Enviromax Combi C35 Pressure Loss and Central Heating weirdness

6 Dec 2019
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United Kingdom
Hello I'm a new poster here,

Boiler- Enviromax Combi C35 with Riello RDB Burner

The central heating has been consistently loosing pressure over time and I have sorted many leaks since moving in the house but have discovered one small leak on a radiator that looses less than 15ml of water over 24hrs. I have this leak on the to be fixed list but if I am going to drain down the system I want to fix the other issue at the same time.
this leak has been like this for some time and not affected the time the gauge takes to rise the way it does described below until now but I'm sure that's not down to the leaking radiator.

When the boiler fires for hot water I can wait some time for the boiler to produce hot water at the kitchen tap and its not that hot. Also if I repressurise the central heating circuit the pressure gauge rises like it should but if I open the filling loop tap fully open to pressurise the heating when the hot tap is running the pressure gauge does not move at all.

This leads me to think the plate heat exchanger is knackered internally as in the flow of the central heating pressure circuit is crossing through a leak internally and going either into the heat store tank via the plate heat exchanger and/or out of the hot tap which cools when the filling loop is open

The thermostatic mixing valve was replaced on the 26th of April 2017 by a plumber but it cant be turned either by the knob or with pliers on the shaft,
Do you know why its seized up after such a short period of time?

The pressure vessel is working and pressurised to the correct pressure as per instructions

Also the boiler relays click a lot when the burner is trying to light on water heating or central heating and sometimes the burner will stop after a few minutes after starting and stay off until it seems to have cooled a bit even thought the thermostat in the hall is on heating and the room temp has not risen to the set temp and the heating LED on the boiler is on

If I should replace the plate heat exchanger the Firebird manual lists a 25 plate and a 31 plate exchanger for the C35 boiler so how do I tell which one is installed in my boiler?

Thanks everyone for reading my long description

Please let me know if my diagnosis is good and I should replace the plate heat exchanger as well as fixing the radiator leak because these are really expensive for what it is and I'm running out of money to throw at this

this is my first post on here I wish id found this forum earlier

Last edited:
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Poor hot water is often caused by scaled DHW heat exchangers. To check, what is the difference in pressure between the hot and cold taps? The mixing valve is probably also scaled up if you cannot move it. The pressure gauge probably does not move with the filling loop open and the hot water running because the hot water is taking the pressure. What is your incoming water pressure like? If you need a new DHW heat exchanger, count the layers in the one fitted. The burner cycling is not unusual. The boiler thermostat will cause this if the heat produced cannot be circulated quickly enough. The pump should still be circulating hot water to the radiators.
Thanks Oilhead,

I have fixed it now the hot water is good now comes through within a minute
The problem with the valve being stuck was PTFE tape blocking it up we had it replaced by a professional in 2017 and they just put ptfe around the old olives and far too much of it too That's just lazy bodging rather than pull the old ones and renew properly so over time the tape has blocked the valve.
So I have pulled the olives and replaced the valve with a new one and joints with new olives.

The boiler relays clicking and the hot water pump not running was the red cabled thermister probe for the flow sensing being faulty and switching continuously on off on off and not just on or off depending on temperature I replaced that with a new one and the boiler now works as it should

I've bled the radiators and its dead hot now too but the pressure still drops below 1 bar over time
so I'm going to fill up with some Sentinel Leak Sealer to see if that does the trick because I cant see any more leaks in the system

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