System boiler - S PLAN to X PLAN (PDHW and OpenTherm)

Bucki

Hi All

I recently got my first house and conscious of the expensive bills & efficiency etc. I am trying to have a better heating system or at least to make it more efficient and safe money of course.

The property is 16yrs old and has following heating system:..

- invented Cylinder Water tank

- System Boiler

- old Honeywell thermostat controller

- S PLAN

I got the boiler changed to a brand new Ideal Vogue Max system boiler and have purchased TADO EU wireless kit (as well as Nest Gen3 - depending which one I can get it to work best)

My aim is to get PDHW and OpenTherm.
Has anybody done that on a system boiler?

Yep, I am aware a NO valve needs to be placed (replaced with the NC Hot water).


Furthermore, am I going bonkers with this and wasting time but not gaining much out of it after all?  

I understand S PLAN on system boiler is a bit crap cos boiler will always run at high temp for whatever ur calling for: HW or CH and we know that it's waste of energy if u call for CH and boilers works at Max temp performance. 

Also, the OpenTherm is able to regulate the temp and be more effecient.

That is where it starts to get complicated!
For one, does PDHW only make sense and be beneficial on NEW cylinder tanks which have fast recovery capability?? My cylinder tank being 16yrs old I don't think it be that great after all? Btw it is not insulated on outter shell at all. It's not copper neither.

Anyway long story short ....

What are your thoughts?

Ps: nope I won't and can't get it switched to a combi boiler as it's a town house and many other reasons.

FreeBear

You don't have to run the boiler at a high temperature for hot water - Turn the thermostat on the tank down, and adjust the flow temperature accordingly.

I have an vented gravity fed HW tank here on a very old Baxi back boiler. The thermostat is currently set at 48°C (has been as low as 45°C). Whilst there is a very small risk of Legionella (200-250 cases reported each year in the UK), saving energy has a higher priority for me.

If I had your system, I'd run a smarter control system (Home Assistant) and an ESP loaded with the ESP-Opentherm code. The circuit to interface between boiler & ESP is trivial enough to breadboard... But if you have not used either Home Assistant or an ESP/Arduino, it is a very steep learning curve.

ThisIsWeird

I've always wondered at the solution to the different boiler temps required for CH and DHW - I'll be watching this thread with interest.

Although it seemingly works for FB, I don't think that a ~48o hot cylinder temp is to be recommended, at least not without, say, weekly blasts up to the high 60s. There surely has to be a better solution to either?

On a different note, Bucki, your unvented cylinder will (should) be highly insulated, with a sandwich under that outer 'decorative' metal/plastic skin. I wouldn't worry about significant heat loss from it - it'll be minor, and all usable in your house for your airing cupboard. I'd be astonished if it was 'worth' replacing.

[Deleted User]

I replaced an Ideal boiler with an Atag with OT control and HW reheating priority.

I had two options to convert my ‘S’ plan to ‘Y’ plan. The expensive option was to re-plumb and fit a diverter. The cheaper option was to remove the HW zone valve and then replace it with an inline valve which was wired to work in parallel with the CH valve. When the CH valve opened, the HW valve closed.

OT is just a protocol. When there is a HW heating demand, OT will demand 93C from the boiler. You need to check that this demand is overridden by the boiler maximum temperature control. This is not the case with all boilers. The last thing anyone wants is 93C water flowing through radiators hence the need to separate CH from HW reheating.

FreeBear

ThisIsWeird said: I don't think that a ~48o hot cylinder temp is to be recommended, at least not without, say, weekly blasts up to the high 60s. There surely has to be a better solution to either?

If you are that concerned, yes a weekly blast at 60°C will minimise any risk. That said, at 50°C, 90% of Legionella bacteria will be killed within two hours. At 60°C it is two minutes.

But if you want a weekly blast at a higher temperature, you'll need a smart(er) control system or be prepared to do it manually every time.

tacpot12

I have a system boiler and have a setup that is not too far from what you are looking to do. 

