Check Your Boiler Before Denver's First Cold Night
Here is the short version, so you can stop reading and go do it if you want to.
Walk down to your boiler on a warm afternoon this week. Turn the thermostat up until the system fires. Let it run five minutes. Look at the pressure gauge, listen for water noise, and put your hand on the pipes leaving the boiler.
That is most of it. Ten minutes, no tools, and it catches the majority of what turns into a 6am phone call in January.
I am not writing this to sell anyone a tune-up. I am writing it because the calls I get on the first genuinely cold night are almost never mysterious. They are pressure, air, or a control that failed quietly in June and nobody had reason to notice.
Why does everything break on the first cold night?
Because nothing broke on the first cold night. It broke in April, and the house did not need heat again until now.
A hydronic system sits idle for four or five months. In that time a slow leak drops the pressure, air works its way into the high points, a zone valve actuator gives up, a circulator seizes. None of that announces itself in July. The first cold night is just the first time anyone asks the system to work.
So the failure is not sudden. Only the discovery is.
What should the pressure gauge read?
This is the single most useful number in the room, and almost nobody looks at it.
On a typical house — three stories or fewer — that gauge should read at least 12 psi cold. Below that, water cannot reach the top of the system, and the parts of the house that go cold first are the ones farthest up.
If it reads near zero, stop and call someone. A boiler with no water in it and no low-water cutoff to catch that is genuinely dangerous. I have seen the writeups on units that hit 260°F because there was nothing in them to absorb the fire. That is a rare failure, but it is the one on this list that can hurt somebody, which is why the pressure gauge is the first thing I check on every service call before I touch anything else.
Low pressure is usually not the problem itself. It is the receipt for a problem — a weeping relief valve, a failed expansion tank, a fitting seeping somewhere you cannot see. Topping it up and walking away means doing it again in six weeks.
Why is the relief valve dripping?
If there is water on the floor under the relief valve, the instinct is to replace the relief valve. Usually that is the wrong part.
Nuisance discharge is most often an expansion tank problem. The tank has a rubber diaphragm with air on one side. When that diaphragm fails, the tank fills with water, has nowhere to put the expansion when the system heats up, and the pressure climbs until the relief valve does its job.
There is a test for it that costs nothing: knuckle-tap the bottom of the tank and then the top. A healthy tank sounds hollow at the top and solid at the bottom. A waterlogged one sounds like a full jug all the way up.
Replacing the relief valve on a waterlogged tank buys you about a month.
Both pipes are hot but the house is cold. What does that mean?
Put your hand on the two pipes at the boiler — the one leaving it and the one coming back. The difference between them tells you more than the thermostat does.
Both hot, rooms still cold. The boiler is making heat and the heat is not getting out. Dirty fin-tube, furniture or carpet over a radiant floor, a fan coil that is not running.
Big difference, return pipe noticeably cold, far rooms cold. Not enough flow. A tired circulator, a partly closed valve, a plugged strainer.
Both pipes near-equal hot, everything cold. No flow at all. Dead circulator, closed valve, blockage.
Both pipes cool and it never satisfies. Something is wrong at the source, or the system is just starting from cold.
You do not need to fix any of these. Knowing which one you have turns a vague "the heat isn't working" into a specific question, and a specific question gets you a better answer from whoever you call — including me.
How long should I let it run before deciding something is wrong?
At least five minutes.
A single glance at a gauge is a snapshot of a system that spends its whole life cycling. A 140°F reading on a boiler set for 180°F can be a perfectly normal cold start, or a slug of cool water from a zone that just opened. Watch it settle before you judge it.
Zone valves need patience too. The thermal actuators on a lot of residential zone valves take up to three minutes to fully open. If one room lags the others by a couple of minutes, that is the system working, not failing.
What about noise?
Water noise is air. It is the most common thing I get called about and the most fixable.
A system that gurgles or sounds like it has gravel moving through it has air trapped in it, and air in a hydronic system is not just annoying — it stops flow where it collects and it corrodes things it sits against. Chronic air usually traces back to something structural: pressure too low, an expansion tank plumbed on the wrong side of the circulator, a vent that has been capped, or a slow leak that keeps pulling fresh oxygenated water into a system that was never supposed to get any.
Purging it is a twenty-minute job. Finding out why it keeps coming back is the part that matters.
The three things I look for that you cannot see
Some of this needs a gauge, a meter, and a look at the water. Worth knowing they exist so you can ask.
Short cycling. A boiler that fires, satisfies, and shuts off in ninety seconds, over and over. It is almost always an oversizing problem, and it quietly eats igniters and controls for the life of the equipment — there is a whole post on why this keeps happening in Denver basements.
Dirty water. A pint drawn from the system tells you a lot. Slightly gray is normal. Black is not, and black water is the single most common reason circulators die.
What else is on the chimney. If the boiler is old enough to be on the replacement list, the water heater sharing its flue is part of that conversation whether or not anyone brings it up. That one catches people.
What if the check turns up something real?
Then you found it in September, which is the whole point.
Parts are available, nobody is working in an emergency, and you get to decide between repair and replacement on a normal timeline instead of at 11pm with the house at 54 degrees. A boiler that is going to need replacing is a very different conversation in September than it is in January, and not only because of scheduling — the design work that makes a replacement worth doing takes time that a no-heat call does not leave room for.
If everything checks out, you have spent ten minutes and you know the system is ready. That is a good outcome too.
The honest version
Most systems pass this. I am not trying to talk anyone into work they do not need — a system that fires, holds pressure, runs quiet and heats the whole house is a system to leave alone.
But the ones that fail this check in September were going to fail in January regardless. The only variable is whether you found out on your schedule or on the coldest night of the year.
Go turn it on.
📞 (303) 908-5658
Ryan Van Inwegen, licensed hydronic contractor (LIC00254926)
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