Cast iron radiators - inefficient?

[Maria D]

Hi All,
We've been living in our Edwardian house now for a few months and so far, so good. I'm impressed with the woodfibre insulation we put in attic rooms. It's working well. For a variety of reasons (cost, speed, availability of tradespeople, retaining original features etc) we didn't do it elsewhere. During the renovation, we refurbished the existing cast iron rads. A couple were too damaged so we couldn't use them, so in some rooms, we bought new rads. But this seems to have created a bit of an imbalance in heating output. So, when one bedroom (with a new rad) is nice and warm, another bedroom (with an old cast iron rad) just isn't heating up as well. Retaining the heat is a whole other issue which I won't go into here! I would appreciate opinions/advice about possible solutions. We love the look of the cast iron rads and have done are best to keep as many of the original features of the house. Thanks in advance.

 

[Mulder]

Might be a balancing issue. You don't want the water to flow through the radiators too quickly - or too slowly. You adjust this by balancing the inlet and outlet valves.
Unbalanced systems can also mean that too much heat flows through the radiators nearer the boiler and further ones don't receive as much of the hot stuff.
Probably a question for a plumber, or you can do it yourself by watching some videos. Easy test would be to shut all the other radiators off and see how hot the cast iron rads get.
Cast iron rads take longer to heat up, as more mass than the new, thin steel ones, but they retain the heat for longer too.
Look up similar sized/columned modern cast-iron rads and see what BTU you can expect.

 

[Maria D]

Thanks for the advice. I had to calculate the BTUs myself and then I was restricted to the rads I had initially. I'll ask the plumber for his opinion also. I'm concerned that in a couple of the rooms, the existing cast iron rads just aren't big enough. I'll try balancing them better and see how it goes.

 

[montyjohn]

All radiators have the same energy efficiency. For a given flow rate and a given delta T drop, the output is the same.
If you mean space efficiency,. then yes a Type 22 radiator is going to take up a lot space for a given output vs a cast iron raidator.

Balancing is done by the lockshield.
How many turns open are the new radiators. Depending on where they are on the system, I would start with a quater turn from closed and see how you get on.

If the new radiators fail to get warm, increase in increments of 1/8th to 1/4. I fine it unusual that I need a modern radiator more than half a turn open.

The other valve assuming it's a TRV should just be set to 4 or 5 depending on the room use. If it's not a TRV, leave it fully open.

 

[paulc]

Balancing is done by the lockshield.
How many turns open are the new radiators. Depending on where they are on the system, I would start with a quarter turn from closed and see how you get on.

If the new radiators fail to get warm, increase in increments of 1/8th to 1/4. I fine it unusual that I need a modern radiator more than half a turn open.

When I started balancing the radiators here, even an 1/8th of a turn would be too much. Turning the pump speed down to the lowest setting helped enormously.
Then picked up a couple of cheap digital thermometers to measure the flow & return temperatures on each pair of radiator tails. Following the advice from Heat Geek, I aimed for a temperature drop of about 30% of flow temperature across each radiator. Combined with a low pump speed and running at a reduced flow temperature, my gas fired heating system appears to be running at fairly good efficiency (a relatively new condensing boiler).

 

[Cubist]

It is a 'balancing' issue but one with little to do with your systems direct flow controls.

The real root of the problem is more fundamental insofar that it concerns the mass of the radiators. Your modern radiators are no doubt manufactured from thin, sub 2mm, pressed steel and your cast iron rads will have walls in their thinnest sections that are at least 2mm and in parts up to 5mm or more thick. The modern radiator will reach its peak performance level and begin transferring heat to the atmosphere of the room much faster than a cast iron radiator would or could. Conversely a modern radiator will cool much faster - by shedding its internal heat to the rooms atmosphere - while a cast iron radiator would take much longer to do so.

You could overcome this problem with a zone control system that employs a thermostat in each room and give precedence perhaps to the rooms with cast iron rads but fine tuning via any lock shields is unlikely to do much for you.

 

[Kearn]

I’m with @Cubist on this one, try moving your stat around to change when the boiler is triggered - could be that if it’s in the room with the new rads, the air temp is turning the stat off before the cast rads have even managed to heat up…

Also, if the old cast rads have been refurbished, were they thoroughly flushed? They have a habit of filling up with gunk which again limits their ability to be fully efficient.

 

[fernicarry]

The thing about balancing CH is that what it really means is slowing everything down to match the slowest room. Either because the heat output is undersized in that room or as said because it has a lot of thermal mass so needs to run for a while to build up some inertia.

