Adding resistance to a DHW circuit?

[DIYedboy]

My boiler (suprima 50L) is 3 feet from my cylinder, I have 2 2port valves and the system is open. I need to change the pipework because it has multiple problems. Whist considering this thanks to the good advice I've been given from Kevplumb, Doitall and others I thought if my HW pipework run from boiler to cylinder and back is no more than 6 feet it'll return back to the boiler as hot as it left virtually (and presumably trip out the boiler thermostat if it has one?). Then when CH and HW are on together the water won't bother with the fag of having to drag itself round the CH circuit cos it's only 3 foot from home!

Currently there isn't any restrictor valve on the HW circuit, would I also be right in thinking that the cylinder coil is like a radiator and therefore the water leaving the coil should be XYZ degrees cooler than when it entered?

 

[kevplumb]

View media item 11248
gate valve

open app 1/4 and see how it goes

 

[DIYedboy]

Kev, when I fit that, am I supposed to measure the temperature drop in n out of the cylinder like with balancing a rad?

 

[kevplumb]

probably but tbf i don't think anybody bothers

its only 1 rad if you think about it on its own run



no doubt im about to be told i'm wrong

 

[DIYedboy]

In the summer when I'm just using HW, I presume this restrictor will stop the boiler from 'cycling'? or am I wrong?

 

[kevplumb]

the amount of insulation on the cylinder and the stored water temp / usage will have a greater bearing

 

[DIYedboy]

I don't understand your answer, so let me give you a scenario:
It's 6.30am in the summer, on goes the HW, the tank is insulated but cold, no-one will use any water for an hour, will the water return to the boiler at virtually the same temp thereby activating the thermostat and shutting down the boiler?

 

[kevplumb]

not till the water in the cylinder warms up

 

[DIYedboy]

Nice one!...............if I don't fit your gate valve restrictor, would the returning water temp be hot enough to shut the boiler down?

 

[Onetap]

Nice one!...............if I don't fit your gate valve restrictor, would the returning water temp be hot enough to shut the boiler down?

Gate valves only provide a significant resistance on the last 1 turn or less, they're fairly useless for regulating. I'd suggest an in-line 1/2" lockshield radiator valve, they're intended for regulating/balancing. I've used double regulating valves , but they're not sold for domestic use and I happen to have a few spare .

You need to keep a minimum flow rate in summer mode, when the heating circuit is off. If it is open vented, you must NOT fit any valves between the boiler flow and the open vent, which is often at the cylinder. Similarly, no valves between the boilker return and the cold feed connection.

The boiler burner will turn off when the return water temperature gets too hot, the heat input will raise the flow temperature over the setpoint of the thermostat on the flow pipe. The boiler pump will keep going until the DHW thermostat reaches it's setpoint and the boiler and pump is shut off.

The boiler will probably cycle on and off as the water temperature approaches the set temperature. It will happen less with a modern BS cylinder which has a larger heat transfer area than cheap, old 'short-coil' cylinders.

 

[DIYedboy]

Well, well, that's interesting, thanks Onetap
My system is open vented, the HW circuit don't have a cylinder stat so can't be shutdown
The boiler is a 10yr old backboiler

 

[twgas]

My boiler (suprima 50L)

My system..........The boiler is a 10yr old backboiler

So what do you have?

 

[DIYedboy]

Sorry twgas, I'm being a nob, I'm trying to finish my uncle's heating job off (backboiler no stat on cylinder) and gettin mixed up with the job I need to start on my house (Suprima, 2no 2ports open vent) which is what this post is about

 

[bengasman]

Kev, when I fit that, am I supposed to measure the temperature drop in n out of the cylinder like with balancing a rad?

Not really worth measuring a cylinder as the flow and return will vary greatly depending on the temperature of the cylinder, and how many rads are open at the same time.
Also depends on the type of cylinder; modern cylinders can absorb a lot more per minute than old ones.

 

[doitall]

Well DIYedboy knows how to confuse us thats for sure.

Are the primary pipes to the cylinder gravity, or pumped.

Can we have a pic.