My boiler is an Intergas HRE OV - the OV stands for Open Vented, but the vented/unvented arrangement make no difference to PDHW and OpenTherm. My system is an S-Plan System. I didn't want the expense of converting it to a X-Plan arrangement. It uses Weather Compensation and the boiler's own ability to modulate rather than rely on OpenTherm. 

The way it works is:

• The Outdoor Temperature sensor is connected to the boiler via a relay using NC contacts, and the power from the Cylinder Stat (the call for heat to the Cylinder) is wired to operate the relay (relay coil uses 230V). This means that when there is a demand for heat from the cylinder the Outdoor Temperature sensor is disconnected from the boiler. The boiler then just operates at its set point (70C in my case as my partner wants the DHW hot! Our cylinder is quite new and well insulated.) I agree with FreeBear that you don't need it that hot, but it does multiply the amount of hot water available somewhat if you have a thermostatic shower valve, and a well insulated tank will not lose to much. They lose more the higher the temperature is set to. 

• When there is no demand for heat to the cylinder, the boiler can read the  Outdoor Temperature sensor and using its weather compensation curve will lower the flow temperature to some lower figure. Even with a flow of 70C, the return is never above 50C so the boiler is always operating in condensing mode. The only downside to this arrangement is that when there is heat demand from the cylinder, water at 70C is pumped around the CH system. Once this demand is statisfied, the flow temperature drops to whatever the weather compensation curve determines.

• All our rads except the bathroom one have TRVs, and there is a manual thermostat in the hall way. (I wanted to replace this with a programmable stat to have a set-back temperature for overnight, but my partner wanted to retain the manual thermostat). The TRVs and manual stat stop the rooms from overheating at all times (but especially when the water in the CH system is at 70C). 

• The boiler modulates its gas valve to maintain the flow temperature (70C or as set by the weather compensation curve), so it effectively sensing the heat demand of the whole house, whereas one OpenTherm thermostat can really only sense the demand in one location. 
  

To my mind, OpenTherm is flawed unless it is implemented with a thermostat in every room, and a central computer (or microcontroller like the ESP32) that aggregates the heat demand from every room to determine the total demand, which it then sends to the Boiler by OpenTherm. 

I think my arrangement is better, and it also required no plumbing changes, just an extra cable from the loft (where the DHW cylinder and S-plan valves are) down to the boiler on the ground floor, a £25 mains relay, and an outdoor weather sensor.  (This is the relay I used: Easy Relay 240V Mains Relay (230V AC 50/60Hz Coil) in White or Red Single Gang Box - Discount Fire Supplies)

Intergas have a wiring diagram for this arrangement on their website, as well as one for X-Plan.

Bucki

Hey All

Thanks for your comments.
I note somebody suggested to turn down the cylinder stat below 70c but is it not recommended cos of that Limenia or whatever that is.

Also it would be very awkward having to turn it up and down all time , as 1) tank is in awkward location and 2) getting to it it's not walk in a park and 3) too much of hassle.

As for the 16yrs old cylinder tank I am just worried that it's years have passed and won't be efficient anymore (I will have it MOTed soon lol).

Just to summarize why I want PDHW and OpenTherm:

Well, my understanding is that it allows system boiler to somehow modulate at separate flow temperature and be more effecient and therefore spend less gass / money.

OpenTherm would regulate the CH temp 
Whereas cylinder HW be a case of On/Off and the cylinder stat is set to 70 or whatever that is as I won't know in all honesty.

By the way, I also got the SmartTRV, as if you know about TADO, they seem to be great to have provided you manage to get it set up on a system boiler (on a combi boiler there is no brainer really).

Idea was to set up OpenTherm using TADO (EU version) with combination of SmartTRVs and hopefully be all happy after all.

However... The challenge so far past few months is finding somebody able to do this work and set it all up. Slowly thinking if after all maybe the PDHW and OpenTherm will not make a difference and be more or less same as on a S PLAN (keeping it standard).

