for all their system boilers:
https://www.worcester-bosch.co.uk/p...ries/greenstar-4000-system-diverter-valve-kit
Intergas use a variation of what Ideal do and they call it X-Plan.
Alpha have an external diverter valve kit. Basically a diverter valve and a cylinder sensor. But it is really just another variation on Ideal and Intergas.
Vokera do not have a hardware solution. They use controls (like Vaillant) on a standard S-Plan. But they don't make the controls themselves. They use EPH controls and OpenTherm.
Baxi is a bit of a mystery to me. I think some have claimed you can do it in the same way as Vokera, but others say it doesn't work. Something to do with their implementation of OpenTherm.
This guide covers similar ground:
PDHW Pt 3 - Heating Academy
Lets look at 4 pipe vs 2 pipe The boilers referred to in this article all have the functionality to control the diverter valve from the software in the boiler, ie there are some terminals to attach the DV valve cables to. Not all boilers have this (Ideal and Vokera being a good example) however...
heatingacademynorthampton.co.uk
Boiler needs to be able to fire at 2 different rates depending on CH or DHW. So needs 2 x inputs for demand.
X plan on Intergas was originally called D plan and undocumented "officially".
2 guys from on here first implemented it and then Intergas finally adopted and supported it.
X plan on Intergas was originally called D plan and undocumented "officially".
2 guys from on here first implemented it and then Intergas finally adopted and supported it.
That's very cool! Which posters?
They're no longer on here.
You can read more here...
Thread 'Intergas eco rf D plan help' https://www.diynot.com/diy/threads/intergas-eco-rf-d-plan-help.512963/
Thanks, that's really interesting. I can't pretend to understand it all, though. It's complicated!
It basically uses a cylinder sensor to create a DHW demand which then causes the boiler to fire at max rate and move the 3 port diverter to move over to supply the cylinder with very hot water.
A cylinder with a large coil and therefore fast recovery will use this heat very quickly to replenish the DHW and therefore CH will only be starved for a short period and not noticeable through heat loss.
I'm a big fan of the ecorf. Fitted quite a few of them and a few using the D plan.
I saw a video last year of the urban plumber doing this on an Intergas boiler, but he used two 2 port valves. When the DHW valve opened it sent a signal to close the CH valve. The use of the diverter valve here has thrown me and I have lost my tentative understanding of how it works!
Urban plumber is on here too.
Or he was... Probably too busy with Heat Geek now and ASHPs.
I've been meaning to go on an install with him but hasn't happened yet... Been too busy to go to sarf landan!
Yes, I've also installed a gash using a NO and a NC 2 port. Same sort of principle... When de-energised, both the 3 port divertor and the NC 2 port will spring close to shut off water supply to the cylinder.
I've just watched his video again. He mentions that it is pin 5 which makes it all work.
What does pin 5 normally do on these boilers when it isn't being used for D-Plan? In the manual it never seems to get connected and doesn't seem to do anything! Although, maybe that's because the manual only shows the standard S-Plan and Y-Plan electrical diagrams with just a switched live coming into X2.
It seems like this boiler can operate in two different ways. The standard S-Plan way where the zone valves basically control the boiler firing. But also in reverse, where the boiler controls the zone valves.
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