Navien combi boiler and evohome opentherm problem

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Hi all,

I just had a new Navien NCB-34WDLE boiler installed to replace my 11 year old Ferroli Optimax HE31C unit. The Navien was recommended to me by a trusted gas engineer, and a few hours reading the literature and testimonals confirmed the choice. The unit does seem very solid and well made (although it's a shame that the pressure vessel is positioned at the back - let's hope it holds up for the foreseeable). It's whisper-quiet in operation too and as far as I can tell, it's using about 10-15% less gas as per the meter readings.

I've had an Evohome setup with an Opentherm gateway for just over a year. It worked quite well, even with the old Ferroli (hint - you have to disconnect the built in timeclock before the Ferroli will respond to Opentherm commands). However, it does not work with the Navien. Central heating is controlled fine, but hot water is disabled and cannot be configured from the front panel. The installer spoke to Navien who are investigating the matter but the first impression is that there is an interoperability problem.

I am a little surprised that I appear to be the first person in the UK to try to use a Honeywell Opentherm gateway with a Navien combi (or at least, the first person to complain about it). Still, Navien are investigating the matter and they will report back to the gas engineer with a response over the coming days.

My guess here - this is pure speculation on my part - is that the Navien, when in Opentherm mode, is expecting the Opentherm gateway to send it a command to enable DHW and set the DHW max temp, but the Honeywell gateway is not sending this so the water stays off. I guess the Ferroli did not require this command to be sent, but the Navien does. If this is the case, I guess that Navien will need to issue a firmware update for their controller to add an option permitting the Opentherm DHW to be locally overridden.

I am not sure whose "fault" this is exactly - is it Honeywell's fault for failing to support controlling DHW temp from their Opentherm stats, or Navien's fault for not catering for the scenario where the Opentherm controller is not DHW-aware ? I doubt there is a "right" answer to this one, but it does highlight that the industry sorely needs an Opentherm certification process which is enforced by the regulators.

I just thought I'd post this in case anyone is considering this combination of boiler and controller. I'd be interested to hear if anyone else has heard of this problem. I will report back here with Navien's response.
 
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It's on Application version 02.00.17.03 and Wifi version 02.00.17.00.

I see that there is talk of a new evohome firmware release coming along imminently. I wonder if it will solve this ..
 
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Just out of wonder, why was you changing the optimal? Was it reliable or was it poor?
Ferroli has Honeywell electronics so the opentherm works as intended by the original creators.

Also did you look into intergas or atag? These is normally first choice when considering opentherm.

My understanding is that a combi shouldn't need the opentherm to control the dhw and should be fully controlled by the boiler itself, for the reason being that you want it to heat up as quickly as possible and that it generally isn't on for very long to get the benefit of open therm on the hot water Even if running a bath the boiler should self modulate to a given hot water temperature.
 
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Yes, I considered Intergas. There is one authorised Intergas installer and no authorised Atag installers as far as I am aware in this part of the UK (N Ireland), and furthermore, local plumbing warehouses do not seem to stock these boilers or their spare parts. It's a shame because I really like the Intergas single hex design (although I do wonder why no other manufacturers have adopted it).

I did not know the Ferroli had Honeywell electronics. I decided to replace it because it had been repaired a few times already, there were pressure problems (adding air did not help) and I didn't want to wait for it to fail in the middle of the winter.

I agree that a combi should not need opentherm to control the DHW. Apparently, though, the same boiler works fine with the Nest, so there is clearly a difference in what different Opentherm controllers do.
 
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Yes, I considered Intergas. There is one authorised Intergas installer and no authorised Atag installers as far as I am aware in this part of the UK (N Ireland), and furthermore, local plumbing warehouses do not seem to stock these boilers or their spare parts. It's a shame because I really like the Intergas single hex design (although I do wonder why no other manufacturers have adopted it).

I did not know the Ferroli had Honeywell electronics. I decided to replace it because it had been repaired a few times already, there were pressure problems (adding air did not help) and I didn't want to wait for it to fail in the middle of the winter.

I agree that a combi should not need opentherm to control the DHW. Apparently, though, the same boiler works fine with the Nest, so there is clearly a difference in what different Opentherm controllers do.

Ah ok, just wondered as the ferroli done well to last 11 years lol

Fingers crossed you find a quick solution to your problem
 
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It's on Application version 02.00.17.03 and Wifi version 02.00.17.00.

I see that there is talk of a new evohome firmware release coming along imminently. I wonder if it will solve this ..
You're on quite an old version. I'm hearing that the new version is giving trouble too, possibly more so, but this is only just being investigated
 
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No, evo doesn't control DHW temp, never has. It's a heating control not a combi control. When we were working with Honeywell commercial with evohome I discovered this, the Danfoss version OT didn't either, they're heating controls. You're certainly not the first person to know of it, this was 5 years ago, maybe more.