Maybe I am just wasting too much time but won't get much benefit after all anyway?!


@tactop12

I see you are on a system boiler + S PLAN?
So all you done is added Weather Comp to your boiler and no further changes?

In that sense the CH will be regulated by the Weather Comp and HW is on a On/Off/schedule programmer.

Indeed, the downfall of system boiler is that it only has one temperature flow and when calling for HW same flow is pumped to CH as well, right. Whereas converting to X plan it will brake that flow and work independently but on a X plan then u won't be able to have HW and CH on at same time lol

My ultimate goal is to make boiler work low condensation mode for CH and not simply use the default 70c flow when calling for CH cos that be insufficient and waste, right?


So yeh, I got all the stuff kit ready but installing wiring wise is a nightmare. Then having this worry in my head saying "but is all worth it after all"? 😣


Note: intergas seems good with X Plan and diagrams but have not found anything for Ideal + TADO EU model.

tacpot12

Yes, I have a System Boiler and S-Plan.

Legionella is a bateria that lives in water, and if it gets into the your lungs can kill you. There are various techniques for reducing the risks. One is heating any stored water above 50C. Tthe higher the temperature is above 50C, the quicker the bacteria are killed, so 60C or 70C are often used as disinfection levels, but as long as the water is above 50C for long enough, it will kill the bateria.

You could uses a programmable cylinder stat such as this one: ESi Electronic Dual Cylinder Thermostat - Mr Central Heating and have it set to bring the tank up to 70C for 30 minutes to an hour every week. (30 minutes is enough to kill all bacteria at 70C, but you should probably running it for longer as the very bottom of the cylinder will be a few degrees cooler than 70C.). The manual reminder us that doing this might cause scalding accidents in the home because most of the time the water in the cylinder will be at the thermostat set point, but every now and again it will be at 70C. It recommends thermostatic mixer valves be fitted to all taps. 

The 16yo tank will need replacing at some point. Copper ones seem to last about 25 years or so, and when they go, they leak, so early replacement is not the worst think you could do. Stainless steel should last longer. You need 50mm of insulation as a minimum, but more is better if you can find one with more.

The changes I made were to add weather compensation and the relay to connect or disconnect the weather sensor depending on whether there is a DHW demand from the cylinder (DHW Demand = Disconnected / No DHW Demand = Connected). This works because the Intergas boiler is happy to use the weather sensor if it is present and reverts to the flow temperature set on the front panel if there is no weather sensor connected, and it doesn't mind if the weather sensor disappears after the boiler has been turned on at the electrical isolator. I don't know if the Ideal Vogue Max would be so tolerant of this, but a call to Ideal's technical support should get an answer.

Our DHW heating is on a simple ON/OFF schedule, with the cylinder stat closing the valve that controls flow into the cylinder's coil when the cylinder is up to temperature.

You are right that any boiler can only produce water at one temperature, but you can influence the temperature on modern boilers using Weather Compensation, or OpenTherm, and you can even have OpenTherm AND Weather Compensation. X-Plan and S-Plan are names for the different arrangement of valves in the system, and don't imply any control over the flow temperature of the water in the system. The difference between the two plans is that X-Plan is a Hot Water Priority design, where you can have water flowing to the cylinder OR water flowing to the central heating system - but not both at the same time. So if you can change the flow temperature at the same time as you change where the water is going you can have different temperature flows going to the cylinder and CH system (at different times).

With S-Plan, water can flow to just the cylinder, or just the CH system, or both. The issue comes if you fit OpenTherm or Weather Compensation on S-Plan because there is a hot water cylinder, and because the water in the sytem can only be at one temperature, if the OpenTherm or Weather Compensation calls for a flow temperature of 50C (because it is either warm in the house, or warm outside), the cylinder's demand for heat will NEVER be satisfied, bucause water at 50C cannot heat a cylinder to 70C, so your pump will always be pushing water through the cylinder coil when there is no benefit of doing so.

