PDHW dual temperature using EPH CP4

[MNW67]

My guess is to avoid cycling due to hysteresis of a standard thermostat there should be a smart thermostat which smooths the cycling, but I don’t know how that could work in practice…because as you say weather compmand opentherm would be likely to conflict each other.

I’d be interested to know as my setup suffers a bit of cycling

I've been trying to think this through. If you have a basic thermostat like the RTS1 (that doesn't use a TPI type algorithm AFAIK), I don't see why that would cause frequent cycling. The hysteresis with these type of thermostats is often quite large, so presumably that means it can take quite a long time for the temperature of the room to drop far enough to make it come on again. Is there any way to check whether it is the thermostat which is causing the cycling e.g. does it make a click when it turns on and/or off.

On a mild day your heat loss might only be 1kW. The minimum output of your boiler is 3.9kW. In this scenario, the boiler would only need to run a quarter of the time. So something like 2.5 minutes every 10 minutes might be normal.

 

[Mister Banks]

Weather compensation on its own doesn’t set room temperature, so that has to be done with a room stat.

My guess is to avoid cycling due to hysteresis of a standard thermostat there should be a smart thermostat which smooths the cycling, but I don’t know how that could work in practice…because as you say weather compmand opentherm would be likely to conflict each other.

I’d be interested to know as my setup suffers a bit of cycling

To control room temperature when utilising weather compensation then I would think that a thermostat is required in each room otherwise how would you stop rooms over heating due to external heat sources such as solar gain or the oven being on. But then you need something to turn down the radiator/heat emitter such as a smart valve.

 

[MNW67]

To control room temperature when utilising weather compensation then I would think that a thermostat is required in each room otherwise how would you stop rooms over heating due to external heat sources such as solar gain or the oven being on.

Why is that any different to a boiler running without weather compensation at a single flow temperature? One room thermostat and standard TRVs do a decent job for most people. Have I missed something obvious!

 

[Mister Banks]

Why is that any different to a boiler running without weather compensation at a single flow temperature? One room thermostat and standard TRVs do a decent job for most people. Have I missed something obvious!

I don’t disagree, I’m just questioning the value of weather compensation especially with a gas or oil boiler.

 

[MNW67]

I don’t disagree, I’m just questioning the value of weather compensation especially with a gas or oil boiler.

Comfort and efficiency.

 

[Notch7]

I've been trying to think this through. If you have a basic thermostat like the RTS1 (that doesn't use a TPI type algorithm AFAIK), I don't see why that would cause frequent cycling. The hysteresis with these type of thermostats is often quite large, so presumably that means it can take quite a long time for the temperature of the room to drop far enough to make it come on again. Is there any way to check whether it is the thermostat which is causing the cycling e.g. does it make a click when it turns on and/or off.

On a mild day your heat loss might only be 1kW. The minimum output of your boiler is 3.9kW. In this scenario, the boiler would only need to run a quarter of the time. So something like 2.5 minutes every 10 minutes might be normal.

Yes, I think I posted some erroneous info

The boiler is short cycling but I think that’s being caused because it’s getting upset because it can’t get the flow temperature down low enough for demand and it’s unable to reduce power output low enough. I think this maybe due to the fact there’s only basically 6 rads on (bedroom one has TRV low) and the return temp is sometimes only 5 deg below flow. I’m doing a garage conversion which will add 3 rads and I’m looking at increasing size of some rads.

You are correct on the cycling caused by the stat, it does happen but it’s only every 10 mins or so.

 

[Cleph]

Weather compensation on its own doesn’t set room temperature, so that has to be done with a room stat.

Thanks @Notch7 , @MNW67 and @Mister Banks , but I really don't think weather compensation is going to solve the problem of requiring two different target flow temperatures because the weather compensation bit has no info about whether the boiler is providing HW or CH. @Notch7 said previously that your boiler has the benefit of a separate switched live for hot water, which is evidently working well from the figures you gave - I wish mine was like yours. On my boiler the SL2 is not described as a hot water switched live, though I would be interested to find out what it actually does - if anyone has any inside info on the effect of applying live to SL2 in the Ideal Logic Max Heat 2 H18 it would be very helpful if they could post it here. That might be my solution. Or maybe there is some other way of getting the dual temperature operation that I haven't thought of.

