Combi boiler and cast iron radiators

[ethans]

Hi,

I am having my current system replaced (10 year old Potterton with hot water tank) in a 4/5 story house with old cast iron radiators throughout. My plumber is recommending a mega flow because of the cast iron radiators - he is saying that a pressurised system can cause leakage and damage to the cast iron radiators as they are porous and can't take pressure. He did say that you can put in a combi boiler, but its bad practice and could cause the damage. I asked him how big a risk, and he couldn't really answer. Naturally, I am concerned by the extra cost associated with a Megaflow (and being sure the risk is certain). One other point to mention is that the current system is pressurised (which he pointed out), but as I have only owned the house for 6 months its impossible to tell whether they have had problems in past. Any advise would be much appreciated.

 

[ChrisR]

Can understand your concern. When your plumber speaks, does the noise come from the front, or round the back somewhere?

There's
1)Primary water - the stuff in the boiler and radiators. Can be pressurised as yours is, or maintained from gravity tank in the loft. Heated in the gas boiler

2)Secondary/ domestic/ hot tap water. The stuff in the Megaflo. Can also be pressurised as it would be in the Megaflo, or a traditional copper cylinder supplied from gravity tank in the loft. Either way, it's heated by a coil of pipe inside the hot water store.
Pressurised hot water means you don't need pumps or a store of cold water above, but you do need a good mains supply, in terms of pressure and flow, which can be overly restricted by the pipe from the road. It also means you use the mains for cold water instead of the tank .

Either one, neither, or both, can be pressurised. Though there can be concerns over leaks from old radiators, after 6 months you would have seen it, in frequent loss of pressure in your heating system.

This is basic stuff - you need another plumber.

 

[gas4you]

If cast iron was porous then we would all have leaks from heating systems all the time

The risk of getting a leak is minimal. This would be more likely due to the chemical cleansing during the powerflush at the end rather than being a pressurised system, especially as it is pressurised already

The fact that it is pressurised already means that hopefully the water in the system will not be tto bad a condition as it hasn't been open to atmosphere as in a f&e tank.

There are no problems fitting a combi. I've never heard of a combi installation being bad practice

A combi has disadvantages in how it can be used i.e. don't expect to use more than 1 hot tap at a time, if boiler breaks down you lose everything until it is fixed etc.

An un -vented cylinder and a system boiler is the best option if you have good cold mains pressure, but it is the most expensive, needs more installation work and needs to be fitted by a plumber who is G3 qualified.

I agree with Chris that your plumber sounds incompetent and hasn't a clue what he is talking about, let alone qualified to fit un-vented.

Go with what YOU want fitted it is YOUR money and house, but get another installer who knows what he is doing

 

[Agile]

I agree with your plumber that older c.i. rads were not designed for pressurised systems.

However if its OK now then it will probably be Ok in the future.

More concerning is if a combi is going to be able to supply all the hot water you need.

That will depend on the mains water flow into the house. Be aware that a combi will only supply one tap at a time! Didn't your plumber tell you that?

Tony

 

[namsag]

Was there not something on here ages ago about old cast iron rads shattering on a pressurised system.

 

[ethans]

Thanks everyone for your advise, it is unbelievably helpful (as my knowledge in plumbing is poor).

Sounds like the general consensus is that I should be fine with a combi boiler on the old rads as irrespective of opinion on the matter, the system has coped with it now for many years so it should continue to in the future (I think, see point on red tank below).

However, on the point about a tank versus combi, you now raise another question - I currently have a cylinder and boiler (although he said it was pressurised as there is a red tank with a valve and meter with a bar reading that apparently adds pressure (?) - he explained it differently, but this was my interpretation). We have only one master bathroom (with bath / shower / toilet / sink on the first floor) and then a second loo (just toilet and sink on the ground floor) with the boiler (10 year old Potterton) / cylinder (looks ancient) in the lower ground and two old aspestos cold water tanks in the loft (3rd floor), which I am naturally keen to do away with very quickly. He seemed to think a Combi wouldn't make any difference to what we have now as our water demands are limited to one major bathroom. He also said that the megaflo would naturally be much better (and did say he would need to replace the pipe from street to house to increase mains pipe size), but would recommend the combi boiler for our house if we hadn't had the issue of cast iron rads.