Either with X-Plan or S-Plan you have to find a way to have the water at 70C (or at least a bit higher than the set point on the cylinder stat) when it is flowing to the cylinder, and a lower temperature when you only need the central heating on.

You lol'ed at the fact you can't have the CH and DHW on at the same time with X-Plan, but you are correct and this an important point. If your cylinder takes a long time to heat up from cold (and a large cylinder might), then your CH system is OFF while this is happening. I took the view that it was better to have the Central Heating ON and the cylinder being recharged if this was what was necessary, and accept the compromise that during this time the water in the CH was hotter than it needed to be. The fact we have TRV means that this doesn't cause any lack of comfort. So a larger cylinder might mean S-plan was preferable. (Ours is a small cylinder, but I stuck with S-Plan because it didn't need any plumbing changes).

You said that your ultimate goal is "to make boiler work low condensation mode for CH and not simply use the default 70c flow when calling for CH cos that be insufficient and waste, right?"

You are correct. Condensing boilers are more efficient, the cooler the return flow is. So the ideal arrangement is one where the flow to the central heating system is closely matched to the heating load of the house, and the flow to the cylinder is also matched to the heat the cylinder requires to recharge it. This wil keep the flow temperature to the lowest it can be, and this will result in the lowest return temperature. (We need to forget about matching the flow temperature to what the cylinder needs because there are no OpenTherm thermostats for DHW cylinders - the thermostat I linked to above is the most sophisticaed thermostat for cylinders that I know of).

OpenTherm + Weather Compensation is the best way to acheive this for the CH system. OpenTherm thermostats calculate a flow temperature based on the different between their setpoint and the temperature in the room. The greater the difference, the higher the flow temperature that is called for. Adding Weather Compensation allows the thermostat to use logic like the following:

• Very Cold Outside + Big Difference between Indoor Room Temp and Setpoint (e.g. when the heating first comes on on a Winter morning)= Max Flow Temp

• Very Cold Outside + Small Difference between Indoor Room Temp and Setpoint (such as might occur when the heating has been on for a couple of hours in Winter) = Medium Flow Temp

• Cool Outside + Big Difference between Indoor Room Temp and Setpoint (e.g. when the heating first comes on on an Autumn morning) = Medium Flow Temperature

• Cool Outside + Small Difference between Indoor Room Temp and Setpoint (such as might occur when the heating has been on for a couple of hours in the Autumn) = Low Flow Temp

To my mind OpenTherm on its own is better than Weather Compensation on its own, but I wasn't allowed to change our thermostat for an OpenTherm one.

It is worth the effort to get the best performance you can out of the boiler, plumbing and controls that you have, because it will save you quite a lot of gas. My parents are paying £500 a month to heat their home, whereas I pay £250 a month to heat mine. They are similar properties in terms of heat loss, but they have a G-rated back boiler and gravity hot water system that means they can't ever have the heating system on without also heating their DHW cylinder (which is poorly insulated). I've been on at them for ages to upgrade, but they can't bear the thought of the mess to make the changes, even though there will not be that much mess.

As far as I know, OpenTherm boilers will still modulate the flow temperature DOWN if the return is too high. So if the OpenTherm thermostat gets the heat demand slightly wrong, e.g. it calls for a flow temp of 52C and actually 51C would be enough, the boiler will modulate down to 51C, so even with an OpenTherm thermostat, it is able to sense the whole house heating demand.

IN SUMMARY 

S-Plan is simpler to setup because it doesn't need any plumbing changes, but you still have to find a way to take the OpenTherm thermostat out of the control loop when there is DHW demand. I suspect that the TADO (and other OpenTherm thermostats luke Honeywell's Lyric T6-HW) revert to their maximum flow temperature when there is a DHW demand, but I don't know how they do this. The TADO manual should tell you if TADO + S-Plan is an option. I don't know the TADO device, so the above is all I can tell you.

ThisIsWeird

Bucki

@tacpot12

May I just say, you seem to know hell a lot about this stuff. Probably you are an engineer.

