Hello. I have an oil fuelled combi boiler and the hot water starts hot, then goes cold for a few seconds and then back to hot... Is this right? Thank you.
Oil fired combis I think produce DHW at ~ 80C and reduce it to 60C with a TMV, if the HW from your hot tap is very hot (80C) then the TMV may be faulty and the shower(s) TMVs over compensate with cold water initially and then stabilize.
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Reply to the thread, titled "Oil Combi Boiler Hot/Cold/Hot" which is posted in Boiler Advice Forum on Plumbers Forums.