We've an oldish oil fired boiler with gravity indirect HW and pumped CH. When we moved in, no full independednt control of HW and CH, so fitted C Plan with 2 port valve because we often want CH without HW. We also have open fire with semi redundant back boiler - in the past connections to rads have been blanked off reasons unknown, but connections to HW cylinder still intact. Two problems occur. 1) When fire is lit, water boils and hammers because no flow route if C plan valve is closed - so I simply fitted an 'override' switch to force the valve open when we use the fire - but there must be a better mechanical solution e.g. spring loaded valve bypass?. 2) When fire off, back boiler gets warm if boiler HW is used - some form of syphonic back feed? Could anyone please suggest a fix to cure both these problems?
Honeywell C Plan Problems with Additional Back Boiler
- Thread starter shirelets
- Start date
Sponsored Links
1 post a drawing 
2 holler for doitall
he loves these
2 holler for doitall
he loves these
New to this, unsure how to post a drawing onto the forum without a bit of help!
The valve is conventional C plan type with limit switches so has to be driven in either direction. It is not spring return.
Sorry, caused confusion about rads. What I meant is back boiler 2 x pipes to CH / rad circuit are blanked off, so rads only heated by oil boiler, not open fire back boiler.
Sponsored Links
Thanks for the offer to help and invitation to email.
Attached hopefully is sketched and scanned piping schematic of my system (don't think I can post this on the forum).
Certain you are familiar with Honeywell C Plan - basically it shuts off gravity feed to cylinder and also inhibits oil boiler firing when there is no demand. It seemed ideal for our requirements as we often use CH without needing any DHW.
We use the inefficient old open fire from time to time in winter because it's nice to sit in front of - although probably would be cheaper to use the CH. Getting 'free' HW from the semi redundant back boiler is a bonus, but I wonder if the warm grate resulting from backfeed when we heat water using the oil boiler, negates any gain! Hence part of my original question.
As my background is electrical and control, it was easy enough to fit an override switch to force the 2 port valve open and inhibit the oil boiler from firing, thus allowing the open fire back boiler water to flow through the cylinder. However this is an unconventional 'botch' that I don't like and we have to explain to friends when they house / dog sit! Hence the rest of my original question!
I don't know why the open fire back boiler connections to the CH system have been blanked off yet the connections to the HW cylinder remain - done by the previous occupants.
Maybe it'd just be simpler to blank off the open fire connections to the HW cylinder as well and drain the open fire back boiler - but then the grate will run very hot I suspect.
Attached hopefully is sketched and scanned piping schematic of my system (don't think I can post this on the forum).
Certain you are familiar with Honeywell C Plan - basically it shuts off gravity feed to cylinder and also inhibits oil boiler firing when there is no demand. It seemed ideal for our requirements as we often use CH without needing any DHW.
We use the inefficient old open fire from time to time in winter because it's nice to sit in front of - although probably would be cheaper to use the CH. Getting 'free' HW from the semi redundant back boiler is a bonus, but I wonder if the warm grate resulting from backfeed when we heat water using the oil boiler, negates any gain! Hence part of my original question.
As my background is electrical and control, it was easy enough to fit an override switch to force the 2 port valve open and inhibit the oil boiler from firing, thus allowing the open fire back boiler water to flow through the cylinder. However this is an unconventional 'botch' that I don't like and we have to explain to friends when they house / dog sit! Hence the rest of my original question!
I don't know why the open fire back boiler connections to the CH system have been blanked off yet the connections to the HW cylinder remain - done by the previous occupants.
Maybe it'd just be simpler to blank off the open fire connections to the HW cylinder as well and drain the open fire back boiler - but then the grate will run very hot I suspect.
The first thing you should do is change the MV to an V4043B normally open type.
1257 is 22mm.
1265 is 28mm.
The disconnected rads provided a heat sink, but would be problematic to re-connect into the oil boiler circuit.
You can't just disconnect the back boiler, it would need filling with vermiculite, or dry sand.
1257 is 22mm.
1265 is 28mm.
The disconnected rads provided a heat sink, but would be problematic to re-connect into the oil boiler circuit.
You can't just disconnect the back boiler, it would need filling with vermiculite, or dry sand.
Sadly don't think I can change to 4043B version as that doesn't have the aux contact which closes when valve fully open, as required by C Plan. Only B & H versions have aux contact it seems. Ahthough not relevant, present valve is actually a Satchwell SM3203 variant equivalent to Honeywell.
Probably getting out of my depth, sorry! Have looked at C Plan data from Honeywell which specifies the 4043H as it needs the aux contacts to interface tank stat etc. And looked at 4043B data which states it definitely doesn't have the aux contacts available. Also contacted Honeywell about the problem in general, for which they advocate their H2 panel designed for interfacing conventional boilers and back boilers, solar panels etc. Seems to cover everything, but £500 and lots of plumbing rework! So thanks for your input, but guess I'll have to stick to my override switch and warm grate from backfeed after all.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