I read your post 3 times and still processing and trying to understand. Apologies, I am not the brightest out there ...


Let's address bit by bit:

Legionilla seems to be dangerous though not very common but still wouldn't risk it. In that aspect I cannot go below 65degree on a cylinder tank otherwise risk is high.

Also from sounds of it, if the hot water remains still in tank for a time that is when is also chance for bacteria to build up. Now, question is "how often will I empty the tank within a week"? Well I don't know?? For one the water tank will always be refilled or topped up by the water supply anyway and I wouldn't know how to measure that. Never the less, I forgot how many letters that tank holds (think 150-200L).

That device you recommended, would that fit any cylinder tank? And is it simply unplug and plug it in? Or does the tank need to be emptied?

Replacing tank?

I was not thinking about it cos I been renovating a lot lately and cant efford it really but it is not copper and not has any insulation on outer shell. I was thinking in my head to somehow make my own insulation and wrap the boiler around but probably 1) won't make a different 2) may risk of overheating? 3) I am just overthinking it lol.

So for now that cylinder needs to say and will have to get somebody to check it out soon. 

If I replace it in near future then it be one of the fast recovery and we'll insulated ones for sure.

This is the tank I have: model PP170B
any thoughts?
https://manualzz.com/doc/23422371/santon-premier-plus-sales-leaflet-pdf

Programmable cylinder stat

The one you suggested can be programmed to reheat automatically on XYZ days or only once temperature drops below 65degree? Hoping it also has the max temperature to make it stop when it is satisfied.


S Plan vs X Plan

Yes, so I understand that X Plan is needed if you want OpenTherm as by doing that you are splitting HW and DW to function desperately meaning one at a time = they won't work both at same time.

I agree with you, there may be a time that I may need both at same time. Can't think of a a scenario really but who knows. Whilst on other hand I would have thought to somehow program the Immersion Coil to come on during off peak time and heat the water (if cheap rate) or as last option to keep it on a separate schedule to make it run during silly hours where CH is less likely to be used a lot.

The logic I fail to understand is how OpenTherm would work in S Plan on a system boiler cos all research say that can't be done unless converting to X Plan.

Whereas you have added Weather Comp to control your CH on a S PLAN - system boiler. Unless that is bespoke to that boiler or something or maybe that's doable only on weather Comp?


Smart Thermostat

I purchased both: NEST G3 as well TADO UK model which supports HW/CH but no OT and the TADO EU version which supports OT but has no connection for HW/CH rather a Relay connection instead.


I seen somebody on YouTube who allegedly made it work on Ideal Vogue + OpenTherm + PDHW on a system boiler using NEST G3 but Unfortunately, I can't seem to follow it's logic.

By the way, my plan is also to add TADO SmartTRVs anyway. As it's my first house I thought make it all effecient .... And I hit the dead end with this ever lasting project.

FreeBear

Bucki said: Legionilla seems to be dangerous though not very common but still wouldn't risk it. In that aspect I cannot go below 65degree on a cylinder tank otherwise risk is high.

Also from sounds of it, if the hot water remains still in tank for a time that is when is also chance for bacteria to build up. Now, question is "how often will I empty the tank within a week"? Well I don't know?? For one the water tank will always be refilled or topped up by the water supply anyway and I wouldn't know how to measure that. Never the less, I forgot how many letters that tank holds (think 150-200L).

Legionella is dangerous, and is "notifiable" within England & Wales - This means cases should be reported by doctors and other health care professionals when detected. Thanks to this reporting requirement, we can see that in 2020 (the latest report I can find), there were just 295 cases out of a population of some 59 million. So it would be fair to say the risk is very, very small - You are probably at greater risk from a gas explosion.

Just maintaining a water temperature in the HW tank above 50-55°C for a few hours is sufficient to kill the bacteria (if it is present). There is no need to empty the tank on a daily/weekly basis, and it would do nothing to reduce the risk.

Plenty of other dangers in a home that can kill for you to worry about. Electricity, gas, food, pets, etc.