Atag iS32 installed + Opentherm Evohome

[fergie]

After much toing and froing, I settled on the Atag IS32 system boiler for my new install.

The main reason for this is that Hengus and others had got Atag to work well with OT Evohome. It is also an extremely efficient boiler with warranty extended to 10 years. I'm also told that it is designed to last 18.

Though the IS32 is maybe slightly on the big side, I felt the IS24 (replacing a struggling and leaky Glowworm 24) was going to be too small - especially as I heat a lot of water for showers and filling a 160 litre bath (210 litre storage tank). I'm also planning an extension + garage conversion in the future.

My original setup was evohome standing alone (no boiler control) and controlling 12 radiators. I had a Glowworm 24 heat only boiler in S plan with fully sealed system - pump, expansion and storage tank in the airing cupboard. I found the evohome reduced gas usage by about 20%, but that was one month of test data toward the end of winter.

My new set up is the Atag iS32 controlled via Opentherm with hot water priority. Though the installer removed the old pump (it was making a lot of noise which partly swayed the choice to change to a system boiler), he decided it was best to keep the expansion tank. He seemed to be fine with the wiring side of things and was happy also installing the Evohome hot water kit. I took care of the pairing and evohome programming set up. I have 11 zones + hot water control.

The main difference is the new boiler is whisper quiet. I also no longer have any clanking etc as the pump starts firing up. The storage tank reheats extremely quickly and the evohome seems to be settling down well with the new boiler.

[DBMandrake]

My original setup was evohome standing alone (no boiler control) and controlling 12 radiators. I had a Glowworm 24 heat only boiler in S plan with fully sealed system - pump, expansion and storage tank in the airing cupboard. I found the evohome reduced gas usage by about 20%, but that was one month of test data toward the end of winter.

I'm curious as to why you were using Evohome without any form of boiler control before ? Apart from a timer what turned the boiler off when the rooms were up to temperature ?

If you have an S-Plan system and you want to keep the hot water control on the old timer it's still possible to use the boiler relay from the Evohome to drive the heating zone valve and thus fire the boiler via the zone valve. (And let the hot water timer fire the boiler through the hot water zone valve)

A moot point now that you have an all new system, but it did strike me as odd.

[fergie]

Simple answer is we knew the boiler was getting replaced. Just took longer than expected to find the right boiler and then an engineer that was able to fit it.

I felt it was pointless wiring an on/off bridge to the old boiler when it was going to just get ripped out straight after.

Regarding the 2nd point, do you mean have the boiler control hot water rather than via evohome?

[HenGus]

Welcome to the Atag club. My installer also kept the existing expansion tank as he concluded that the size of my system dictated the need for it. I am interested in how you came to the conclusion that you would need a 32kW boiler? I also replaced a 24kW cast iron boiler and the calculations that my installer carried out suggested that my 12 zones/19 radiators would be more than happy with an IS18. The deciding factor was the HW re-heat time - hence, I ended up with an IS24 by way of a compromise. Another factor in choosing the size of a boiler, of course, is the modulation ratio with a minimum modulation output on the IS18/24 or 5kWs compared to, I think, 6.9kWs on the IS32.

I confess that I haven't had much opportunity to test my boiler with full CH turned on. From what I have observed, it is slow to get into condensing mode, and I only have the P0 profile set to 70C Set. I suspect that this is an Evohome thing. I would though be interested to compare notes over the course of the coming months. What I have seen to date though is a 30% further reduction in my HW re-heating costs (one period a day with a temp differential of 5C). I suspect that this is down to the fact that I am no longer paying for a pilot light. With such low usage, it does make me wonder whether I would be better switching my gas to a zero standing charge supplier for 6 months?

[fergie]

Ultimately it was my decision. One installer specced the 24 and another the 32. I went with the 24 spec installer, but we agreed to fit the 32. If there was a 26 or 28 option I would have picked that. Part of my decision was knowing it could modulate down and also not wanting to find at a later date (especially after adding an extension) that I had to shell out another 2-3k for the boiler to be changed again. Also the extra hot water heating power is worth it.

Though my house is 9y old and well insulated, being aprox 220m2, the 24 glowworm simply couldn't cope at the height of the Scottish winter. Yes there is an argument that with evohome you can downsize the boiler further, but I also felt it's better to have something efficiently giving 75% effort rather than struggling at 100%- esp in terms of stress and reliability.

Is there a way to log stats from the boiler rather than manually checking it?

[HenGus]

Ultimately it was my decision. One installer specced the 24 and another the 32. I went with the 24 spec installer, but we agreed to fit the 32. If there was a 26 or 28 option I would have picked that. Part of my decision was knowing it could modulate down and also not wanting to find at a later date (especially after adding an extension) that I had to shell out another 2-3k for the boiler to be changed again. Also the extra hot water heating power is worth it.

Though my house is 9y old and well insulated, being aprox 220m2, the 24 glowworm simply couldn't cope at the height of the Scottish winter. Yes there is an argument that with evohome you can downsize the boiler further, but I also felt it's better to have something efficiently giving 75% effort rather than struggling at 100%- esp in terms of stress and reliability.

Is there a way to log stats from the boiler rather than manually checking it?

A couple of points. When you call for HW, Evohome/Opentherm will stress your boiler as it demands 92C; however, the maximum flow temperature should be limited to what you have set on the boiler. Equally, whenever a HR92 is outwith 1.5C of its set temperature then, once again, Evohome will ask for max flow temperature. That is why I said that Atag/Opentherm seems to be slow to get into its condensing stride. That is not to say that the boiler isn't modulating down as heating demand decreases. I am pretty new to condensing boilers but I sense that the key to maximum efficiency is (a) the correct installer profile and (b) the max flow temperature that the user sets via the RH rocker switch. I learnt the hard way that the set boiler flow temperature cannot be below the maximum temperature set in the HW parameters.

As the Atag One controller does provide the installer with boiler information then there must be some way of getting at this information. I suspect though that the information to the installer comes via the controller.

My installer is adamant that to get maximum efficiency out of an Atag then Evohome zoning should be used with care. He is not a great believer in high zone temperatures in some rooms which then sit alongside cold adjacent rooms/zones. Bearing in mind how Evohome/Opentherm and boiler reacts whenever there is a large heating demand, then he could be right. This coming Winter could be an interesting experiment.

[fergie]

Yes winter will be very interesting. For me it's not just about efficiency- it's also about the level of control I have at room level.

My evohome is still relearning the rooms / zones, but it did great when working in standalone along side the hall thermostat controlling the boiler.

[fergie]

Just to update, I had a look at what the boiler did at 6pm when the evohome switched on for hot water. The tank sensor was reading 15deg (210L tank) and the boiler gradually eased up to 70deg over about 10 mins. The water temp was rising about 10deg per 10 min, but 55 to 65 took a bit longer. Aprox an hour over all to go from 15 to 65.