Help and advice for the correct replacement pump for my microbore system

[knurly]

I need some advice as to the correct circulating pump to fit to an existing microbore system on my domestic two story semi-detached house.

The system was installed many years ago and works on a baxi Bermuda back boiler, the system is clean and the boiler is well maintained and working fine, currently fitted is a wilo-smart 25/5 pump but I believe this to be the wrong pump for the system as when the heating switches on the pump appears to hold back and just hums rather than circulating, also again at start up I get some kettleling from the boiler and initially soon after start up hot water backfeeds as far as the f&e tank. This only appears to happen with the heating and not so much with the hot water Which I suspect is caused by latent heat from the boiler due to the pump holding back.
When the system warms up and the pump is running constantly the cf and expansion cool back down to acceptable levels.

Its worth note that the boiler stat now needs to be set fairly low again in order to stop overrunning into the expansion tank.

Originally fitted to the system was a two stage (manually variable head) myson pump which worked fine for donkey’s years but has now been discontinued.

The system is pretty conventional with a cylinder on the first floor controlled by a three port mid position valve both the feed and expansion connections are about 150mm apart.

The pump is on the return to the boiler and about 1.3mtr away I changed the pump about 18months ago as the original developed a few leaks and updated the rads and valves sometime after.

The system was in need of drastic updating as although the boiler had been changed a few years back the system was about 30 years old, So I recently changed all the radiators disposed of all the old dual port microbore valves and replaced these with thermostatic ones, apart from of course the towel rail in the bathroom which works from the heating system and has manual valves left partially open,
The system currently and as it is more or less at its maximum as regards to heating output and required a fair bit of balancing in order to get heat to every radiator hence the towel rail valves being only partially open.

I suspect that the pump is either faulty or the wrong pump for my type of system and that I should have fitted a pump with a constant flow rather than the wilo which appears to have an internal invertor, any suggestions as to the make/type of pump that i should have fitted as a direct replacement for the myson would be much appreciated, I am aware that changes to legislation seem to now require invertor pumps but i desperately require advice as to what pump I need to fit in order to make the system more efficient and stop my ceurrent problems.

 

[DaveTidy]

Others may disagree but we were always advised that on high resistant heat exchanger boilers or systems (e.g. mini/micro bore), a 6 metre pump was to be installed and throttle the speed back until the system could cope with the flow, the lower speed the better (less than 1.5 metres/second).

Some microbore systems on a Thorn Apollo boiler (1.5 litre heat exchanger) worked very well on speed one of a 15/60 (6m head) pump. But, I guarantee the next year of anyone discussed rad temps etc the next guy would automatically turn it up to 3 and say it needs it to overcome the pipe size, not always the case.

I remember this being proved with the calculations. We used to fit a Grundfos 15/60 at the time.

 

[Reg Man]

Your system pipework configuration is incorrect. It may have worked originally on that old Myson pump, but will not work on modern pumps. The pump should be located on the flow before your 3 port valve. The open vent and cold feed should tap into the flow just before your pump so they are on the negative pressure part of the system. This will stop your pumping over into the F&E cistern. You may have circulation issues on the micro bore, as they are restrictive and prone to blocking up. You could try a cleanser in the system and flush it through, but its not always guaranteed to solve the problem

 

[happyflyer]

There is a lot going on here and things have been changed, a few point to clear up:

The old Baxi Bermuda if its the cast iron green one that sits on a metal stand has a very low pressure drop, it had four 1" bsp connections one side for pumped and the other from HWS gravity. This system is fully pumped, I hope they cross connected it, it does help, the thermostats were prone to creep, losing pressure and out of range going high.

IMO I would not fit any pump with an internal speed control, there was never a need for it when the system was fitted all those years ago and there isn't one now.

The FE looks like its pumping over at some time perhaps when the 3 post valve is on the move, if it is the water is going over the tank and not in the rads, dare I say normally you would find a combination CF and Vent this usually stopped pumping over, some bright spark a long time back said it was safer to have separate CF and vent and then the pumping over started, I fitted 100 s of systems with combined CF and vent, back in the 60s never had any problems.

What was the original pump not just the make the size or have you binned it.

Baxi used to supply and injection tee also if you wanted to only use one connection side for pumped and HWS.

