The hot is just going cold
When I run my engine for five to six hours, the water is really hot, but when I get up in the morning, it is only warm. Could it be feeding back to the engine and thus cooling down? If so, how do I sort it out?
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QAYes, it certainly could. There are a number of ways to try to solve the problem. The simplest is just swapping the pipes over on the calorifier coil so the hot goes into the bottom of the coil.
You could fit a plumber’s non-return valve in the hot line or re-pipe so the hot ‘into calorifier’ pipe has at least a horizontal section at the calorifier end or, better still, downward length of pipe run/hose run. In the latter case, make sure that either no air can be trapped in the pipe or fit a bleed valve at the highest point.
Make sure any spring (if there is one) in the NRV is very light because the first two options put a little more load on the engine water pump so circulation might be compromised to some degree. If the hot take-off is in the cylinder head and no significant bypass pipework is fitted, then this is not such a major problem but it would be if the take-off were on the skin tank side of the engine thermostat.