navitron
 
Renewable Energy and Sustainability Forum
Welcome, Guest. Please login or register.

Login with username, password and session length
News: Anyone wishing to register as a new member on the forum is strongly recommended to use a "proper" email address - following recent spam/hack attempts on the forum, all security is set to "high", and "disposable" email addresses like Gmail, Yahoo and Hotmail tend to be viewed with suspicion, and the application rejected if there is any doubt whatsoever
 
Recent Articles: Navitron Partners With Solax to Help Create A More Sustainable Future | Navitron Calls for Increased Carbon Footprint Reduction In Light of Earth Overshoot Day | A plea from The David School - Issue 18
   Home   Help Search Login Register  
Pages: 1 [2]   Go Down
  Print  
Author Topic: mystery rayburn plumbing problem!  (Read 2449 times)
desperate
Hero Member
*****
Offline Offline

Posts: 3472


Backache stuff!!


WWW
« Reply #15 on: September 23, 2019, 07:24:36 PM »

I've seen this several times over the years. I'm pretty sure what happens is that when the tank is hot -ish you cannot establish a thermosyphonic flow because you are initially trying to get hot water to flow downhill. So even if the boiler is pretty hot you wont have a difference in density in the two pipes which is what drives a gravity system. Do you find that after a bit of popping and banging in the boiler the gravity flow gets established? If it does then I think we have an answer, a pump might be the long term solution TBH.

Desp
Logged

www.jandhbuilders.co.uk

still a crazy old duffer!
j.w.reynolds.01
Newbie
*
Offline Offline

Posts: 5


« Reply #16 on: September 24, 2019, 05:34:56 PM »

yes it's that, knocking and banging to get the thermosiphon going - it gets up to nearly boiling before enough pressure gradient is (noisely!!!) produced to get things going.

Well I think I'll go whole hog and put on swept bends AND a thermostatic pump incase the bends don't make enough difference in reducing the resistance. Like that i won't have to re drain the system over and over again!

just out of interest does anyone have an explanation if a direct tank adds more resitence to the system, or as i thought less resistance as the internal circulation doesn't go through a coil?Huh

John
Logged
ny
Jr. Member
**
Offline Offline

Posts: 51


« Reply #17 on: September 26, 2019, 09:34:03 PM »

Not wishing to step on anyone's toes but a few observations from my own system and some strategically placed thermometers. Soon after the boiler temp is above the cylinder hot water temp, and regardless of pipework temp, flow starts. I can 'watch' a long slug of hot water being dragged out of the cylinder into the return pipe.
Logged
ny
Jr. Member
**
Offline Offline

Posts: 51


« Reply #18 on: September 26, 2019, 09:54:54 PM »

Also, and again only being able to refer to my own system, almost 14m of horizontal flow/return pipework and no swept bends. Old system, only replaced due to re-siting of cylinders was getting on for a horizontal run of 20m. 28mm pipe and indirect cylinders. Thermosiphon was never able to overcome airlocks in pipework.

Nigel.
Logged
gravyminer
Sr. Member
****
Offline Offline

Posts: 440



« Reply #19 on: September 27, 2019, 01:54:17 PM »

The driver for the thermosiphon is colder water is denser compared to hot water that has expanded ( yeah I know, stating the obvious )

In my experience the horizontal pipework can be a fair bit longer than is being discussed here and even have the hot pipe falling a bit, as long as all air is removed. and the vertical bit of the hot side of the circuit is above the water heater i.e first go up then go sideways.
I have even seen it thermo through horizontal 22mm pipework ( designed to be pumped but still worked with failed pump )

I would suspect an air pocket based on the symptoms described and air bleeds at the ends of any horizontal pipework would be a good starting point.
Logged

gravyminer
pj
Sr. Member
****
Online Online

Posts: 286

Nom de Plume


« Reply #20 on: September 27, 2019, 02:29:47 PM »

Are your flow and return pipes well lagged? It could be that the flow pipe loses its heat to the surrounds before a flow can be established.

It sounds like the thermosyphon effect is kicked off by the rising air bubbles that themselves force a flow. Once started, the thermosyphon takes over. This also fits with insufficient lagging.

I don't think it's likely to be a resistance problem. With 28mm pipe work, 90deg bends and poor pipe cuts included, it only takes a few mm of pressure to get the water moving, hence why thermosyphon can work so well.

