Oil boiler problem Heating engineer in need of help!

[essexheating]

Recently installed a heating system in a large house, 2 x Worcester 18/25 utility system boilers. We have fitted 2 x tiger loops to the oil lines in. The boilers are fed from a 5000 ltr bunded oil tank and the oil line is approx 24 m run, I know its long but was the only place we could site the monster oil tank.
The oil tank is approx 1.75m lower than the oil boilers. We have teed the oil pipes straight after the vlave outlet on the oil tank and both boilers have a seperate feed from there. We have had weeks now where boilers keep locking out. Sometimes hitting the reset works and they fire for a few days then lock out. When we manually bleed them through from the nipple alot of the time air seems to be in the oil line. Oh just to add the oil tank has 3000 ltrs of oil in it which means the oil level is higher than the boilers. Appreciate any help on this one as me and my business partner are staring to pull our hair out!!
Regards Mark

 

[Billy Bob Bob]

i am only gas sadly but welcome to the forums sure some of the other members who do oil should be able to point you in the right direction i have only ever done 2 oil installs

 

[dontknowitall]

There's been some discussion recently about oil waxing and freezing with these latest temperatures (around -9 degrees seems to be the starting point of problems).

Appears to be rather hit and miss as to who and what systems are affected, but this could easily be the problem if they're going okay for a while.

 

[essexheating]

The oil tank is approx 6 months old, the system was a brand new refit. The spec was high so used titan talking tanks, worcester boilers, megaflos tiger loops. We think that as the oil feed is on a back fall to the tank when the boilers are off for a period of time that the oil is possibly falling back towards the tank after the tiger loop and this is how air could be getting into the line. When the boilers re fire they light for a little bit using the oil in the tiger loop then once that has been used the boilers will lock out. Just clutching at straws really but it is puzzling us. Also do you know if they make non return valves for oil. May give them a go if cant solve it soon. Thanks for peoples time and effort. Regards, Mark

 

[essexheating]

Initually the problem was definetly air related. We did consider the oil being cold but we look after many oil boilers in the same area and this seems to be the only one effected. Do you think the incline on the oil feed could cause a problem, the twining off 2 boilers onto 1 outlet from the tank?

 

[dontknowitall]

Until the weather warms up for a week or so, if it was me I'd pin my hopes on the low temperatures. If all's well when it's warmer, then you know some protection of the pipes might be an idea. Don't want to teach you to suck eggs though as I expect you know MUCH more than me!! Thought you might like to know the temp has been affecting other people though.

 

[WHPES]

Either

a, diameter of suction pipe too large - my memory reckons for a long run it should be 6mm dia but I can't rememer exactly. If it is 10mm diameter this is probably the cause of your "air related" problem and there's your problem now.

b, A lot of problems recently caused by ice crystals in filters or lines.


The tank is quite capable of supporting multiple outlets but the sizing of your oil line is critical to prevent trapped air.

Cold weather is only an issue for gas oil and not kerosene.

 

[ecowarm]

We have a problem and have narowed it down to the kbb fire valve sucking air somehow?

Going out to change the fire valve in the next few days

Hope this helps

Eco

 

[WHPES]

Unlikely, especially if it's happening to both boilers

I have checked my OFTEC Installation file and for 25m it's 6mm diameter pipe which is specified. If the supply pipe is 10mm I would pretty much stake my reputation on that being the fault.

 

[tackleburger]

I would agree. Out of interest why is the tank so much lower thus meaning it needs to be 1/2 full all the time? On such a big job I am surprised that the height wasn't accomodated for.

 

[WHPES]

I disagree. The tank doesn't need to be 1/2 full all the time if you have a 2 pipe fuel system with tigerloop fitted! The height of the tank is irrelevant if the installation has been done correctly to standard.

Often it is preferable to have the tank at a lower height to reduce the visual impact. The installation guide recommends tanks to be sited at ground level in preference to being raised.

The OFTEC Installation manual has been developed to prevent such hiccups - "Just whack it in and hope for the best" is not a good way of working. Use the books!

 

[essexheating]

Either

a, diameter of suction pipe too large - my memory reckons for a long run it should be 6mm dia but I can't rememer exactly. If it is 10mm diameter this is probably the cause of your "air related" problem and there's your problem now.

b, A lot of problems recently caused by ice crystals in filters or lines.


The tank is quite capable of supporting multiple outlets but the sizing of your oil line is critical to prevent trapped air.

Cold weather is only an issue for gas oil and not kerosene.

