Upgrading a gravity HW + pumped CH system...

[TimS]

Hi,

We currently have a gravity HW & pumped CH system using a Thermacon Option 60/80 floor standing oil boiler which is currently setup to always run HW+CH. There's a motorised valve on the return pipework from the HW cylinder to the boiler, but nothing on the CH side. It's around 20-30 years old and is open vented with a feed/expansion/header tank in the loft.

The HW cylinder is a standard indirect / vented cylinder and was fitted about 4 months ago. Cyl flow/returns are in 28mm copper reduced to 22mm at the cylinder. HW flow/returns are 22mm.

With summer hopefully coming up we're looking at upgrading the system with the intention of hopefully being able to run HW on its own, or HW + CH rather than always needing to run the radiators.

Is this possible...? Is it a case of adding a motorised valve on the CH side & upgrading the controls / programmer to one supporting HW/HW+CH, or would more substantial alterations be likely?

Thanks for your help,
Tim

 

[D_Hailsham]

The ideal solution would be to convert to a fully pumped system. This would be more expensive but save money in the long run as your hot water would get reheated faster. I will assume you don't want to do this.

You say there is a valve in the HW return, do you also have a thermostat strapped to the side of the HW cylinder? If so, you are almost there as all you will need is possibly some wiring changes and a new programmer.

What programmer do you have at the moment - make and model no?

This system is called a C Plan. Here are some diagrams (click to enlarge):

View media item 827 View media item 73

 

[TimS]

I need to check the model number (at work at the moment) but if memory serves correctly the programmer is a Horstmann CentaurPlus C11 or C17.

I'm not adverse to looking at converting to fully pumped.
Presumably it's generally a little more involved than capping off the 28mm cylinder flow/return pipework / terminals on the boiler, and altering the heating flow/returns along with adding a motorised valve to feed the cylinder from the CH side of the boiler...?
(along with a new programmer, which would be needed either way I guess)

If I was to stick with gravity HW, don't I need to add a motorised valve on the CH side or can you effectively stop the CH from heating the rads by not running the pump?

Thanks!
Tim

 

[TimS]

Sorry, to answer your question there's also a cylinder stat strapped on the side of the cylinder along with a room stat in the hallway away from any rads.

 

[D_Hailsham]

I need to check the model number (at work at the moment) but if memory serves correctly the programmer is a Horstmann CentaurPlus C11 or C17.

Both of those are single channel programmers, which is why you have to have HW and CH on at the same time.

You have two options (this is true for existing system or fully pumped):

1. Retain the existing wall thermostat and replace the Horstmann with a two channel programmer.

2. Retain the existing programmer for HW control and purchase a programmable thermostat for the heating.

The second option sounds more complicated but you get a more accurate thermostat. The only drawback is that you have two time controls. However I have been using this a system for nearly four years and it's never been a problem as the times when you need to alter HW times are very rare. In any case there is the boost button if you need more hot water.

Presumably it's generally a little more involved than capping off the 28mm cylinder flow/return pipework / terminals on the boiler, and altering the heating flow/returns along with adding a motorised valve to feed the cylinder from the CH side of the boiler...?

Yes, it is more complicated. With a fully pumped system the valve(s) always go in the flow side after the pump. There are two ways of doing this, called Y-Plan (left pic) and S-Plan (right pic). The Y plan uses one 3-port mid position valve and the S plan uses two zone valves. Unfortunately they will be 22mm valves, so you existing 28mm valve will not be suitable. The piping to the Feed and Expansion tank will probably have to be altered.

View media item 69 View media item 71

If I was to stick with gravity HW, don't I need to add a motorised valve on the CH side or can you effectively stop the CH from heating the rads by not running the pump?

The pump acts as a valve.

 

[TimS]

Thanks for this - very helpful!

Are any particular 2-channel programmers generally recommended more than others these days?

Not worried about having to make wiring changes if needed - the current wiring is a complete mess & mostly consists of a collection of terminal blocks stuffed into back boxes with blanking plates.. Needs sorting (and relocating to a somewhat more accessible position), so I'm figuring I may as well take the opportunity to replace the programmer at the same time.