I guess Navien interpreted the OT spec as we did; it states quite clearly in the spec that an OT control has to be the master and the appliance the slave. Therefore some manufacturers of combis obeyed the rule disabled all the adjustments other than reset on the boiler control fascia, otherwise the slave could override the master. Other manufacturers did not interpret the spec that way. I wonder was the Ferroli a bithermic model as the Intergas?

Honeywell designed the protocol, the Open Therm Association write the spec, Honeywell didn't read it carefully enough and now the company that hold HW franchise and sells the product say it's the boiler manufacturer's that are wrong although to be fair before they sold it off Honeywell domestic sales force said the same thing. That doesn't change anything and none of this helps you.

I found a way to get over it with our boilers that may work for you as it overcame the problems with the Honeywell trials but it was clunky and because of other issues dropped for the commercial project.

Some combi boilers needs to be told what temperature the customer wants DHW by the OT master, if not it'll deliver it chooses a seemingly arbitrary figure, sometimes hot mostly not and it can't be changed easily. That's why HW hastily introduced room stats for combis incorporating DHW adjustment.

I assume you want to keep evohome so you could try what I did but no guarantees; disconnect evohome, connect an OpenTherm stat that does allow DHW adjustment, set maximum DHW, swap back to evohome. That way our boiler remembered the DHW setting and simply tries to achieve the new DHW temperature. Failing that just fit a stat that is designed for DHW on OT combis.
 
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No, evo doesn't control DHW temp, never has. It's a heating control not a combi control.

Forgive my pedantry, but doesn't the evohome control DHW temp - when using a temperature sensor on a HW cylinder ? I appreciate that this is an entirely different mechanism, but from a regular Joe point of view, if it can set DHW temp on a cylinder, it doesn't strike me as a huge leap for it to be able to set DHW temp on the Opentherm side. Of course that's just my non-expert opinion.

I guess Navien interpreted the OT spec as we did; it states quite clearly in the spec that an OT control has to be the master and the appliance the slave. Therefore some manufacturers of combis obeyed the rule disabled all the adjustments other than reset on the boiler control fascia, otherwise the slave could override the master. Other manufacturers did not interpret the spec that way. I wonder was the Ferroli a bithermic model as the Intergas?

Tthe Ferroli had a regular plate heat exchanger and a diverter. With Opentherm enabled, it allowed me to set both CH and DHW setpoints. I was never able to work out whether it just allowed Opentherm to override the CH setpoint, or if it used the CH setpoint as an upper limit and interpreted the Opentherm CH requests proportionally. But it definitely honoured the DHW setpoint.

The spec is obviously not sufficiently clear if different manufacturers feel able to interpret this in different ways ..

I assume you want to keep evohome so you could try what I did but no guarantees; disconnect evohome, connect an OpenTherm stat that does allow DHW adjustment, set maximum DHW, swap back to evohome. That way our boiler remembered the DHW setting and simply tries to achieve the new DHW temperature. Failing that just fit a stat that is designed for DHW on OT combis.

Yeah, I'm obviously reluctant to walk away having splashed out quite a bit on the Evohome, the OT gateway and HR92s. I guess I could stick it all on Ebay to recover some of my outlay but I'm not sure what other manufacturer is offering an alternative with the same feature set that will not have the same problem.

My options as I see them are :
- try your idea and hope it "unsticks" the DHW Temp.
- hope that either Resideo or Navien (or both?) do a firmware update. This seems far fetched but stranger things have happened.
- add a box between the OT control and the boiler to inject a DHW setting. Involves hacking.
- stick with the plain old relay box and give up on Opentherm. Disappointing but not the end of the world.
 
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Small update : the evohome beta firmware does not solve the problem - to be honest there was no reason to suppose that it would, but it was worth a shot.
 
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For what it is worth...........

As I recall OpenTherm was not an Honeywell original concept but was a version of the eBUS network.

eBUS was developed and was in use in various application before 1998

eBUS_test_spec.jpg

( the file is https://ebus-wiki.org/lib/exe/fetch.php/ebus/spec_test_1_v1_1_1.pdf )

The specification of the eBUS physical layer ( the hardware ) and the eBUS Data-Link layer ( format of data bits ) was more formally published in March 2007

eBUS is polarity sensitive, OpenThermo is not

eBUS and OpenTherm use similar bus interface circuitry but OpenTherm adds a bridge rectifier to allow for reversed connection of the data pair. This bridge rectifier reduces immunity to noise on the wiring but with the short cable lengths in domestic installs this should not be a problem. (eBUS works up to 100 metres )
 
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