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Hot tub floating thermometer

Hot tub hotness is not homogenous

one year ago

A hot tub is apparently “a large, usually wooden, container full of hot water in which more than one person can sit” where normal humans will “soak and usually socialise”. I, on the other hand, was reminded of something to do with heating systems!

This hot tub was heated by a wood boiler which thermosyphoned. Thermosyphoning uses the fact that hot water rises to make the water in pipes flow - the hot water pipe coming from the stove is higher than the cold pipe going to it, so the hotter water flows to the hot tub, and the colder water flows to the stove to be heated.

The floating thermometer was showing 40ºC (not that hot yet, granted), but reach down and the water below was a lot cooler. I moved the thermometer down and sure enough that was showing as 30ºC. I thought that was quite a nice way to directly experience this temperature difference - my upper body was a warm 40ºC, my legs and feet at a cooler 30ºC.

That 10ºC temperature difference is actually fairly typical for wood boiler stoves and that’s why connecting a wood boiler just straight to a thermal store or large hot water cylinder isn’t always the best idea.

Imagine that the water tank is at 15ºC - as the stove warms up it will heat up the tank, from the top down, with water at 25ºC.

Thermal store heated by boiler stove with no load unit 1

Only once the entire tank gets to 25ºC can the stove get it any hotter. When this happens the stove does another pass, heating it further to 35ºC.

Thermal store heated by boiler stove with no load unit 2

35ºC still isn’t going to be hot enough for a shower, so you have to repeat the whole thing until the tank is at 45ºC, and so on.

So it takes a long time before you get usable hot water.

That’s one advantage of using a load unit or load valve. Just the thermostatic valve in a car engine doesn’t allow hot water out for the heating until the water jacket around the engine is warm, a load unit routes the water straight back to the stove until it has reached a set temperature, often we set that at around 70ºC.

With a load unit installed the water heating up the water tank is always at 70ºC or above, so instead of having to repeatedly raise the temperature of the whole tank in 10ºC increments, you heat it to at least 70ºC from the top down and so you get usable hot water much, much faster.

Thermal store heated by boiler stove with a load unit

The hot tub I was in wouldn’t have benefitted from a load unit but I did enjoy the practical example of why load units are often a good idea.