Think you may be right about the pump, where do you live, RegMan has a few good points, but you should be able to stop the pumping over, you must address this ASAP.

Tony

 

[knurly]

Thank you folks for your early responses, it's worth a note that the system doesn’t actually pump over all that happens is that soon after start up the hot water from the system surges back up both the cold feed and the open vent but only to the level of the tank, the hot water just reaches the inlet in the tank with just a small amount entering the tank, the open vent gets hot to the level of the tank but as far as i know never actually pumps over, Its accompanied by a surging noise and the water at the connection point and above is extremely hot hence my thoughts that this is caused by latent heat in the boiler where the pump is possibly not man enough or not circulating enough. This surging only happens on start up or soon after once the system is hot i lose all boiler noise and the temperature/ position of the water in the cold feed and expansion drops to where you would expect it to be. In an attempt to cut down this surging i have lowered the temp on the boiler stat which seems to work although there is still some kettling soon after start up.

I had written a long post in reply and just as i copied it as i guessed the forum timed out my machine crashed so bear with me please.

A few years ago i changed the baxi for a newer version, it does have a cast iron heat exchanger but this is a fairly lightweight and more efficient version the water enters the exchanger at the bottom through an inlet spreader then passes via manifold through to the top section and then out of the flow thus the heat exchanger is effectively part of the pipework. It’s worth a comment that the spreader itself does restrict the flow and must cause a slight pressure build up.

Reg man does have some very valid points but although the c/f and exp are on the centre inlet to the three port and just in front of it they have been there for a long time and previously i have not noticed a problem with them on the positive side, the pump is mounted on the floor below about a metre or so in front of the boiler. the system is very clean has new rads and was completely flushed when i installed al the new rads, i actually removed the boiler completely checked for scaling and flushed that independently. the system was filled inhibited and a boiler noise additive put into a rad. and even the old plumbers trick of putting a squirt of detergent in the system to cut down ketteling.

i still suspect the wilo pump as being the cause of the trouble, all the commercial pumps i know that have inverters fitted normally reduce the speed of the pump as the demand drops and as the system pressure increases but don’t actually stop. The wilo seems to stop completely now and then and just hums to itself even when the system is calling for heat.

The wilo is a wilo-smart a25/4-130 and after some scrabbling about i found the old myson pump which is a [FONT=&quot]Unit two/two speed variable head pump[/FONT], the head is /was adjusted by a lever on the back the settings are min2-3-max the lever is stuck mid point between the min2 and the 3 setting. The type appears to be type c it ran at 0.49amps and 110watts suitable for a max pressure of ten bar.

Thanks again for all of your thoughts and i accept that if you can recommend a suitable replacement for the myson that it is just that a recommendation and may not be the solution, in answer to Tony i live in clacton on sea essex.

As to the head between the pump and the boiler i guess this to be between 5 and 6mt 6 being the maximum.

Best regards Mike.

 

[happyflyer]

Pumps with built in speed control you too clever for their own good, totally over the top for domestic systems, bloody manufacturers with wild ideas
way above their station in life, you don't want speed control in a small systems with little water and heat exchangers that require constant flow across them. Do you know what the pressure drop is through the new heat exchanger, did Baxi produce this new heat exchanger, the old ones weighed about 60 lbs, they suffered from leaking rubbers on all four headers.


Tony

 

[knurly]

Tony the whole boiler was of baxi manufacture the actual heat exchanger i would guess weighs somewhere around 40 lbs and the two headers are joined by a single eliptical port thus allowing the water to travel more efficiently through each header on its way through to the flow outlet the boiler model is a baxi bermuda 401 the pressure drop is stated at 13mbar(5in w.g)at 900l/h(3.3g.p.m) the whole thing weighs 38kg.including casings burners the full monty.... i would think by experiance that although the boiler is clean the actual pressure drop may be a tad higher the water content is stated as 2.2 litres(o.48 gals)

I hope this info helps

Mike

 

[happyflyer]

Mike,

That's pretty low pressure drop I make that 1.3 Kpa @ 0.25 ls, what do they rate the boiler output at ??


