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Hot water boiler temperature

Neal Lewis

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A hot water boiler is used for a combination of domestic hot water, baseboard and radiant heating. The boiler temperature was running at 220 F. How much higher than the normal 180F limit is acceptable?

BTW the whole installation was a clusterF. Radiant tubing stapped every 3 feet, sagging with no metal plates or insulation at the floor joists. No mixing valves, 220 F degree water in tubing under hardwood floors, etc.

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I'm not aware of an official legal limit for residential boilers but the common sense limit is 190. Been that way and been accepted throughout the industry for a long long time. Several books I have do have ratings listed for radiation with water temperatures up to 240 but to my knowledge this is only done on specialty commercial/industrial systems with elaborate engineering/controls to prevent disaster. Having the water over 212 is an explosion waiting to happen if there is a sudden loss of pressure. It may also be pushing past the listed limits of the radiant PEX, the recommended limit for the pumps... I would have called it, no question, in fact I just did about three weeks ago. There's just no reason for it to be that high. It likely wasn't that high originally but migrated upwards due to a failing/dirty hydrostat or maybe they did set it that high due to the high end craftsmanship leading to the floors still being cold at 190.

Having the radiant tubing at that temperature is absolutely wrong. Should be around 125 going through the tubing and radiating about 85 from the floor. On a hack job like what your describing I'd also expect to find potable water PEX instead of the O2 barrier PEX that is required.

OK, just went down and checked the Alumi Pex on my system (my memory is not worth a dam these day''s...) it's rated for a max water temp of 200F.

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I'm thinking you are talking about a natural draft boiler. High efficiency boilers run cooler so they can condense.

A 3 load full modulating boiler can run differently for each load.

For example my configuration looks like this:

Load 1 (In floor) - 130F

Load 2 (Space) - 160F

Load 3 (HWH) - 140F

These are maximum set discharge temperatures, and can vary depending on the demand load and outside temperature.

If the outside temp is -30F and the snow is falling horizontally you could see the boiler at 160 but generally it runs around the 140 mark, the condensing max temperature.

Did you check the PRV for leaks?

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No discharge from the relief valve. The pressure was 15-20 psi. It's an oil fired Weil McClain; standard efficiency.

In the manual for a Weil_McLain WGO/WTGO the Startup - checkout procedure has the statement; "Limit control set to the system temperature requirements (max. 220#730;F)?" But I doubt you'd find a hydronic tech that would recommend running at that temp. in a typical residential setting.

In the end, the radiant issue aside and forgetting about the considerable inefficiency, it's not that different then pulling a hot water loop off of a steam boiler. But if that isn't done in just the right way you are begging for trouble. I've seen the results of it being done wrong several times. It pretty dramatic.

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