SEM:Heat Pump:SG Ready relay - Considerations and documentation reference for setting up for Vitocal 252-A

[tlinnet]

Hi,

I am new to SEM, and trying to configure it.

I have a Viessmann Vitocal 252-A. I have the HA integration Viessmann ViCare.

I was wondering how "SG-Ready relat 1+2" could be setup. Viessmann support it: SG-Ready Heat Pump

I tried calling our Heatpump installer, to hear his opinion.
He was worried about having an external system, controlling the heatpump, if not by original made by Viessmann.
The system might be worned out, if too many heating cycles is done per day.

He suggested me to look into Dynamic Assistant.

But it has a subscription price, and since our Battery solution is sonnen, there is no direct integration between these.
I also worried about, that if I set Holiday mode in ViCare, will SEM still send "SG" signals / "Heat water boiler control" signals to it?
And how would the Heat pump handles these inputs if in Holiday mode?

In any case, here I add the reference documentation. Perhaps someone finds it in the future.
Perhaps someone have some experience on this, and would like to share.

I looked up in the manuals that came with the device:

And installation manual:

Then I tried to look at Amazon for relay devices, with 2 relays, and zigbee. Found MHCOZY

And then figure out how-to power device. Perhaps external power is needed. Perhaps something is available internally.
But that would need to be figured out by our Heat-pump installer

[traktore-org]

Hi @tlinnet, thanks for the detailed write-up and for attaching the Vitocal 252-A manual pages — that's genuinely useful reference material for the next person. 👍

Good news: SEM already has native SG-Ready heat pump control, so you don't need any extra automation glue. Here's how it maps to your questions.

1. How to set up "SG-Ready relay 1+2"

SEM drives the SG-Ready 4-state protocol via two switch entities — one per SG-Ready input pin. You configure them in the integration's Options → "Heat Pump (SG-Ready)" step, in the two fields heat_pump_relay1_entity and heat_pump_relay2_entity.

The relay states SEM writes follow the BWP "SG Ready" standard truth table (relay1:relay2):

• 1 – Blocked / EVU-Sperre: 1:0
• 2 – Normal: 0:0
• 3 – Boost (recommended increase on solar surplus): 0:1
• 4 – Force-on (forced max on very high surplus): 1:1

The MHCOZY 2-relay Zigbee device you found will work fine — it just needs to expose two switch.* entities in Home Assistant, which you then point SEM at. Wiring the relay dry contacts to the Vitocal's SG-Ready terminals (and figuring out powering) is exactly the part to leave to your installer, as you already noted.

One wiring detail worth flagging: if your relays end up wired normally-closed rather than normally-open, there's a per-pump heat_pump_invert_sg_ready toggle that flips both contacts — otherwise BOOST/BLOCK come out reversed.

2. Your installer's "too many cycles / wear" concern

This is a fair concern, and two things should reassure him:

• SG-Ready is the manufacturer-sanctioned interface (Viessmann documents it themselves — it's a smart-grid input, not an external system hijacking the pump). SG-Ready states 1–3 are advisory requests: the pump's own controller still decides, and all of Viessmann's built-in compressor protections (min runtimes, etc.) continue to apply. Only state 4 (Force-on) is a true forced start.
• SEM adds its own anti-cycling guard on top: it enforces a 10-minute minimum run and 5-minute minimum compressor rest before it will flip the pump off/on again, specifically so a fast surplus-control loop can't short-cycle the compressor. It also skips boosting entirely if a configured temperature sensor shows the tank/house is already warm.

So you avoid the "force-on" wear risk by leaving SEM in normal surplus mode (BOOST) and setting a high force-on threshold — SEM defaults force-on to a 5000 W surplus, which is rarely hit.

3. ViCare "Holiday mode" + SG signals

Important to be transparent here: SEM sends the SG-Ready relay signals "blindly" — it does not read back the heat pump's own operating mode (Holiday, Service, etc.). It only optionally reads a temperature sensor and a power sensor. So:

• Yes, SEM will keep sending SG-Ready signals while ViCare is in Holiday mode.
• How the pump reacts is entirely Viessmann's firmware decision. Since states 1–3 are advisory, a pump in Holiday mode will typically honor an EVU-block/normal but is free to ignore a BOOST request (it won't heat a house you've told it you're away from). I can't verify the Vitocal 252-A's exact Holiday-mode-vs-SG-Ready precedence from the SEM side — that's a Viessmann behavior question, and the safest answer is to test it or ask Viessmann/your installer.

Docs reference

There's a short section on this in KNOWN_LIMITATIONS.md under "Heat pump SG-Ready control" if you want the canonical description.

Hope that helps you and your installer move forward — and thanks again for the reference docs. If anything above doesn't match what you see in the Options flow, let us know your SEM version and we'll dig in.