This is for the guys who’ve been called back to a job because the boiler’s throwing a fault code, or the thermostat won’t pair, and you’re standing there wondering what you missed. I’ve been there. More times than I’d like to admit. Below are the questions I wish I’d asked – and answered – before I started my first Vaillant installs.

1. Why did my Vaillant boiler show an F.28 fault immediately after installation?

The short answer: You probably didn’t purge the gas line properly. It’s the most common new-install mistake I’ve seen (and made).

In my first year (2017), I installed a Vaillant ecoTEC plus 832 and it lit up with F.28 (ignition failure) straight away. I spent an hour checking the spark electrode and gas valve. Turned out, there was air in the gas line from the meter because I hadn’t purged it after the new supply run. The fix took 2 minutes: bleed the line at the burner until you get a steady gas flow. I felt like an idiot.

Lesson: Always purge the gas line before connecting the boiler. It saves you the diagnostic headache.

2. Can I install a Vaillant wireless thermostat without the VR 71 module?

Honestly, I’m not entirely sure why Vaillant made this so confusing. My best guess is it’s down to their eBUS communication protocol. But here’s the deal: Yes, you can install a basic wireless thermostat (like the VRT 350) without a VR 71 module if you’re only doing on/off control. But for modulation or weather compensation, you need the module.

My screw-up: In September 2022, I ordered 10 VRC 700f units and didn’t check that the customer’s existing boilers needed the VR 71 interface. $1,200 worth of thermostats that wouldn’t talk to the system. We caught the error when we went to commission, wasted a day reordering, and had to pay for an extra site visit. So glad I now keep a compatibility chart taped to my toolbox (note to self: update it for the new VR 92).

3. What’s the biggest mistake with Vaillant boiler pressure after installation?

Setting it too high. I see this all the time from new guys. They fill the system to 2.5 or 3 bar cold, thinking it’s better to have too much than too little. But when the boiler heats up, that pressure can push past 3.5 bar and the PRV dumps water everywhere.

Rule of thumb: Fill to 1.5 bar cold (1.0-1.2 for larger systems). That gives you expansion room. I learned this after we had a PRV discharge on a customer’s brand-new carpet in Q1 2023. Cost us $890 in redo work plus a 1-week delay for the carpet replacement. The customer was not happy, and my boss was less so.

4. Why does my Vaillant boiler keep locking out on F.75 (low water pressure) when I’ve just filled it?

You might have an air lock in the heat exchanger. It’s a classic new-install problem. The system looks full, the gauge reads 1.5 bar, but there’s an air pocket stopping the water flow through the boiler.

Fix: Open the boiler’s automatic air vent and bleed the pump. I’ve done this so many times I could do it blindfolded. Once, I ordered 20 boilers and every single one had the same issue because we filled the system too fast (venting wasn’t done properly). That mistake affected a $6,400 order and cost us a full day of rework.

5. Is it worth using Vaillant’s own system filters, or can I use a generic one?

I used to think a filter is a filter – save money, buy a generic. But then I saw what happened on a job in Q2 2024. We installed a Vaillant boiler with a third-party magnetic filter, and the boiler started throwing F.22 (overheat) errors within 3 months. The filter was too restrictive for the flow rate, causing the heat exchanger to overheat.

When I compared Vaillant’s VMS 10 filter vs. the generic one side by side, I finally understood why Vaillant specs matter: the internal flow path and magnet strength are optimized for their pump curves. The generic had a smaller bore, restricting flow. That error cost $450 in wasted parts plus the embarrassment of explaining to the customer why we had to re-pipe the system. Now I only use Vaillant’s recommended filters.

6. What about switching from an old boiler to a Vaillant heat pump? Any traps?

Yes, and this one’s big: pipe sizing. Old boiler systems often use 22mm or even 15mm pipes. Heat pumps need larger flow rates because they run at lower temperatures (like 50°C vs. 70°C). If the pipework is too small, you’ll get pressure drops and the heat pump will constant short-cycle.

I nearly made this mistake on a retrofit in November 2024. Almost issued the order for a Vaillant aroTHERM plus without checking the existing pipework. But before I did, I did a quick pressure-drop calc and realized the 15mm runs would choke the system. Dodged a bullet there – was one click away from a very expensive mistake.

Lesson: For any heat pump retrofit, verify the pipe sizes. You’ll probably need to upgrade to 28mm for the main runs. This was accurate as of late 2024, but heat pump tech changes fast, so verify current sizing recommendations.

Bottom line: Most of these problems are avoidable with 10 minutes of extra planning. I’ve got a checklist I now use for every Vaillant install. It’s saved me from at least 6 major callbacks in the past 2 years (plus a lot of embarrassment). If you want a copy, ask your rep – or just make your own based on the mistakes you’ve seen. Better to learn from mine than repeat them.