low flow lockshield valve

[bengasman]

When squeezing down the flow to achieve the required 20 degree drop on radiators for a condensing boiler, I often find it hard to get more than about 15 degrees without shutting the flow completely and if it does work I usually find an annoying whistle coming from the lockshield valve. Which valves can be used without these problems?

 

[doitall]

There ain't one, design and flow rate are the problem.

You need a smaller pump, turn the stat on the boiler down, or fit bigger rads.

Restricting the flow will create noise in the system. the 20° drop is the boiler return not every radiator

 

[brumylad]

I wonder if putting in a radiator at the end of the return pipe plumbed in "single pipe system" style would help

I've never done this incidently

I wonder then how many installations are going to run in condensing mode when producing hot water only. or is it easy to get this drop now with modern rapid recovery cylinders

 

[ChrisR]

This isn't something I've investigated, but I thought the steamers with modulating pumps would do it by slowing them down.

btw a Megafo (etc) coil is 25kW!

 

[bengasman]

A good idea to use a smaller pump and I would if I could; however boilers do come with pumps built in them these days and the better ones are electronically controlled rather than with a three step switch. Unfortunately the controls will increase the pump capacity when the return is cold, i.e. when the boiler comes on in the morning. It is not correct that the 20 degree drop is measured over the flow and return, it has to be over the radiators; this is in order to achieve a return temperature under 55 C at the boiler. More ideas please.

 

[kevplumb]

Table 3. Flow Rate
MODEL MINIMUM FLOW RATE
18sxi 774 litres/hr.
30sxi 1289 litres/hr.
This is equal to 20oC differential at maximum heat input.

gloworm data

 

[doitall]

A good idea to use a smaller pump and I would if I could; however boilers do come with pumps built in them these days and the better ones are electronically controlled rather than with a three step switch. Unfortunately the controls will increase the pump capacity when the return is cold, i.e. when the boiler comes on in the morning. It is not correct that the 20 degree drop is measured over the flow and return, it has to be over the radiators; this is in order to achieve a return temperature under 55 C at the boiler. More ideas please.

If you say so

 

[kevplumb]

It is not correct that the 20 degree drop is measured over the flow and return, it has to be over the radiators; this is in order to achieve a return temperature under 55 C at the boiler. More ideas please.

so educate the great unwashed with a reference for your statment

ie where did the info come from

 

[doitall]

Before you answer, restricting the flow through the radiators will Increasethe return temperature

 

[kevplumb]

never said nowt i didnt

i dig the hole and you fill it in

i give UP

 

[brumylad]

If the system is designed with a larger temp drop (20 not 11'c) then I believe I am correct in suggesting the mean temp of the rads will be less.

Using a Mears calculator then, the temperature difference between the mean temp of the rad and the room will be less and therefore a larger rad is required. Using the calculator again, a larger temp difference between flow and return means lower flow (litres/sec). This also means smaller pipe can be used.

I think this takes us back to the earlier statements made about system design.

 

[PTH]

Whistling valves are often a problem when trying to squeeze flow rates. I always specify good quality valves : Samson ones have a contoured seat which seems to work well. Others worth the extra expense are Oventropp and Herz. Why not use a proper flow control valve such as a double regulating or even a commissioning set on the return to the boiler when putting condensing lumps onto an existing system, that way you can balance the rads in the normal way and give the pump some additional resistance to ensure the right flow rate, hence return temperature. After all, it's only Q=m x c x dT.

 

[bengasman]

In essence, yes the drop is measured over flow and return pipe at the boiler and is designed at 20 degrees to ensure the return temperature to stay under 55 C. however, I have no means to influence the temperature differential at the boiler itself, hence I should have said measuring at boiler by adjusting all radiators.
this 20 degree difference is achieved by heatloss and assuming pipes are properly insulated, this heatloss will occur at the radiator. Any radiator. Therefore, all radiators should achieve a drop of somewhere near 20 degrees. Imagine a system with 2 rads, one dropping 18 degrees and one only 10, I doubt if you have much chance of realising a 20 degree drop at the boiler. If anyone knows how to get the 20 degrees whilst getting only 10 at the rads, please tell.