Work out what you have, then what the symptom means

Steam and hot water look alike from the hallway and behave nothing alike. Getting that right first is the difference between a useful answer and a confident wrong one.

The second question has appeared on the left. Choose whichever line is closest — none of them has to be an exact match for the answer to be useful.

• Look at the gauge on the boiler. A pressure gauge reading in single-digit pounds, with a second gauge showing a water level, usually means steam. A gauge reading tens of pounds alongside a temperature dial usually means hot water.
• Look at a radiator. A small metal vent sticking out of one side, near the top, that occasionally hisses — steam. A small bleed screw you turn with a key, or fin-tube baseboard behind a metal cover — hot water.
• Listen when it starts. Steam announces itself. Hot water does not.
• If the floors are the heat source and there are no radiators anywhere, it is radiant.

If you are still not sure, that is completely normal and it is the first thing we establish on a survey. It takes us about five minutes in the basement.

Steam can only fill a radiator once the air already in it has been pushed out. That happens through the vents — on the radiators, and on a one-pipe system often on the mains as well. Vents clog with age and stop passing air, and the radiator behind them never fills.

• The far radiators going cold first is the classic pattern, because the air in the mains has nowhere to go.
• Main vents matter more than radiator vents and are the more commonly neglected of the two.
• Vent sizes should vary across a system. A house with identical vents on every radiator has usually been "fixed" by someone who did not know that.

This is one of the most satisfying repairs in the trade: cheap parts, an afternoon, and a house that heats evenly for the first time in years.

Water hammer on steam is condensate that cannot drain back the way it should, so steam slams into it. It is not the boiler being old, it is a drainage problem, and it is worth taking seriously because it can damage pipework.

• Pipes and returns need a continuous fall back to the boiler. Sagging, a pipe re-run during a remodel, or a radiator tilted the wrong way after a floor was refinished will all do it.
• A radiator valve partly open on a one-pipe system causes it reliably. On one-pipe, those valves belong fully open or fully closed.
• Excess pressure makes it worse, which is why turning the pressure up is the wrong instinct.

Worth calling sooner rather than later. Severe hammering can move and damage pipework, and on an old system that turns a service call into a repair.

Most residential steam systems are designed to run at a very low pressure — enough to move steam round the house and no more. Plenty are found running several times that, usually because somebody turned it up trying to heat a radiator that was actually a venting problem.

• High pressure makes the boiler cycle on its control, wastes fuel, and makes hammering worse.
• It also masks the real fault, which is why systems get turned up year after year.
• The pressure control itself can drift or fail, and on an old boiler it may never have been checked.

Setting steam pressure is a job with a right answer, and it is one of the cheapest improvements available on an old system.

Cold at the top and warm at the bottom is air sitting in the radiator. Bleeding it out fixes the symptom. The question worth asking is why there is air there at all.

• Once or twice after a refill is normal. Every few weeks is not — that means water is being lost and replaced, and fresh water brings fresh air with it.
• Persistent air often points at a failed expansion tank, a leaking automatic fill valve, or an air separator that has stopped separating.
• Repeated refilling also brings fresh minerals into the system, which is its own slow problem.

Bleed it, then watch whether it comes back. If it does, that is worth a visit — it is usually a small part and a large difference.

When one zone never heats and the others do, the plant is working and something between the boiler and that zone is not. The good news is that it is a short list.

• Zone valves stick, and the motor can fail while the valve stays shut. Many have a manual lever, which is a useful test and a terrible long-term fix.
• A zone circulator can fail on its own, or on a shared control that has lost one channel.
• A thermostat calling and nothing happening at the boiler narrows it to wiring or the zone control board.
• On an older converted system, sometimes the zone was never properly balanced and has simply lost out to the others since the day it was added.

Short cycling on hot water is hard on the boiler and usually not a broken part. The commonest cause in an older building is a boiler sized for the building as it used to be — before the windows, the insulation and the draught-proofing.

• Controls matter more than capacity here: outdoor reset, sensible differentials and a boiler that is allowed to run low and slow rather than flat out in bursts.
• Zoning a system without reviewing the boiler makes it worse, because each small zone asks for a fraction of what the boiler can make.
• Low water flow through the boiler causes it too, which can be a pump, a partly closed valve or a blocked strainer.

Before anybody quotes a replacement, this is worth investigating properly. Some of it is adjustment, and adjustment is considerably cheaper.

In-floor radiant heats a mass of concrete or screed, and that mass takes hours to warm and hours to give the heat back. Controlled as though it were forced air, it will always be chasing — too cold in the morning, too warm in the evening.

• Set it and leave it. Deep setbacks work against radiant rather than saving money on it.
• Outdoor reset suits it particularly well, because it lets the floor temperature track the weather instead of the thermostat.
• A thermostat with a floor sensor behaves far better than one reading only room air.

Most radiant complaints we see are control strategy rather than anything mechanical, and that is good news.

One cold area with the rest working means the water is not reaching that loop. At the manifold, each loop has its own flow setting and usually its own actuator, and either can be the reason.

• Actuators fail closed and give no sign of it. They are inexpensive and quick to change.
• Loops need balancing against each other — a long loop and a short one on the same manifold will not behave alike without it.
• Air trapped in one loop will stop flow entirely, and purging a single loop is a straightforward job at the manifold.

One thing we will not guess at: whether a loop in the slab is damaged. That needs proper testing, and anybody who tells you it is broken without testing is guessing with your floor.

Radiant runs at much lower water temperatures than radiators do, so it almost always sits behind a mixing valve or an injection arrangement. When a boiler short-cycles on a radiant system, that is usually where to look first.

• A mixing valve that has failed towards the hot side sends the boiler straight back to its high limit.
• Low flow through the boiler — a pump, a valve, a strainer — produces the same pattern.
• And the familiar one: a boiler sized for a building with a much larger heat loss than it has today.

Getting this right matters more on radiant than anywhere else, because a boiler running hot into a slab is uncomfortable as well as wasteful.