Add 2nd pump to Domestic Hot Water system

[Fastlearner]

I have a Glow-worm back boiler which is tapped for gravity feed by 4 connections.The central heating has a twin pipe flow system & separate return.I believe this complex set up is because of the shape of the bungalow & the remote location of the pump (15 mtrs from boiler).It works extremely well & the boiler is bullet proof.The hot cylinder (roof space) is compromised & needs replacing.At this point sourcing a gravity cylinder is difficult as most suppliers only stock cylinders for fully pumped systems.I aware that there will be a clamour to upgrade the entire system to conform to modern standards, but am unwilling to change a complex but very effective set up just because of a defective cylinder.Maybe in the future.I like the idea used by JohnD (posted Jan 2010).I would like to place a 2-port valve & pump on the return side of the coil immediately after leaving the cylinder & controlled by a cylinder stat.The cold feed/exp is a T at the flow input to the cylinder.I realise that some balancing of the flow will be required, hopefully at the pump gate valves.My question is: Does my thinking fit with those that have successfully achieved this & do i need an additional programmer to control this set up independently of the central heating pump.

Ron

 

[Agile]

There is a bodge that you could do but the best idea is to upgrade to the proper circuit now!

If you must do a bodge then you can put a pipestat on the flow pipe set to about 60° which powers the pump on the HW return which is again fed through a cylinder stat.

The trick is to close both pump valves to mostly closed. This will allow enough gravity flow over a few minutes to heat up the flow pipe so that it activates the pump. The pump even on the lowest setting will create enough flow through the pump valves to heat the water. Its important to set the valves well closed because that reduces gravity circulation when the cylinder stat is off and enables the proper control of the HW which you will not have at present ( unless you have a Cytrol which you did not tell us about ).

Its not necessary to use a motor valve as that will prevent the flow pipe heating up and operating the pipe stat. Saves £45 too.

This bodge will work quite well and not require any electrical connection with the existing boiler control system.

Tony

 

[DP]

OP, chances are the cylinder that you fit will be high recovery, therefore gravity flow will be almost nil. Be aware, some cylinders use microbore coils in shape of a pancake at the base of the cylinder, others have long length of 22mm pipes in a coil shape close to the cylinder body- gravity circulation through this is almost nil (have been there and got the tee shirt).

Easiest option would be to fit a pump, gate valve and a 2 port valve between the tee (that connects to the header tank) and the cylinder. Go for a timer that gives you the option of independent timebase for CH and HW (or instal a programmable room thermostat leaving the programmer to control HW run times). Now you will be able to heat the water before you rise in the morning (prior to heating kicking in)

Set the pump speed to minimum and the gate valve for 11 degree drop across the flow and return with heating running as well.

An example of above can be seen here>>> //www.diynot.com/forums/viewtopic.php?t=210872

 

[Fastlearner]

Thanks for input Tony,
My reason for adopting this route was that i was advised by RM Cylinders Tech dept. that a cylinder designed for a fully pumped system would result in air locks due to the coils being wound tighter.( pumped coils are 22mm.Am not sure if gravity coils are 28mm)If this was the case then i presume that the gravity circuit would not function.If i understand your suggestion correctly it would require this function operational in order to activate the pipestat and then the pump.Is the pipestat that you refer to clamped externally to the flow pipe and is its location critical.Thanks

Ron

 

[Agile]

The problem is that to transfer heat the water hgas to circulate under a very small head of water which needs larger pipes like 28mm.

With a 22 mm coil or the microbore type there will be gravity circulation which will heat up the flow pipe but the actual flow rate will be so small that the return will be cold ( or at the temp of the cylinder ).

A gravity circulation takes about an hour to heat a standard cylinder but a pumped flow willl take about 20 min. thats a big advantage if you often run out of hot water. If not the you could fit a gravity cylinder and always convert it to pumped later.

Gravity should really be fitted with a Cylrol or other temperature control to avoid over heating.

Tony

 

[Agile]

The problem is that to transfer heat the water has to circulate under a very small head of water which needs larger pipes like 28mm to move sufficient volume of water.

With a 22 mm coil or the microbore type there will be gravity circulation which will heat up the flow pipe but the actual flow rate will be so small that the return will be cold ( or at the temp of the cylinder ).

A gravity circulation takes about an hour to heat a standard cylinder but a pumped flow willl take about 20 min. thats a big advantage if you often run out of hot water. If not the you could fit a gravity cylinder and always convert it to pumped later.

Gravity should really be fitted with a Cylrol or other temperature control to avoid over heating.

Tony

 

[Fastlearner]

Thanks for input guys.
Just to back refer to suggestion by DP.
In my case the cold feed/expansion T's off the flow pipe.Therefore the installation you recommend would be on the flow immediately before the cylinder.The forum link which you cited is the site which got me started on this topic and those considered successful appeared to suggest the return side more appropriate.What are the relative merits/demerits of installation on flow or return side.
Tony/ The main purpose for my interest at this time is that i was supplied with a high recovery cyl which is clearly not suitable.The supplier is currently trying to source a gravity cyl. but i am pessimistic given market demand
Ron

 

[DP]

It is of little or no relevance where the pump and valve is fitted as long as it is not fitted on the feed/ expansion pipe or vent pipe. The branch that connects these pipes to the cylinder are suitable points for insertion of lockshield, and zone valves as well as the pump.

 

[Fastlearner]

Hi guys,
Just a follow up to earlier proposals.My intention is to install Honeywell cyl stat(L641A1039) & 2 port valve(V4043 with 5 wires --grey,brown,orange,blue,earth)My dilemma is that i cannot figure out how to wire the 2 pumps to work independently via a single live to the boiler.Does this require secondary switching at this interface to avoid both systems operating together.Also, i did a test today hoping to understand the switching arrangement( gravity to fully pumped)in the programmer.I expected that moving switch over with existing untouched setup that the CH circuit would be de activated, and that the HW circuit would function as normal on demand.Unfortunately neither circuits were active.Would appreciate some help.
Thanks,
Ron