Tony

 

[tamz]

If you want a fixed speed pump this would do
Flomasta Central Heating Pump | NoLinkingToThis
or one of these
Antares 25-50 Domestic Pump
The 25 just means it has a bigger bore and won't make any difference

You do realise you will be breaking the law fitting this :lol:

 

[tamz]

Mike,

That's pretty low pressure drop I make that 1.3 Kpa @ 0.25 ls, what do they rate the boiler output at ??


Tony

A 401 is 40,000 btu max.

 

[happyflyer]

A 401 is 40,000 btu max.

Tamz thought it was around 11 kws, pretty small output 0.25 is about 11kws,

 

[happyflyer]

If you want a fixed speed pump this would do
Flomasta Central Heating Pump | NoLinkingToThis
or one of these
Antares 25-50 Domestic Pump
The 25 just means it has a bigger bore and won't make any difference

You do realise you will be breaking the law fitting this :lol:

Scottish Court hey, what law is that Tamz ??? PSR pump safe registered now is it!

 

[tamz]

You could near drive it with a bike pump :wink:

 

[tamz]

I dunno Tony. No doubt the same save the planet that stops them selling them.

 

[knurly]

As Tamz said the maximum output is listed at 11.7kw or in old money 40.000Btu/h this appears to be variable dependent on the gas pressure setting but it's currently set at 7.3in wg which is 40.000, i have to say the boiler copes with the load quite well so i dont have a problem there, one thing that throws me a little is that the old pump was set on step two with the variable head lever at about mid point between min2 and 3 so i suspect i need a pump that has some adjustment with regard to the head possibly if i had set it at max on step two it may have given me other problems and i reduced the head/pressure via the lever but i installed this system about 33 years ago and so memory fades when everything works fine.
apart from the pipework just about everything has been updated several times but works fine apart from the current problem
unfortunatly i have no idea how the wilo pump is intended to work never having dealt with domestic pumps of that type, again i suspect because of the head and the pressure drop across the boiler that it needed a more powerfull one possibly something like an A25/6 as i assume the /6 refers to head,but dont really want to rish pumping over hence the search for advice.
it could also be that the wilo pump is faulty as i doubt that they are intended to basically stop pumping completely if they sense an overpressure and hum to itself as this seems to at times.
or again this may be down to me selecting the wrong pump as a replacement for the myson.
I never used to like wilo pumps to be honest so am a bit predjudiced there.

As to scottish law i assume this is the same law all my old pipe fitters used to quote to me with delight with regard to a plumber's knowledge of heating (S**t runs down hill and payday's thursday)

Thanks again for the help guys much appreciated.

Mike

 

[happyflyer]

Mike,

The pressure drop across the boiler is very low as you would expect from a BB unit, its not a condensing boiler, they have a relatively high pressure drop, for what it will cost you I would change that speed control pump, you don't know what's going on in there when its running.

 

[tamz]

As to scottish law i assume this is the same law all my old pipe fitters used to quote to me with delight with regard to a plumber's knowledge of heating (S**t runs down hill and payday's thursday)

Friday pay day up here :lol:

 

[Billy Bob Bob]

jesus dont you guys know bermudas? as tam said 401=41,000 btu or the more comon 552= 52,000 btu,in the op,s case pump should ideally be on the flow

 

[knurly]

Problem solved just by changing the pump to a new grundfoss alpha2 L after a very helpfull chat with Tony i decided to bite the bullet and change the pump to the grundfoss one.

I had replaced the old myson one with the wilo about a year or so ago and wilo state that it was the correct replacement for the old unit2 myson one. But obviously not or if it is this ones faulty and has been from the start.
On fitting the grundfoss everything worked fine from the moment of switching it on, it comes preset to a mid range proportional pressure and has the option for three fixed speeds as well.

SO no more kettling no more surges or blowing over no sucking air in the system and absolutley no need to move the cold feed and expansion or modify any pipework at all. The system works as sweet as it did when i installed it 36 years ago.

THANK YOU all that gave me helpfull advice it really is much appreciated and a special thanks to Tony for his patience for putting up with my mithering.

And Tamz stop p**ing in the clyde on a friday as it makes my scotch taste funny!!!

 

[happyflyer]

Only too happy to help Knurly, hope that wind didn't hit too hard

Best Regards

Tony