I’ve made enough expensive mistakes as an HVAC tech to fill a filing cabinet. This is the one I pull out when a customer asks why my quote isn’t the lowest one in the pile.
My name is Ray, and I handle residential heating service for a small company. For the past eleven years I’ve installed and serviced boilers, heat pumps, and tankless water heaters. I keep a logbook of my own misses because my first boss told me that a mistake you don’t write down will be repeated. He was right. This one is page one.
The call that came back around
A rental property manager called me on a January morning. A tenant had reported hissing from the utility closet and water on the floor. The boiler was a three-year-old Vaillant ecoTEC Plus combi, and the pressure gauge was sitting at 2.8 bar. The pressure-relief valve was dripping, and the little copper discharge pipe had fresh water underneath it.
My first move was too fast. I checked that the filling loop was closed, watched the pressure for a few minutes, then told the manager the relief valve was faulty. I replaced it, refilled the system to 1.5 bar, ran the boiler, and left. On the way out I said, “I think so,” when he asked if it was fixed. That phrase should have been my first clue.
“Vaillant boiler pressure too high” is a symptom, not a diagnosis
It looked like a simple valve job. It wasn’t. The next morning the same property manager called again. Same sound. Same puddle. This time I did not order parts. I set a Lasko fan on the floor to dry the spill area, then closed the boiler service valves and drained the primary side down to zero.
On the inside of the boiler there is a small expansion vessel, and on that vessel there is a Schrader valve. If someone searches “Vaillant boiler pressure too high,” this is the check too many people skip. I put a tire gauge on it. It read zero. Not low—zero. There was no air cushion left in the vessel to absorb the water as it expanded.
Why does that matter? Because water expands when it gets hot. In a sealed heating system, that extra volume needs somewhere to go. An expansion vessel is that somewhere. Without air inside it, the water presses directly against the rubber bladder and the system pressure climbs until the pressure-relief valve opens. I had replaced the relief valve in order to avoid the actual cause. The new valve was simply waiting for the same pressure spike to make it leak.
I recharged the vessel to the pressure on the rating plate, refilled the system to 1.5 bar, and ran the boiler through a full heating cycle. Pressure went up slowly to 1.8 bar and stopped. It held there the rest of the week. The fix was not a part. It was ten minutes with an air chuck and a willingness to look like an idiot in front of a property manager.
Where does that leave my Vaillant combi boiler review?
If you ask for a Vaillant combi boiler review, I’m not going to recite the brochure. My field review is shorter. The boiler did what it was designed to do when the system went wrong: it vented pressure before anything inside was damaged. That is not a product flaw; that’s a safety event. And it happened because someone—me, in this case—stopped at the first wet part instead of asking why it was wet.
A good boiler is a well-installed and well-serviced boiler. The sticker price and warranty years matter, but the knowledge behind the installer matters more. If you ask me, that applies to Vaillant and to every other brand I’ve touched.
On the money side: I refunded the relief valve and did not charge for the follow-up. That wasted a morning and put a small dent in a good relationship. The real cost wasn’t the valve. It was the extra phone call the manager had to make to tell me what I should have found the first time.
How does a heat pump work? And what about a tankless hot water heater?
The manager brought up the owner’s plan to future-proof the building. How does a heat pump work? Instead of burning fuel to make heat, it uses refrigerant and a compressor to move heat from the outside air into the water. The U.S. Department of Energy sums it up this way: a heat pump moves heat rather than generating it. That is why a well-designed system can be much more efficient than resistance heating.
But the expansion question doesn’t disappear. A hydronic heat pump still needs an expansion vessel or expansion tank in the system. It still needs a technician who diagnoses pressure, flow, and sizing instead of chasing the cheapest quote. If you look at a heat pump quote and it does not include a hot water cylinder or radiator upgrades, the quote is probably incomplete.
He then asked about a tankless hot water heater. This is the other half of the confusion. A tankless hot water heater is great for unlimited showers, but it does not heat your radiators. A combi boiler acts as a tankless domestic hot water heater and a central heating boiler in one box. A tankless hot water heater alone cannot replace the heating side. If someone tells you it can, you are talking to someone who is either confused or quoting you a problem.
The logbook version
If you are a tech who wants the checklist without my storytelling:
- Check expansion vessel air pressure before replacing a pressure-relief valve. It takes five minutes and costs nothing but a little water.
- Read the pressure cold, then again after a full heat cycle. The pattern tells you whether the issue is expansion or something else.
- Compare what’s included when you compare prices. The cheapest quote only looks cheap if you ignore the likelihood of a callback.
That last point is why this story is still in my notebook. The cheapest fix I sold that day was not a good deal for anyone. The good deal was the second visit, when I finally listened to the system instead of listening to my own assumptions.