Thankyou all for your advice. Especially WHPES. We know that the oil tank can be sited lower than the boiler as the oil pump will pull the oil through. We run the pipe feed as one line initually 1/2 the way in 15mm and twinned it into 10mm there after then fed into the tiger loops 1m away from boilers. After having problems we considered that the demand from the 2 boilers running at once may have been exhusting the oil line and then collecting air so ran 2 oil lines in 15mm from the oil tank outlet then reduced half way down pipe run to 10mm.
I know this may seem a stupid error on my part and here may come a even more stupid question but here it goes, how certain are you that by reducing the oil line to 6mm will resolve the problem. Really appreciating all the comments. Regards Mark

 

[WHPES]

About 99% certain. With 15mm you have absolutely no chance at all of it working properly. Your modification will have actually made it worse.

Try your replacement line out on the surface before you go to the trouble of burying it.

Go all the way from the tank outelt in the smaller diameter (look at table in OFTEC file ffor sizes) and your problem should be over. I would recommend running the pipe inside some plastic pipe for protection.

BTW the easiest place to get 8mm or 6mm plastic coated soft copper is a hydraulics merchant or possibly automotive spares wholesaler as plumbers merchants have to order it in and it costs loads.

Anyone who has ever tried to use a bit of hosepipe to try and syphon water from one container to another will remember the frustration of the water being unable to maintain suction and air getting in and stopping it (remember science clas at school). Reducing the diameter of the pipe prevents the syphon being lost (although the flow rate is less without mechanical assistance) as it's all down to speed, viscosity and surface tension.

One of my customers has rented property which has a massive tank and four properties take their oil from it, each with its own meter.

Do a bit of bedtime reading of your OFTEC books. Follow what is given there in future and you won't go wrong again.

Assuming you are OFTEC registered, don't forget there is a support line for members so don't be afraid to ask questions.

I would expect that 6mm would be fine for the shared section but you will need to work it out from the combined requriement of fuel (i.e. USGall/hour). You could possibly run in 8mm to the tee and then 6mm from the tee but we are now in the realms of fluid mechanics.

PS as well as doing heating, I am actually also a qualified pipeline and fluid engineer so that is my technical slant!

 

[essexheating]

WHPES, May i reccommend you get some sleep as just woke up myself and seen what time your post come in. I got to be brief as im about to be late for work. Yes we are oftec registered. Found your last post very interesting. It makes sence what you said. Where would you recommend twining the pipes from? Come straight out the tank in 15mm then immedietly twin to 6mm or further along the line?
Also just curious, what type of work do you do being a qualified pipeline and fluid engineer.
Sorry for all the questions. Been plumbing fresh from school. Me and my business partner are 24 so still green to many things but trying to make a go of it. And it seems to be working. Really apreciate your help on this one. Better go now as am late, and as we all know its not like plumbers to ever be late!

 

[WHPES]

On night call-out service hence the hour. Just about to turn in and get some zzzzzzzzzzz.

Assuming both boilers are typically 0.75 US Gall/h that's 1.5 when both are firing that's just over 1 Imperial gallon which is 8 pints i.e. filling a typical builders bucket 1/2 full. You know the speed that a bucket will fill with water from the mains (fed by 15mm) so imagine how slow it will have to be running to fill at that rate - about 1 pint every 7 1/2 minutes, so it's not a lot when you think about it.

Personally I would run in 6mm all the way from the tank and tee off later. For 8mm and 6mm I would advise using flared fittings throughout as opposed to compression as they are much more reliable. Pipework will need protection where it is vulnerable. Remember never use compression fittings underground and soldered joints are not allowed at all. Try it out on the surface to make sure it's going to work. Check the pump vacuum with it running (use that pump test vacuum gauge which hardly every sees daylight!)

Never use more than 10mm pipe in any case. 10mm is fine where the line is in positive pressure (ie the base of the tank is above the pump level.) I use 8mm as a default where the tank level is a little higher if the run is short. The narrower the bore, the easier it is to prime the pump.

Where the pipe run is short, just do a two pipe system to and from the tank as its a lot easier and cheaper than messing about with a tigerloop..

I do pipeline work - private water supplies etc - sprinker systems etc but there's not enough of this around here to make a living on its own so plumbing and oil boiler work is the main part of the business.

 

[Dannypipe]

We did a very similar job to the one described. This was back in November. We had to use the existing 10mm supply as the huge plastic tanks were about 25M away and the supply was buried under a concrete drive. We split the supply as it rose and put on two tiger loops.

We then linked the system into a sequenser to split the demand in summer months.

I don't want to tempt fate, but it's worked perfectly since fitted. So don't know where that stands against the pipe sizing debate.

One thing we were replacing a 70KW internal boiler which was working well, on 10mm. The client wanted to free the room up and turn it into storage. So the old boiler was running on 10mm just fine.