Also, I don't think it will be airlocks, because the boiling/bubbles gets it going - this wouldn't clear an airlock, just add to it.
Logged
BobLazar22
Newbie
*
Offline Offline

Posts: 9


« Reply #21 on: September 27, 2019, 09:37:47 PM »

The driver for the thermosiphon is colder water is denser compared to hot water that has expanded ( yeah I know, stating the obvious )

In my experience the horizontal pipework can be a fair bit longer than is being discussed here and even have the hot pipe falling a bit, as long as all air is removed. and the vertical bit of the hot side of the circuit is above the water heater i.e first go up then go sideways.
I have even seen it thermo through horizontal 22mm pipework ( designed to be pumped but still worked with failed pump )

I would suspect an air pocket based on the symptoms described and air bleeds at the ends of any horizontal pipework would be a good starting point.

My WBS was installed in place of an old Baxi back boiler. Had to make a few changes to the pipe work with regards to the vent/expansion and fill pipe, but the primary circuit is unchanged. 22mm flow and return, thermosiphons brilliantly up to the vented indirect tank that is up in the attic on the opposite side of the house. Loads of elbows and a long horizontal run before the tank. Heats all but the downstairs radiators on gravity, the pump takes care of them. I think what helps the most in my case particularly is the placement of the feed connection. I think it helps push the flow up and away from the stove. Could be wrong but it works a treat. I hope you can get to the bottom of your issue.


* boilerstove.png (14.67 KB, 1891x854 - viewed 117 times.)
Logged
Mostie
Sr. Member
****
Offline Offline

Posts: 498



« Reply #22 on: September 28, 2019, 02:44:52 PM »

About 10 years ago I was talking to an old heating engineer and when i mentioned using sweeping bends on my gravity system, he laughed and said it would make no difference as there is no pump, so i didn't use them, instead i bought sharp 90Deg 28mm bends, well it all works with no issues, there is even a horizontal section right out the side of the Yorkshire stove where it goes straight through the brickwork and then upstairs where it splits, one side goes to DHW tank and the other goes to heat leak rad.
Logged

2x Solis PV = 1.875 kW, Mitsubishi inverter heat pump. Yorkshire Boiler Stove.
j.w.reynolds.01
Newbie
*
Offline Offline

Posts: 5


« Reply #23 on: September 28, 2019, 09:11:12 PM »

thanks for the extra replies - like most thing it's probably a bit of everything, but if other long and bendy thermosiphons work without pumps it gives me hope.

The flow and return pipe are admittedly not well lagged in there entirity so its sound like this would be a good first step for me to address. Would it be better to lag them both well, or am i right in thinking that if i only lagg the flow pipe this could create even more impetus for to get flow going if the return is not insulated and so losing heat???

John
Logged
Nickel2
Hero Member
*****
Offline Offline

Posts: 1729


Method mixed with Madness


« Reply #24 on: September 29, 2019, 09:48:13 AM »

If it is not a silly question, would the fitting of a non-return flap-valve in one leg be a viable option?
This is the idea: When the boiler is cold, the flap is shut. As the boiler warms, the water expands; it can only go one way, and that is up the required pipe. As the 'up' pipe warms with the water in it, it becomes less dense on the correct side and the thermo syphon is then started.
(I am not a plumber, please don't shout at me if it's a rubbish idea! fight)
« Last Edit: September 29, 2019, 10:07:03 AM by Nickel2 » Logged

1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
pj
Sr. Member
****
Online Online

Posts: 286

Nom de Plume


« Reply #25 on: September 29, 2019, 01:05:26 PM »

If it is not a silly question, would the fitting of a non-return flap-valve in one leg be a viable option?
This is the idea: When the boiler is cold, the flap is shut. As the boiler warms, the water expands; it can only go one way, and that is up the required pipe. As the 'up' pipe warms with the water in it, it becomes less dense on the correct side and the thermo syphon is then started.
(I am not a plumber, please don't shout at me if it's a rubbish idea! fight)

This suggestion has been tried elsewhere, unfortunately thermosyphon does not have the strength to open the valve, it only operates on a few mm of head.

As far as pipe lagging goes, I'd try lagging the flow pipe first, especially if it goes through any cold open areas. If this does solve the problem, then the return pipe will also need lagging, otherwise when the boiler is off, the return pipe will cool quickest, and suck all the heat out of the store via thermosyphon Undecided
Logged
Pages: 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.20 | SMF © 2013, Simple Machines
SMFAds for Free Forums
Simple Audio Video Embedder
Valid XHTML 1.0! Valid CSS!