Vaillant combi, system or open vent: which fits which home

What is the difference between a combi, system and open vent boiler?

Where the hot water comes from, and what the property has to find room for. Vaillant's own explanation is plain enough. A combi provides hot water and central heating from one unit and heats water when a tap opens rather than storing it, so there is no cold water tank, no external hot water cylinder and no feed and expansion tank. A system boiler is not connected to the cold mains: a coil in the cylinder carries central heating water at high temperature through the cylinder, heating its contents, so several taps and showers can be supplied at once. It runs a pressurised heating circuit topped up by mains water, with the expansion vessel usually inside the boiler. An open vent boiler works with a cold water tank in the loft feeding a hot water cylinder, plus a feed and expansion tank that keeps the level in the heating circuit up.

Which Vaillant gas boilers are combis?

Most of the current range. Vaillant's gas boiler listing puts the remastered ecoTEC plus down as a wall-hung combi and system boiler, the ecoTEC pro as a wall-hung combi, and the ecoFIT pure as a wall-hung combi, system and open vent boiler. The ecoTEC exclusive is described on its own page as the premium combi and system boiler, though the listing card for it leads with system. Vaillant aims the ecoTEC pro at small to medium homes wanting instant hot water without a water tank, and sells the ecoFIT pure as the cupboard-size option, compact enough to fit a kitchen cupboard, with a rear flue option for awkward positions.

Which Vaillant boilers are system boilers?

The ecoTEC plus, the ecoTEC exclusive and the ecoFIT pure. All three appear in system form on Vaillant's current gas boiler listing, so the system option runs from the value end of the range right to the top of it. That is worth knowing when a customer wants to keep a cylinder but lose the loft tanks, because it rarely forces a change of tier to do it. Vaillant also notes that solar panels can work alongside a system boiler where the correct cylinder is connected, which is a fair thing to raise on a house already carrying panels.

Which Vaillant boilers are open vent?

Two of them, the ecoTEC plus and the ecoFIT pure. Vaillant sells a dedicated open vent version of the ecoTEC plus and describes it as suitable for existing buildings with centralised or decentralised heating, which in practice means a replacement for a system that already has tanks in the loft. The ecoFIT pure is listed as combi, system and open vent, so the compact range covers all three configurations on its own. The ecoTEC pro and the ecoTEC exclusive do not appear in open vent form on the current listing, so check Vaillant's own product pages before promising a heat only unit at those tiers.

When is a combi the wrong call?

When the demand is bigger than the mains can serve. Vaillant is candid about the limits. A combi has a limited water capacity, so on the average unit only one tap or outlet can be used effectively at a time, and because the water is drawn straight off the mains the pressure drops rapidly when several taps and showers run together. Vaillant puts combis at properties with one bathroom and one to three bedrooms, and says larger households will want hot water at a rate a combi cannot meet. On those jobs the cylinder conversation is the honest one, even when the customer arrived asking to lose theirs.

What is the practical difference between system and open vent?

Space, and how the pressure is held. Both store hot water rather than heating it on demand, so at the tap the customer will not notice much between them. The difference sits behind the wall. An open vent boiler keeps the heating circuit topped up from a feed and expansion tank, with the expansion vessel outside the appliance. A system boiler runs a sealed circuit pressurised from the mains, with the vessel usually built in. Vaillant's own summary is that a regular boiler uses more space and often needs more complex pipework, which is why system boilers are the more common choice in modern properties with several bathrooms.

What should you check before quoting a change of configuration?

Whether the property can take it, before the model choice comes up at all.

• The data plate: read the existing make, model and configuration off the appliance rather than from what the customer believes is fitted.
• Incoming mains flow and pressure: this decides whether a combi can meet the hot water demand at all, since a combi draws straight off the main.
• Radiators, bedrooms and bathrooms: Vaillant lists all three as the factors that set boiler size once the type is settled.
• Cylinder sizing: Vaillant works to roughly 50 litres of stored hot water per person, and warns that a depleted cylinder can mean a 30 to 40 minute wait while it reheats.
• Loft and cupboard space: moving from open vent to system frees the tanks, and moving to a combi frees the cylinder as well.

Whichever configuration goes in, the record afterwards is the same job every time. Manifold builds the CP12 and the service record from the check on your phone, reads the make, model and GC number off the data plate with PlateScan, and keeps the appliance history against the property so the next visit starts with what is actually fitted.