Oh and you can get check valves for oil (seem to remember you asked that at some point) you can get combined check/fire valves.

 

[Bartdude]

I have encountered this problem and I currently have my problem boiler running on a 8mm oil line on the surface. When I spoke to Tigerloop they were adamant that 10mm pipe would be well within limits(this is a 24 kw boiler,25m run and 1.5m lift).

WHPES, is 10mm too large for the OPs' pipe run? I agree that best practice is to run all suction lines in 8mm but is it technically wrong? I don't have your grasp of fliud mechanics!

Have the oil lines been pressure tested? If compression fittings have been used are liners fitted?that's a requirement now,but rarely heeded. Personally I don't think flared fittings can be beat,especially on a suction line

The problem has to be related to air entering the oil line somehow either through a leak or incorrect pipe diameter causing loss of the siphon effect. Tigerloop don't recommend a check valve to be fitted,that's a red herring and would do more harm than good.

 

[WHPES]

Yes it is technically wrong. 10mm is defnintely too large for the OP. You might get away with 10mm if you are lucky and if the suction head isn't too high. However the OP has 15mm diameter pipe which you would struggle to get away with. With larger pipe, it's not just the cross sectional area increase but the reduction in proportion of pipe wall against the overall area subjecting the flow to friction.

If you go into more detail there are viscosity/pipe diameter/flor rate charts. The higher the viscosity the larger diameter pipe you need. For example bakers syrup requires a larger bore line than cooking oil due to viscosity.

If the line is too small for the viscosity of the liquid and flow rates, it puts strain on the pump. If you suspect a poor supply issue to the pump, use your vacuum gauge to check before you start making alterations to the line.

Conversely if the line is too large, your vacuum readings will be very low and you will get air problems.

Ever looked at the fuel line on your car? now that is really small and its for exactly the same reason. If an air bubble get in, its rapidly sucked through because of the high flow rate.

I had assumed the line had been pressure tested on installation as per OFTEC specs which is why I never mentioned possibility of leakage or air ingress.

Dannypipe - what was the suction head for the system you mention?

 

[Billy Bob Bob]

see where your coming from whpes when i had a tiny split in the diesel line on my old van this was pulling in tonnes of air and causing starting running issues and the supply pipe was about 4mm

 

[quality]

I have to agree WHPES it that boyles law that prevail as preesure and volume are inversley proportional to each other, in other words less volume (6 mm pipe) higher pressure equals higher flow
seems self explanatory when you look at the logic

 

[Bartdude]

I've found this on the very good Danfoss website:

Oil Suction Line Calculator

It would seem that 8mm would be ideal for the OPs' system according to this.

 

[Dannypipe]

Dannypipe - what was the suction head for the system you mention?

Boiler is aprox 1 meter higher than base of tanks.

Like I say, we inherited the oil supply and cut it just below where it entered the property and split it for the two boilers.

I'm very interested in what you guys are saying as I mainly deal with gas, and find oil installs interesting, even if it is a mucky fuel! I can see bio fuels being popular with some people in the future. So the rules apply there too.

Everything you've said has made sense. I'm just curious as our install was very similar to the original posters and works well.

 

[WHPES]

Boyles law is for gases. For oil, the main factors are Bernouilli's Principle for flow and pressure relationships for change in diameters, the Darcy Weisbach Equation for calculating pressure drop along pipes (which is used to work out pump sizes and pressure settings). Two pipes with the same cross sectional area as a larger pipe will not have the same flow rates due to greater friction because of more pipe wall in contact with the fluid. The shear stresses in the liquid as it flows are annular. The liquid in contact with the pipe wall is stationary whereas the liquid at the centre of the pipe is moving fastest. The gradient of velocity is non linear due to the geometry of the pipe.

These kind of calculations are absolutely critical in larger engineering applications. In sprinkler systems for example, the pipework must be capable of delivering sufficient water at the correct pressure to any part of the system.

No point in me putting much down here - Wikipedia will have plenty of stuff if you want to know more.

 

[quality]

yes gases are pretty slow to react but with fluid dynamics the same laws apply but are amplified somewhat

ps whats Wikipedia

 

[quality]

Just googled wiki thing interesting stuff even if not all correct

Try Charles law for gases and then look how a gas a fluid or both all react the same time but different depending at what pressure temp and volume gases or fluids are subject.

simple terms cold engine no pressure hot engine positive pressure

liquid volume is smaller because pressure is higher hence so is the temp

it is very similar on an open circuit but not as easy to measure

try standing on top of your missus then your 3 year old it the same physics

thats the good thing about physics they do no change only our understanding of them does