 

[Notch7]

Yes, Vogue and Logic Max Heat 2 seem different. In summary, the Logic Max Heat 2 offers terminals: SL1 (primary on/off from room stat), SL2 (Frost Stat

On my boiler the SL2 is not described as a hot water switched live, though I would be interested to find out what it actually does - if anyone has any inside info on the effect of applying live to SL2 in the Ideal Logic Max Heat 2 H18 it would be very helpful if they could post it here

I’m not sure SL2 is a frost stat -I can’t see that description in the manual

Here is the weather comp manual

https://idealheating.com/uploads/documents/Weather_Compensation_Kit_User_Guide.pdf

On the logic system boiler the wiring diagram for S plan on logic with weather comp shows BLR on goes to SL1 and hot water off goes to SL2.

And the manual 2.30 general checks shows the weather comp will set boiler temp when not calling for hot water

https://idealheating.com/uploads/documents/logic-max-system-2-installation-and-servicing-guide.pdf

 

[MNW67]

I really don't think weather compensation is going to solve the problem of requiring two different target flow temperatures because the weather compensation bit has no info about whether the boiler is providing HW or CH.

This screenshot from the user guide for your boiler shows that, when the weather compensation sensor is attached, there are separate demands for CH and HW. When CH and DHW are both asking for heat at the same time, it gives "Hot Water Priority". In this scenario, the CH demand says "Ready" and the HW demand says "On".

https://idealheating.com/uploads/documents/logic-max-heat-2-user-guide.pdf

However, when there is no weather compensation sensor attached, this is what you see i.e. just one flow temperature.

I think your installation manual may just be wrong about SL2 being for a Frost Stat. This is not unusual. On several occasions, with different manufacturers, tech support have confirmed small mistakes in an installation manual.

Finally, I have just found the below diagram in the installation manual for the regular version of the Logic Heat2 (not the Max version manual). It says "Note. If an outside sensor is connected then HW OFF to SL2 IN". This is exactly how it has been done on previous versions of the Logic, as shown in the Weather Compensation Guide linked to above. However, the main wiring schematic in the regular version manual also shows SL2 as "Switched Live Frost Stat" just like it does in your manual. It seems pretty clear that this reference to the Frost Stat is just a mistake.

Just to make sure, I think the best thing to do would be to contact Ideal tech support and ask them how to do weather compensation on your boiler and ask them to supply the applicable wiring diagrams.

 

[Cleph]

Thanks both for your further thoughts. It is mysterious about SL2 and whether this is Frost Stat or not. @Notch7 - the reference to Frost Stat is on page 26 of the installation manual you linked (zoom in on the left small circle) and on page 28 on the Primary PCB. But I note the caution from @MNW67 that this could be an error.
So from the screenshots that @MNW67 posted it appears that weather compensation apparently changes the boiler function, so I wonder how those two flow temperatures are set and am suspicious that they cannot be set independently (because both show 60°C, which is too low to get the hot tank above 55°C) - it is unclear to me whether this will override the target flow temperature set via OpenTherm (55°C for CH).
Although this might be the route that Ideal force one to follow, I am still very doubtful about it because of this paragraph in the user guide, page 4, as linked by @MNW67:

This is absolutely not what I want, because my room temperatures are set to different levels at different times of day (we like it warmer in the evenings) and are different between my two heating zones (bedrooms cooler). That's all handled by the EPH programmable stats.
Maybe all would be clearer if I could find some info about what SL2 actually does, with and without weather compensation, and I suppose I could try Ideal again to see if they will be at all helpful - but they weren't last time I tried, so I'm pessimistic.

