Pumping from thermal store, connections, flow rates, storage and recovery times

[craigy]

Hello all,

I have been a member for a while but fresh to posting.
I am a plumber with 5yrs experience, qualifications in hot and cold water and licences unvented for your info.

This is the first time I have come across this type of setup in a house so require advice on my action plan.
The type of system in the house I am working on is c/h fully pumped and vented, gravity hot water heated indirectly via coil and mains fed header tank for supply to h/w cylinder. The cylinder is a combination type with the cold supply bang on top of the hot cylinder and fully jacketed(thermal store?) it is a 40/120 cold store/hot store cylinder.
All cold draw offs in the house are mains fed. Incoming mains pressure is 3 bar and flow rate is 16ltr/min off peak.
i am installing a mixer shower with over head square shower head (36sq") and diverted to hand held.
i am putting in a negative head pump 1.5bar.
Knowing the cold storage requirements were not there and the 40ltr for the hot storage was insufficient I have installed 2x18 gallon tanks in the loft and linked them with 22mm copper, I am feeding one tank from the mains with a equilibrium valve delivering 12ltr/min. From the other tank I plan to tap into and feed the pump and just above tap into and send a supply down to the combination cylinder cold storage tank with another equilibrium valve setting both at the same level.
i have considered increasing the overflows in case of overloading should both valves fail.
when I have tested the flow rates from the tank under gravity conditions from the downstairs tap I get 8.5 mins of 12 ltr/min at 60 degrees then it starts to drop, the cold tank which supplies the hot is almost empty at this point as the valve could not allow enough in. Changed the washer and it's much better. I plan an Essex flange from the cylinder to feed the pump.
The only issue I think will lye in the heat recovery, when the temps are mixed I appreciate I will get more than 8.5 mims but I know the pump will pull faster. The boiler is a glow worm regular gas fired piped up in 28 for the big run and down tom22 for the rest.
all my pipe work and connections are 15mm.
Would a circulating pump help the hot water recovery.
should I have the heating element on whilst the coil heats the water too.
the element is not linked to the boiler by the way.
help appreciated.
craig

 

[Whyme]

dont think thats a thermal store..if it is and you put an essex flange on your going to be drawing off the heating water.

 

[Best]

Firstly, do you really know for sure if you have a thermal store?
Usually a thermal store does not supply the hot water from its store.
The hot water is mains water, either from a coil of heat exchanger.

 

[Whyme]

tank on top. standard combination cylinder me thinks..also why a negative head pump ???

 

[craigy]

If it's standard combination cylinder then will my idea work?

 

[Best]

Thats not a thermal store cylinder.

That is what I thought. Sounds more like the hot is coming from a combination cylinder.
Cylinder would need changed to a standard indirect cylinder & fed with cold supply from reasonable sized cwt in attic.

 

[Whyme]

Er the float valves in the tanks are float valves not those little plastic heat intolerant ones designed for toilet cisterns are they.?

 

[Best]

If there is good enough pressure & flow on the mains going into house, then by far the best & easiest & often cheap solution is to get an unvented cylinder fitted by qualified person. No shower pump or cwts needed.

 

[craigy]

That's right they are not plastic valves.
the price of the unvented would be 500 and i could install and commission in a day. Problem is discharge as stack is internal and 5 metres away and joists running unfavourably. The site is central and on the first floor and reaching outside would be a time consumer.
the tanks, pump, flange and pipe work orme to 400.
really my only option.
what are the chances of this setup working?
replies appreciated thanks very much

 

[craigy]

The pump is situated in the loft that's why neg head

 

[craigy]

I know it's indirectly heated from the heating circuit. No sign of port valves but they could be under the floorboards I guess

 

[Whyme]

strapped tanks with a pump work fine, i would increase the size of the joining pipe though..

 

[Whyme]

If its gravity hot water your realistically going to have go with it cos to upgrade to properly fully pumped is out of your price remit by the look of your posts.

 

[craigy]

Will use 28 or 40 then thank you.

 

[craigy]

Would Keeping the boiler running for the duration of the shower and having the heating element running simultaneously help minimise heat recovery times?

 

[stevetheplumber]

if its a combination cylinder a pump will empty the tank in less time than it takes to shower youll need a new cylinder and a 50g tank

 

[Whyme]

I dont know what the cylinder is first thermal store then fitting tanks in loft then maybe combination?? Can you post a pic craigy ?

 

[craigy]

I'll post picture tomorrow, just to clarify tanks in the attic are 1 currently for central heating and 2 new ones totalling 36 gallons which will feed pump and cold store which supplies hot cylinder. Just looked up the cylinder it is standard indirect combination cylinder with coil heat exchanger and additional rod element situated at the bottom. With 36 gallons I've worked out nothing will be emptied by the pump, it's really the amount of hot water (120ltr) running out in 8.5 mins if only hot water drawn, combined with cold in the mixer that will increase the amount of time the hot will last. How long to recover or how effective would the boiler be if left on for hot water and circulating through the coil whilst showering and even having the electric element switched on too. Could increase the kW of element too?

 

[Best]

My guess is you need to replace the combination cylinder with a standard indirect cylinder feed by cold water tank. As said above, ideally no smaller than 50 gallon (actual) tank & if you can't fit that in & are going to use two cold storage tanks, then do take the cold draw off for pump from one of them & the feed to the new cylinder (28mm) off the other tank. You perhaps should fit ball valves to each tank, as this, IMO will guarantee each tank fills properly & water lowers evenly & doesn't stagnate. Obviously overflows need to be fitted from both tanks properly - I prefer larger type 32 or 40 mm o/f pipes & these can be teed together.

 

[craigy]

Hello all who replied to my post I appreciate your input.
the job finished with the setup I proposed working fine thankfully. On the budget I was on it was worth a go. I thought I would end up replacing the cylinder after having my first attempt at fitting an Essex flange with the split washer as no way of getting your arm in the cylinder, hairy moment. Cylinder wall thickness was paper thin and top end hole cutter still tried to chew it.
2x18gallon cws linked with 28mm, both supplied individually from the mains via brass ball valves and individual overflows.one tank supplying cold to pump and the other I took a 15 mm feed to the combination cylinders cold storage and sited a new float valve next to the origonal to work in tandem, I increase the over flow to 40mm. 1.8 bar pump was sited at the base of the combination cylinder.
Both cws tanks get to about 1/3 full after 17 mins when shower is on and blending with hot.
the hot last 12 mins when mixed to 40 degrees c. That's with pre heating the hot for 45 mins.
if I leave the hot water on whilst the showers running you get a slower decrease in temp after 15 mins.
Thanks for everyone's input, I wish I could have done the unvented route or had a bigger attic for the Cws or even had a bigger budget. This time it worked, I would do it again though. I'll tell the customer "add £1000"