Adding heatpump into DESS and the potential impact

So I've been running my narrowboat setup on Victron DESS for a couple of seasons now — Multiplus-II, Cerbo GX, and a 20kWh Fogstar Drift battery bank. Works a treat for the most part, juggling grid top-ups at the marina against solar from the roof panels.

Here's where it gets interesting though. I'm seriously considering adding a small air-source heat pump for the winter months rather than running the diesel stove constantly. Fuel costs on the cut are no joke lately.

My question is around how DESS handles the additional load prediction. From what I understand, DESS uses historical consumption data to decide when to buy cheap overnight units versus when to lean on stored solar. But a heat pump is wildly variable — a crisp January morning moored up in the Midlands is nothing like October in a marina. The algorithm surely can't model that accurately without some manual input?

Has anyone actually integrated a heat pump into a DESS-managed system and found a sensible way to flag that load change? I'm wondering whether:

• You can set consumption overrides manually in VRM to account for the extra draw
• Or whether it just takes a few weeks of "learning" and hammers your battery in the meantime
• And whether a small dedicated Renogy panel array purely for the heat pump circuit even makes sense as a partial offset

Also aware there are murmurs about feed-in rules changing again, which might shift whether DESS optimisation is even worth the faff long-term for a boat that moves around anyway.

Keen to hear from anyone who's navigated this, particularly if you're on a mobile setup rather than a fixed property.

Running a similar setup on my narrowboat — Multiplus-II 3000 with a 15kWh Fogstar bank. Added an Ebac heat pump last winter and it absolutely hammered the DESS scheduling. The problem isn't the heat pump itself, it's the startup surge convincing the Cerbo that demand has spiked and it starts pulling from grid at the wrong time windows.

What's your heat pump rated at? Compressor startup on mine was pulling nearly 3x the running draw and the DESS logic couldn't distinguish between a genuine sustained load and a momentary spike.

Ended up having to set a higher minimum SOC threshold than I wanted — felt like a bodge rather than a proper fix. Has anyone actually managed to tune the ESS minimum SOC and assistant mode settings properly to account for cycling appliances rather than steady loads?

Hey @ExChippie72 and @BurnWalker — worth flagging that heat pumps can really upset DESS scheduling if your COP drops during cold snaps, since the actual draw becomes unpredictable. I'd strongly recommend setting a dedicated ESS minimum SOC floor slightly higher than you'd normally run — maybe 40-50% rather than 20% — so the system isn't caught short when the heat pump pulls more than expected overnight. Also check your Multiplus-II's transfer switch settings; some heat pump compressors have startup surges that can momentarily confuse the inverter's load detection. Have either of you looked at integrating the heat pump into Node-RED via the Cerbo to get smarter scheduling around your solar forecast? Makes a noticeable difference in shoulder seasons.

@ExChippie72 this is something I learned the hard way on my shepherd's hut install before I moved the same logic across to the boat. The critical thing DESS doesn't natively account for is the thermal lag between the heat pump kicking in and your space actually reaching setpoint — so the inverter sees a sustained high draw right when it's trying to transition between scheduled charge windows.

What fixed it for me was setting a dedicated AC input current limit slightly above the heat pump's peak startup draw, then letting the Multiplus-II's PowerAssist fill the gap from the Fogstar bank during that spike. Takes the scheduling instability completely out of the equation.

Check your ESS Assistant version too — older versions handle sustained resistive-looking loads poorly. Victron pushed a meaningful update mid-2023 that improved this considerably.

Hey @ExChippie72 — one thing worth considering that nobody's touched on yet is the interaction between your heat pump's defrost cycles and DESS. When defrost kicks in, you can get a sudden reversal in power draw that the Cerbo's ESS assistant doesn't always anticipate cleanly, especially if you've got tight SOC windows set. I found setting a slightly wider discharge floor — maybe 25% rather than 20% — gave the Multiplus-II enough headroom to handle those transient spikes without the system getting flustered. Also worth checking your AC input current limit is sensibly configured so grid assist cuts in smoothly rather than the battery taking the full hit. Are you on a marina hookup or primarily solar-charged? That'll change how aggressive you want DESS to be with scheduling around the heat pump load.