Thanks for your help
Tim

 

[mysteryman]

These valves are all available in 28mm as well as 22mm.

The Y Plan is better for several reasons - it is just as easy to fit and wire, and there is always a port open so residual heat can escape from the boiler after shut-down.

'The pump acts as a valve.' Please explain.

 

[TimS]

Thanks MysteryMan

These valves are all available in 28mm as well as 22mm.

The Y Plan is better for several reasons - it is just as easy to fit and wire, and there is always a port open so residual heat can escape from the boiler after shut-down.

'The pump acts as a valve.' Please explain.

Assuming 22mm copper usually be fine up to the cylinder converting to Y-plan doesn't look like it'd be particularly difficult.

The pump's currently located next to the boiler, which is on the ground floor directly under the cylinder. I guess I could simply cap off the boiler's current feed outlet for the cylinder; add a 3-way valve after the pump with one leg feeding the cylinder & the other the CH ?

I'm assuming I'd also need to replace the current 28mm cylinder feed on the flow side with 22mm from the valve?

Would it be fine to leave the HW cyl return plumbed in 28mm ?

 

[mysteryman]

I think that is the best way to do it.

 

[D_Hailsham]

Are any particular 2-channel programmers generally recommended more than others these days?

Not worried about having to make wiring changes if needed - the current wiring is a complete mess & mostly consists of a collection of terminal blocks stuffed into back boxes with blanking plates.. Needs sorting (and relocating to a somewhat more accessible position), so I'm figuring I may as well take the opportunity to replace the programmer at the same time.

Honeywell kit is very good. The "RF² Pack 2" is probably the best solution as it is a two channel programmer with a wireless room thermostat.

'The pump acts as a valve.' Please explain.

Do you remember the Grundfos "Pump Plan"? It was two pumps in one. with a common entry port and separate exit ports for each pump. The common entry connected to the boiler and the two exit ports connected to heating and hot water. You could set the speed separately for each pump, e.g 3 for CH and 1 for HW. When one pump was not running it acted as a closed zone valve for that circuit.

In any case a non-rotating pump acts as a closed valve as water cannot get through the pump.

I ran a C plan system for many years with the pump in the heating circuit and never had any problems with the rads heating up when they shouldn't.

 

[mysteryman]

'In any case a non-rotating pump acts as a closed valve as water cannot get through the pump.'

This is not true. An impellor type pump is not positive displacement, and water can pass through.

Yes, I do remember 2 pump systems, including the older SMC type. There were anti-gravity valves included, weren't there?

My apologies to the original enquirer, we're getting a bit esoteric here!

 

[D_Hailsham]

An impeller type pump is not positive displacement, and water can pass through.

If that is true, why do the radiators in a pumped heating/gravity HW system not heat up in the summer when the pump is turned off?

Yes, I do remember 2 pump systems, including the older SMC type. There were anti-gravity valves included, weren't there?

I was not aware of that, but a quick google found this: Grundfos Pump Plan valves sticking.

You learn something new every day.

 

[transam]

An impeller type pump is not positive displacement, and water can pass through.

If that is true, why do the radiators in a pumped heating/gravity HW system not heat up in the summer when the pump is turned off

The radiator's would & do heat up ( upstair's) unless a nrv is installed , gravity circulation will travel through a circualing pump .

 

[D_Hailsham]

The radiator's would & do heat up ( upstair's) unless a nrv is installed , gravity circulation will travel through a circulating pump .

Thanks for reminding me! It's many years since I had a gravity/pumped system and I had forgotten completely about the valve on the flow pipe.

Hopefully the OP has an NRV (aka anti-gravity valve).

 

[TimS]

Cheers guys - the existing system seems to have a valve on the return but not the flow....

Think I'll be converting it to a pumped Y plan by the sound of things.
What's that immortal phrase now... "how hard can it be"....?

Thanks for your help!