SEM:Heat Pump:SG Ready relay - Considerations and documentation reference for setting up for Vitocal 252-A #603

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.