I replaced a domestic cast iron calcified cast iron boiler on a Rayburn Royal, with a thin walled stainless steel model expecting to get a cylinder of piping hot water. Instead I've got water which is barely above tepid. I was called in initially to deal with a problem of diminishing water temperature in a fairly recentely replaced primatic cylinder (about 5 years) My first instinct was to drain the cylinder because the water had on occasion got near boiling which I thought might have affected the performance of the primatic cylinder.This made no difference hence the boiler change. Now the water is not as hot as it was before. The circulation to the cylinder is good because the water temperature is about the same by the cylinder as it is by the Rayburn. The pipework is undersised ie 22mm instead of 28mm but that has not been an issue in the past. The pipe run is 5metres (lagged) continuousely rising. The Rayburn oven is acheiving the correct temperature so the heat is there.
A Rayburn Royal should never be connected to a Primatic cylinder, only a twin feed indirect cylinder. You have answered your own question, the fault is the cylinder, the pipe size & there's every chance the gradients are also wrong.
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