Cascade boiler system mess

[shipbadger]

The first thing I must say is that I had nothing to do with the installation of this system.

My customer has a system comprising of a pair of Greenstar 24Ri boilers plumbed in cascade, on an open vent system. The circuit is. for want of a better description, like a very large bungalow; with a maximum head of less than 3metres. There are 20 radiators on the heating circuit, all fed off drops on a circuit loop of about 60 metres. The pump, which is situated just 1.2 metres below the base of the F&E cistern is a Grundfos UPSD 40-80 F 250, which I think is way too powerful.
The customer has reported to me that they have replaced many radiators due to rusting. When I have observed the system the circuit water is circulating rapidly through the F&E cistern, leading I believe to excessive oxygen entrainment and consequent rusting. There is no low loss header fitted.
What would be the best thing to do with this mess, with consideration given to the fact that a complete refit is out of the question. There is no possibility of increasing the available head. My initial thoughts are to convert the system to a sealed system, but I would value the help of people here, some of whom seem very expereienced with this type of set-up.

 

[Boilerman2]

From your description it would seem that the pump is a bit big, but the most obvious problem is the lack of head for operation of the pump and system, The obvious thing to do would be to convert to a Sealed system running at about 1.5 Bar, at a guess a 100 Litre Expansion Vessel would be more than adequate, and it might be a good idea to fit a pressurisation unit to maintain the cold fill pressure

 

[Biggles..]

I'd also consider fitting a drv to slow down the flow,and open the bypass accordingly to reduce system noise and degridation.

The pump is probably not that big kpa wise compared to the heatoutput and load,even though they've fitted a 1.5inch port.ev should be 1litre per kw,so 50 would blitz it.

 

[Agile]

My first thought is that its unlikely to ever need 48 kW and the boiler output is way too high.

If I was rearranging the system I would fit two 15-60 pumps with NRVs so that each boiler/pump would be a self contained heat source and for all but the coldest weather just one boiler would be adequate.

It would then have redundancy and in the event of any failure the heating would be continued on the other set.

Taking it further I could arrange for one set to have a c/o relay to allow powering from an external generator input plug.

Tony

 

[tamz]

My first thought is that its unlikely to ever need 48 kW and the boiler output is way too high.

If I was rearranging the system I would fit two 15-60 pumps with NRVs so that each boiler/pump would be a self contained heat source and for all but the coldest weather just one boiler would be adequate.

It would then have redundancy and in the event of any failure the heating would be continued on the other set.

Taking it further I could arrange for one set to have a c/o relay to allow powering from an external generator input plug.

Tony

I concur except for the last bit.

 

[Agile]

The particularl advantage of my approach is that each boiler has an appropriately sized pump.

Currently in the usual situation with only one boiler working the pump is grossly oversized and the delta T will be too small.

My friend in a small village was without mains power during extreme weather some years ago for seven days but managed to power his boiler initially from an inverter and later from a small generator.

Tony

 

[htgeng]

I'd be err-ing on the side of keeping it OV. The pipework might not cope as a sealed system. What I would do is modify the pipework around the boiler so that it cannot pump over in any situation. Changing the location of the f/e, using an aerjec and you mentioned a LLH are all ways you could modify it. Because you have two heat sources you could also consider a dunsley neutralizer box which would be a ready made LLH. That would completely eliminate pumping over. I would be quoting for about 12 hours work and the parts - a couple of pumps for each boiler, the box, repiping around the boilers, 28mm magnaclean, flushing/inhibitor.

 

[Biggles..]

I'd be err-ing on the side of keeping it OV. The pipework might not cope as a sealed system. What I would do is modify the pipework around the boiler so that it cannot pump over in any situation. Changing the location of the f/e, using an aerjec and you mentioned a LLH are all ways you could modify it. Because you have two heat sources you could also consider a dunsley neutralizer box which would be a ready made LLH. That would completely eliminate pumping over. I would be quoting for about 12 hours work and the parts - a couple of pumps for each boiler, the box, repiping around the boilers, 28mm magnaclean, flushing/inhibitor.

what about controls,changing from a single pump to independant and sepeartaing the boilers temperature control from the system control.

were not sure how this system is entirely setup,could be one boiler is turned right down so both boilers fire first off but one is soon satisfied.

rather than wasting money on dunsleys(over the top when mhs do a 60KW compatible LLH for peanuts) and reconfiguring,why not just fit the right sized pump..id be looking at UPED 25-60 for that sort of boiler load,pumps modulating so will reduce risk of pumping over.