I am not conerned about water outage if the boiler should fail (intend on getting the best the market has to offer, which I am presuming means we won't have much disruption over the coming five years we intend to live here?). I am concerned about maintaing good pressure / hot water as we have with our old sytem.

Any further thoughts / advise?

 

[ethans]

With regards to the point on cast iron rads shattering, I couldn't find anything on the site, has anyone found anything? This comment fills me with fear, as our entire system has cast iron rads (which I really want to keep, as they look fantastic and produce an enormous amount of heat).

 

[doitall]

First you must check the flow rates of the mains water, and that applies to combi or unvented, I would suggest 25 l/m being the minimum at peek times.

Then you must check if the heating is really pressurised, if you have a small and large asbestos tank, perhaps the heating side is gravity.

As for the cast iron radiators, they are fine on a pressurised system and we install them all the time.

You may have issues where the odd fitting leaks, but they can be re-made. Radiators can be broke down and re-gasketed if a section weeps.

Tony's comment about them not being suitable for sealed systems should be ignored, as above any older type column rads can be re-made if necessary.

 

[JohnD]

ethans, do you mean that you don't want a system with a cold water tank in the loft, and a hot water cylinder?

I quite like this old-fashioned arrangement, and there are some very good system boilers around that will do it (you haven't got to convert to a combi or a Megaflow if you don't want to).

When the time comes that my cast-iron boiler expires, I have my eye on the stainless Vitodens. As the system boiler has fewer bits in it than the combi, it can be expected to go wrong less often.

I am just a homeowner.

 

[ethans]

Thank you doitall, sounds like I can relax about the cast iron issue, although I am not sure how to figure out flow rates of mains water.

JohnD, this system hasn't been presented as an option. Although I am keen to preserve the history of the house, I am also keen to support progress with regards to energy efficiency. Will the system you refer to be as energy efficient as the two systems being discussed here?

 

[Nige F]

Can understand your concern. When your plumber speaks, does the noise come from the front, or round the back somewhere?


.

Does his breath smell

 

[petetheleak]

Was there not something on here ages ago about old cast iron rads shattering on a pressurised system.

Never heard so much tosh in all my life.If it was gonny leak it would have done it by now .If anything it would crack or leak at where the sections join together.

 

[Rotunda82]

Maybe make some enquiries ( with a different plumber !! ) about a condensing heat only boiler and fast recovery cylinder - this would still make for a very efficient system - as stated you dont HAVE to have a pressurised heating or hot water system if you dont want to

 

[gas4you]

Thank you doitall, sounds like I can relax about the cast iron issue, although I am not sure how to figure out flow rates of mains water.

JohnD, this system hasn't been presented as an option. Although I am keen to preserve the history of the house, I am also keen to support progress with regards to energy efficiency. Will the system you refer to be as energy efficient as the two systems being discussed here?

A combi is more efficient in that it only heats the actual water you are using, other than heating and storing a tank full all the time.

You could go with the system boiler option, efficiency of the boiler will be the same as a combi, then replace your ageing hot water cylinder with a new fast recovery cylinder that will be physically smaller in size/water content but re-heat the water faster so you still get the same amount of hot water to use.

This coupled with an upgrade to your heating controls possible change to a fully pumped system and thermostatic valves on the radiators will give you maximum efficiencey and running cost savings whilst retaining a gravity hot water system with new tanks in the loft.

I wonder why your plumber did not give you this option. Was it something you hinted that you didn't want or did you just ask for a combi to be installed so he thought it was not worth mentioning

 

[Rotunda82]

A combi is more efficient in that it only heats the actual water you are using, other than heating and storing a tank full all the time.

Depandant on your hot water consumption as a combi has to fire on its maximum for the duration of heating your hot water