Different boiler temps required for hot water vs radiators.

[siricosm]

Hello,


I have a non-combiboiler (Vaillant), non-condensing, non-vented with an integratedexpansion tank, that is used to heat radiators in my house. It ismaybe 10 years old and I am hoping to get a few more years from it.All of my water is heated with electrical point-of-use heaters, whichis expensive. I would like to also use my boiler for hot water byadding a cylinder and zones to my existing system. I have read a biton the forum about the S and Y plans. I have my boiler set to 70C,but in my house the boiler temperature rarely rises above 40C, unlessthe boiler has been off for a long time, and the house is very cold.In order to prevent the growth of bacteria, I would like the cylindertarget temperature to be 55 or 60C. It seems the hot water requires amuch higher boiler operating (return) temperature than the radiators.


From what I have read there are a few possibilities:


S-Plan: When onlythe cylinder calls for heat, all is fine, the boiler can go to 60C+.If only radiators call for heat, everything is also fine. If bothcall for heat, both zones will be open and heat will be taken fromthe cylinder (assuming it is up to temperature) to heat the rads.Maybe this is not a problem? In any case some kind of bypass isrequired since the Vaillant pump runs for quite some time after theflame stops, and both zones will be closed.


Y-Plan: This wouldseem ideal if there were no middle position, in middle position youhave the same issue as with S-Plan, in that heat can be taken fromthe cylinder to the rads. Maybe it is possible to find a valve withno middle position? I also have read on the forum that some kind ofextra valve is required for an unvented cylinder for Y-plan but Idon't really understand why.


Which brings me tomy question. If one makes the assumption that the cylindertemperature will always be hotter than the radiators, why not let thecylinder run wild (always open) and use only one 2-port zone valvefor the radiators. In this configuration no bypass is needed sincethe cylinder loop is always open.


Am I missingsomething?

 

[Chalked]

Every boiler must have in interlock, ie it must be able to shut off the boiler with a there instant of sorts.
Your hot water should be set for 60 degrees and your boiler ( if connected and wired properly) should be able to easily reach 70degrees.
An s plan is far better than a y plan.

 

[siricosm]

Sorry, I missed one point. I was thinking of putting a relay in place of the end switch for the hot water zone valve. it would behave like the zone valve electrically, but without the hydraulic switch, hot water loop would be always open.

 

[SimonG]

I'm missing why? S plan all the way and twice on a sunday.

 

[lame plumber]

I'm missing why? S plan all the way and twice on a sunday.

coz Y is crap and breaks down more

 

[siricosm]

I am not sure if one 3 way is more complicated than 2x2-way valves, but it it seem to me that the system can be made with 1x2-way, and no need for the bypass valve.

Am I missing something?

 

[Geyser]

Ye a g3 registered installer!

 

[siricosm]

No, I am not an installer, but I would like to be able to have an informed discussion with one when the time comes. Are you an installer? and if so, can you answer the question?

 

[PhoenixServices]

You can't have a cylinder open circuit really. Could boil over and inefficient. Also not using a standard system and wiring may confuse people who come to work on it.

 

[SimonG]

Engineer.

 

[Nostrum]

If you only had one valve then you'd be doing exactly what you appear to be trying to avoid ie potentially robbing heat from the cylinder.

The zone valve on the cylinder is to turn that circuit off when up to temperature, preventing any heat being further transferred to or from the cylinder.

No disrespect intended, but I would suggest you stop reading and start phoning, as you sound like you're going to get into a whole load of bother. As the saying goes, a little knowledge is dangerous!

 

[siricosm]

I don't really see how an indirect cylinder can boil over, however I do understand the wisdom of having a standard system.

 

[siricosm]

No worries, I am here for information.

As I said previously, I would like to understand my options before I start phoning installers.

 

[Geyser]

You can get external 3 way valves that link to the boiler and provide hotter water to cylinder then lower temp to rads. I use atags, not sure about your model of vaillant. Your installer should know!

 

[siricosm]

Agreed, but I would like to avoid this because it is expensive, and I don't know how long my Vaillant will last. I need to install something now, knowing that I will need to upgrade my boiler to a newer condensing model in 5 years or so.

 

[dancinplumba]

C plan EVEN

 

[Nostrum]

An S plan is a much better solution, it's more logical, simpler and more reliable generally.

Heat is never taken from the cylinder to heat rads, I'm not sure where you have gained that assumption.

The cylinder will require a min flow temp of around 70, return of 60ish to ensure the bottom of the cylinder can reach 55.

The radiator circuit is another matter. If you're saying you can run them at 40 comfortably then great, however when you add a cylinder you'll need to have the boiler temp set to satisfy the cylinder. Short of turning the boiler stay up and down like a yo yo, you could blend the radiator circuit down with a mixing valve if required. If not then just run them at a higher temp. With your current boiler, there's not a huge saving to be made running a lower temp.

Unvented cylinders need to be able to be shut off from the heat source in the event of overheating, a 3 port valve doesn't achieve this, a zone valve does.

 

[siricosm]

When both heating and hot water zones are open, I don't see how heat will not move from the cylinder to the rads. I am not sure if this is a problem since eventually the house will heat, the rad zone will close and the boiler can then heat the water. However, I would prefer the priority be the other way round.

I have my boiler set at 70C. It rarely goes over 40C. This will need to change if a cylinder is installed.

 

[siricosm]

C-plan looks exactly as I envisioned, with the zone valve on the rads. Is this a common setup?

 

[Dotty]

Look up balancing.

 

[chris watkins]

I have my boiler set at 70C. It rarely goes over 40C. This will need to change if a cylinder is installed.

We do not understand this statement please explain what you mean?

 

[Nostrum]

Because the zone valve to the cylinder will only be open when the cylinder is below set point eg 55 degrees. When the flow temp is greater than 55, heat will only be transferred into the cooler medium, ie the water in the tank.
The laws of physics prevent it doing any different.

When the cylinder is satisfied eg > 55 degrees, the zone valve will close, preventing circulation, although this is still preventing it from being heated further as opposed to any heat being removed.

If your boiler stat is 70 yet you can only get a flow temp up to 40, something is wrong.

 

[Nostrum]

A C plan would contravene current building regulations.

 

[siricosm]

Look up balancing.

My current setup seems reasonably well balanced, no hot or cold spots in the house. All the rads have thermostatically controlled regulators. What do I need to know about balancing that can help me in this?

 

[chris watkins]

C-plan looks exactly as I envisioned, with the zone valve on the rads. Is this a common setup?

You will be having work carried out that means your new heating system & controls will need to be compliant with Building Regulations AD Part L1b.

The C-plan system will not meet these & it will not be suitable when you have to install a new boiler.