S-plan or PDHW with replacement system boiler

[ediy]

Hey. I’m looking to replace my 18yr old and occasionally temperamental Worcester system boiler with something efficient and reliable, possibly a Viessmann 100-W system boiler. With a smart thermostat that would ideally give modulation control (pref using OpenTherm). I plan to keep the Megaflo cylinder and would hope to avoid changing pipework to the Megaflo or radiators. It is an S-plan setup at the moment. I’m struggling to get quotes for anything other than a drop in replacement keeping the S-plan setup.

Am I right in thinking/hoping that a boiler with an internal diverter valve would only require some relatively minor re-plumbing in the garage (see photo) and then a PDHW setup could be achieved, allowing high flow temps for HW cylinder recharging and lower and variable flow temps for modulated CH control? So allowing the boiler to operate in condensing mode most of the time.

Am I overthinking this? Are the gains vs the S-plan setup likely to be marginal?
Any advice welcomed.

 

[Madrab]

Am I right in thinking/hoping that a boiler with an internal diverter valve would only require some relatively minor re-plumbing in the garage (see photo) and then a PDHW setup could be achieved, allowing high flow temps for HW cylinder recharging and lower and variable flow temps for modulated CH control?

In simple terms - Yes.

 

[Notch7]

Hey. I’m looking to replace my 18yr old and occasionally temperamental Worcester system boiler with something efficient and reliable, possibly a Viessmann 100-W system boiler. With a smart thermostat that would ideally give modulation control (pref using OpenTherm). I plan to keep the Megaflo cylinder and would hope to avoid changing pipework to the Megaflo or radiators. It is an S-plan setup at the moment. I’m struggling to get quotes for anything other than a drop in replacement keeping the S-plan setup.

Am I right in thinking/hoping that a boiler with an internal diverter valve would only require some relatively minor re-plumbing in the garage (see photo) and then a PDHW setup could be achieved, allowing high flow temps for HW cylinder recharging and lower and variable flow temps for modulated CH control? So allowing the boiler to operate in condensing mode most of the time.

Am I overthinking this? Are the gains vs the S-plan setup likely to be marginal?
Any advice welcomed.

I found it is very worthwhile having PDHW, I had a basic Ideal vogue boiler fitted which has 2 temperature inputs and weather comp.

at the moment when outside temps are around 0 deg, the boiler flow temp is around 50deg, when the outside temp is say 10 deg the flow temp is around 37-40 deg. Our boiler is well into condensing mode all the time when heating.

aside from the efficiency the house feels much more comfortable, the background heat of low temp rads makes it really nice


In terms of the hot water priority, we have the hot water timed to come on half an hour before the heating in the morning and afternoon so it doesnt interrupt the heating at the beginning of the cycle -esp useful in the morning as the house cools over the day


Weve found it best to have nearly all the TRVs set to max to allow the heating to warm up the fabric of the house -for max efficiency you want maximum transfer of heat from radiators to house when using low temps

 

[Madrab]

Only thing I'd add to that is make sure that the system temps for CH is set to allow the rads to supply enough heat to warm the house properly (delta's), if the syst temp is set too low then the rads may not have enough output to offset the heat loss.

 

[Notch7]

Only thing I'd add to that is make sure that the system temps for CH is set to allow the rads to supply enough heat to warm the house properly (delta's), if the syst temp is set too low then the rads may not have enough output to offset the heat loss.

yes Ive experimented quite a bit with the weather compensation curve and "notional room temperature" to get that right

luckily the house has rather large K11 rads in all the rooms, so it hasnt been much of an issue

 

[stem]

Not relative to your question and just an observation, but the nomenclature in your photo shows the frost protection pipe thermostat fitted on the incoming cold water mains supply pipe.

The idea of the frost protection system is that the wall mounted frost stat switches 'on' when the ambient temperature drops to a value slightly above where freezing might occur and starts the central heating. Once warmed water (i.e. not hot, but warm enough to eliminate a freezing risk) gets back to the boiler it is picked up by the pipe stat which is fitted to the return pipe which switches the heating off. However, as your pipe stat is on the incoming cold water mains that will never happen so the heating will continue to run.

 

[muggles]

PDHW is the only sensible way to go

 

[Exedon]

PDHW is the only sensible way to go

Totally agree we have now done a fair few using vouge system boilers on X plan. Married to a unvented cylinder with high performance coil you get very quick recovery times.

 

[ediy]

Thanks for the useful replies.
So shouldn't be a plumbing change apart from in the garage. How about wiring? I think in the Viessmann case it needs a low voltage stat, so would need a Viessmann Cylinder Demand Box. Is it probable that can be fitted at the garage end, using the existing signal from the Megaflo CL250HE?

 

[Mister Banks]

As I understand it from online posts it can be a right pain in the a*** to get DHWP to work with Viessman boilers due to the demand box requirement. There is a good thread on the Buildhub forum.

 

[Madrab]

There are certainly easier boilers to implement PDHW with.

 

[muggles]

As others have said, I'd be looking at an alternative to Viessmann for this