 

[Notch7]

This is absolutely not what I want, because my room temperatures are set to different levels at different times of day (we like it warmer in the evenings) and are different between my two heating zones (bedrooms cooler). That's all handled by the EPH programmable stats

It doesn’t actually control room temperature, it’s badly described, it’s used by the boiler to make a comparison with the outside temperature so the boiler can calculate best flow temperature. It’s a notional temperature setting.

 

[MNW67]

@Cleph

That quote about how weather compensation works on the Ideal boilers is extremely confusing. I don't know why they have written it like that. With weather compensation, you still set the actual room temperatures using the room thermostats! The "Room Temperature Setpoint", which you choose with the boiler knob, does not affect that actual room temperature. Instead, it just adjusts the flow temperature based on the curve you choose and the outside temperature. Below is the information from the Ideal Weather Compensation Kit guide. It is very, very confusing. I would imagine that Ideal were trying to simplify things with their description of how it works, but they've just made it worse.

When HW is called for, the flow temperature defaults to 80C

I am not actually recommending that you fit weather compensation! I have no idea how well it works on a Logic or how well the Logic handles low flow temperatures. I also don't know if it would work with your current wiring or how it would interact with your EPH controls. But I thought you should have all the information to make a decision.

 

[Notch7]

Thanks both for your further thoughts. It is mysterious about SL2 and whether this is Frost Stat or not. @Notch7 - the reference to Frost Stat is on page 26 of the installation manual you linked (zoom in on the left small circle) and on page 28 on the Primary PCB. But I note the caution from @MNW67 that this could be an error.
So from the screenshots that @MNW67 posted it appears that weather compensation apparently changes the boiler function, so I wonder how those two flow temperatures are set and am suspicious that they cannot be set independently (because both show 60°C, which is too low to get the hot tank above 55°C) - it is unclear to me whether this will override the target flow temperature set via OpenTherm (55°C for CH).
Although this might be the route that Ideal force one to follow, I am still very doubtful about it because of this paragraph in the user guide, page 4, as linked by @MNW67:
View attachment 333533
This is absolutely not what I want, because my room temperatures are set to different levels at different times of day (we like it warmer in the evenings) and are different between my two heating zones (bedrooms cooler). That's all handled by the EPH programmable stats.
Maybe all would be clearer if I could find some info about what SL2 actually does, with and without weather compensation, and I suppose I could try Ideal again to see if they will be at all helpful - but they weren't last time I tried, so I'm pessimistic.

Ideal have very very little information on weather comp or PDHW anywhere on their website or manuals.

It’s pretty poor TBH, although typical of the U.K. industry I think. Something like 90% of condensing gas boilers fitted in this country don’t actually Err condense hardly at all.

 

[Mister Banks]

Ideal have very very little information on weather comp or PDHW anywhere on their website or manuals.

It’s pretty poor TBH, although typical of the U.K. industry I think. Something like 90% of condensing gas boilers fitted in this country don’t actually Err condense hardly at all.

A lot of plumbers install boilers and have no interest or knowledge on how to set them up. They sling the boiler on the wall, join the pipes and leave. Most heating systems in the uk seem to be either s plan or y plan, neither system is good for a condensing boiler or heating system efficiency in general.

 

[Cleph]

I am encouraged to hear that my room stats would still work if weather compensation is fitted, but I guess the question is whether adding a weather compensation kit would disable the OpenTherm control from the EPH stats. Here's the page from the EPH user guide for setting OpenTherm parameters:

And the climatic curves from the next page are:

Which go in a similar direction to the Ideal WC curves that @MNW67 showed above (if you allow for the reversed axis).

If the Ideal WC Kit worked sensibly, i.e. took account of the OpenTherm information from the EPH stats, then it would be great. But if it's not sensible then it will override the OpenTherm values and I will have spent money unnecessarily. I guess I need to think of a question, but if anyone has experience of OpenTherm with the Ideal WC kit I'd welcome any input.