Coil running cold? Start here
Before you call: what to notice
Is the heat working?
If the radiators are cold too, the boiler is the likely cause, and that part is outside this service.
One tap or two?
Notice whether hot water fades only when two things run at once, or even with a single shower.
Hot then cold, or never hot?
Starting hot and fading suggests capacity; never really hot suggests a control, valve or scale problem.
Any change lately?
A coil that used to cope and now does not is worth mentioning, as is a new bathroom or more people at home.
If you smell gas, leave the building and call the gas utility and emergency services from outside.
If a carbon monoxide alarm sounds, leave the house and call emergency services from outside.
What a tankless coil is
A tankless coil is a length of copper tubing, usually on a plate bolted into the side or top of the boiler, sitting in the hot boiler water. Cold household water enters the coil, picks up heat as it travels through, and leaves hot enough to use. There is no tank and nothing stored. The boiler has to be kept hot so the coil is ready whenever someone turns on a tap.
Coils were fitted to a great many oil boilers across Connecticut, and they remain common in the rural east, the Quiet Corner, Tolland County and the Litchfield Hills. They work, but they have limits, and those limits become more obvious as coils age and households change.
Why a coil runs out of hot water
It can only heat so much, so fast
A coil heats water as it passes. At a modest flow it keeps up; ask for more than it can heat and the outlet temperature drops. Two showers, or a shower and a washing machine on hot, can exceed what the coil can deliver, so the water turns lukewarm. That is the coil working as designed, not failing.
Cold inlet water makes it worse
The colder the water entering the coil, the more heat it needs to pick up in the same short trip. In a Connecticut January, and year-round on many wells, the coil has less to give at any given flow.
Scale builds up inside
Minerals in the water deposit on the inside of the coil, especially with harder well water. Scale insulates the copper from the boiler water and narrows the passage, so both heat and flow fall. A coil that ran a comfortable shower when new may struggle after years of this. See water heaters in private well homes.
The mixing valve or controls
Most coils have a mixing valve on the outlet that blends in cold water to temper very hot coil water. A stuck or failing mixing valve can send too much cold, too little, or swing between them. The boiler's own temperature control also matters, and it is not something to adjust yourself.
Why the boiler runs all summer
Because the coil only makes hot water while it sits in hot boiler water, the boiler has to stay warm around the clock, including in July and August when no one wants heat. Households are often surprised to hear it firing on a summer afternoon. Turning the boiler off for the season turns off the hot water too. If you would rather the boiler rested in summer, the hot water needs a different arrangement; see the options below.
On a private well?
Coils and well water
Scale is the main enemy of a tankless coil, and some well water lays it down much faster than town water. If your coil's output has fallen steadily over the years, scale is a likely part of the reason. Tell us about the well when you call, so the technician can consider descaling, replacement, or moving to stored hot water.
Scale from well water is a common reason coils fade.
The options
| Option | What it does | Best when |
|---|---|---|
| Repair the mixing valve or controls | Restores proper temperature if a part has failed | The problem started suddenly |
| Descale the coil | Clears mineral build-up to restore some output | The coil is sound but scaled |
| Replace the coil | A fresh coil restores its original capacity | The coil is leaking or badly scaled, and its capacity was once enough |
| Add an indirect tank | Stores hot water heated by the boiler | The boiler is staying and the household needs more hot water |
| Switch to a standalone heater | Makes hot water independently of the boiler | The boiler's future is uncertain, or you want it quiet in summer |
More on each: replacing a tankless coil, indirect water heater installation, and hot water after a heating system change.
What we cover, and what we do not
The local installers work on the hot water side: the coil, its mixing valve and connections, descaling, coil replacement, and adding or changing the hot water source. Repairs to the boiler, its burner and the heating system are outside this service. If the technician finds the boiler is the cause, they will tell you so.
What to leave alone
- The boiler's temperature control and any burner reset
- The mixing valve setting
- Oil or gas lines, wiring and the flue
- The boiler's relief valve and any drain on the boiler
How the visit works
The technician confirms the boiler is running and heating normally, then checks the coil's output at the tap, the mixing valve, and the flow through the coil. They look for signs of scale and of leaks at the coil plate. They explain whether the problem is a failed part, scale, or simply a coil being asked for more than it can give, and price the options before anything is done.
Each option priced so you can compare them.
What affects the cost
- A mixing valve or control repair costs far less than a coil or a new tank
- Descaling depends on how heavy the scale is and how the coil is plumbed
- Coil replacement depends on the boiler model and parts availability
- Adding an indirect or a standalone heater involves the most equipment and labour
- Access around the boiler in older cellars
How long it takes
Diagnosis is usually done within the visit, and mixing valve repairs often happen the same day. Descaling and coil replacement need the boiler shut down for a time and may need parts ordered for older boilers. Adding an indirect tank or a standalone heater is a planned installation.
Tankless coil questions
How do I know if my hot water comes from a tankless coil?
If there is no water heater tank anywhere in the house and you have an oil or gas boiler, the hot water almost certainly comes from a coil in the boiler. You will usually see hot and cold water pipes connected directly to a plate on the boiler, often with a mixing valve nearby.
Why does my shower go cold when someone runs the dishwasher?
A coil heats water as it flows and can only handle so much at once. When a second appliance draws hot water, the total flow exceeds what the coil can heat, and the temperature drops. It is a capacity limit rather than a fault.
Can a scaled coil be cleaned?
Often, yes. Descaling can restore some of the lost output. How much depends on how heavy the scale is and the condition of the coil. A badly scaled or leaking coil may be better replaced.
Why is my boiler running in summer?
With a tankless coil, the boiler must stay hot to make hot water whenever a tap is opened. That means firing through the summer. If you would rather it did not, the hot water needs a different source, such as an indirect tank or a standalone water heater.
Should I turn the boiler temperature up to get more hot water?
Leave the boiler controls to the technician. Adjusting them can affect the heating and the boiler itself, and it may not solve a problem caused by scale or a failed mixing valve.
Is it better to replace the coil or add an indirect tank?
If a fresh coil would give the household enough hot water and the boiler has years ahead of it, replacing the coil is simpler. If the household regularly needs more than a coil can deliver, an indirect tank usually serves better. The technician can walk through both.
We are replacing the boiler. What happens to the hot water?
If the coil is in the old boiler, it goes with it. The hot water then needs a new source, such as an indirect tank on the new boiler or a standalone heater. See our page on hot water after a heating system change.
Water is leaking near the coil plate on the boiler. Is that the coil?
It may be the coil gasket or a connection, or it may be something else on the boiler. The technician will trace it. If it is the coil or its connections, it is on the hot water side and within this service.
Do well homes have more coil trouble?
Often. Scale is the main cause of a coil losing output, and some well water deposits it faster. Mention the well when you call so the technician checks for it.