Also if you have a single thermostat, often placed in some arbitrary position that suited the guys doing the install, then the temperature at that point is what controls the whole lot. The classic example being the thermostat in the hall next to the front door where the temperature bears no resemblance to that in the living room.

Zoning room by room basically lets you throw that out the window. Let each room run as fast as it can for as long as it needs to reach its set point. The downside of this is that the slowest zone may end up running for longer periods on its own and that could drop the demand below the minimum that your heat source is happy with.
This house has a room stat in each room controlling actuators on the rads, but the heat comes from a thermal store (sometimes called a buffer store) which means the system can drop right down to almost nothing without hurting boiler efficiency. The boiler only comes on occasionally to top up the heat store when demand is low, or it can run virtually non-stop when demand is high. Circulation is by adaptive pumps that sense the pressure drop in the system and work harder when more zones are open (I think these are probably standard/mandated now).

Thermal stores do have other downsides like standing heat losses but in terms of getting the heat to where its needed as fast as possible I haven't come up with a better solution.

 

[Sunshine99]

I've also had numerous problems with some lovely old radiators ... balancing certainly can be a big issue, largely now mitigated by the fact that I've gone to a smart system so each rad takes care of itself.

Radiator size has been an issue: the room I'm sitting in now has a titchy little old rad, fine for a bedroom but not for the study that this room now is. I now regret not replacing it when I refurbed this room - I would have got something bigger in a reproduction style which might not have been an exact match but would have been good enough.

The other problem was flushing: both pipes and rads were horribly blocked and even many hours of flushing didn't clean it all up. Some pipes had to be replaced and at least one rad has never performed properly (and will be replaced if I ever get round to refurbing that room generally, though that feels unlikely now).

 

[Flyfisher]

Also if you have a single thermostat, often placed in some arbitrary position that suited the guys doing the install, then the temperature at that point is what controls the whole lot. The classic example being the thermostat in the hall next to the front door where the temperature bears no resemblance to that in the living room.

Is this really still a thing in new heating systems these days? I can see it might work for a small, well insulated, house where the occupants want to keep everywhere at roughly the same temperature but for a larger property I don't think it would provide adequate control. We keep our heating bills down by not heating some rooms at all at certain times and a single thermostat just wouldn't work - and that's before the 'thermal balancing' issues of having a mixture of UFH, cast iron rads and wood stoves - whereas a combination of zones and TRVs seem to work for us. Horses for course I suppose.

Thermal stores do have other downsides like standing heat losses but in terms of getting the heat to where its needed as fast as possible I haven't come up with a better solution.

We have a thermal store and you're right about the standing heat losses, although it need not necessarily be a downside. In our case, the thermal store is located where the heat loss leaks into an adjacent room and also into an upstairs bedroom when the window is open (sounds odd, I know, and a bit difficult to explain without a detailed drawing), so the heat is not actually lost in the sense of being wasted.

 

[LadyArowana]

Also if you have a single thermostat, often placed in some arbitrary position that suited the guys doing the install, then the temperature at that point is what controls the whole lot. The classic example being the thermostat in the hall next to the front door where the temperature bears no resemblance to that in the living room.

Now that the battery powered thermostats “talk” to the boiler they don’t actually need to be fixed anywhere. The plumber thought I was a bit mad when I said I would just put it on the shelf but I can move it around if its usual spot is getting too warm and other places where I am aren’t. The fancy WiFi ones are fixed and wired in though so I have to do a sneaky override to get past the keeper of the app who likes it a bit cold.

 

[montyjohn]

Is this really still a thing in new heating systems these days? I can see it might work for a small, well insulated, house where the occupants want to keep everywhere at roughly the same temperature but for a larger property I don't think it would provide adequate control. We keep our heating bills down by not heating some rooms at all at certain times and a single thermostat just wouldn't work - and that's before the 'thermal balancing' issues of having a mixture of UFH, cast iron rads and wood stoves - whereas a combination of zones and TRVs seem to work for us. Horses for course I suppose.

The problem with zoning off rooms, particularly with an oil boiler is they don't modulate down.
So a boiler sized for the whole house is going to short cycle if only heating a couple of rooms.
This makes them less efficient.
Add on top of that the rooms you are trying to heat are losing heat to the cold rooms you may find you struggle to get those rooms up to temperature.

It depends a lot on the shape of the house but heating the whole house means the boiler can run more efficiently and the heat loss Vs the volume you are trying to heat is a lot less.

So the savings may not be as large as you might expect when heating fewer rooms.

This is amplified massively with ASHP.

Gas modulates down well so is less of an issue. Less likely to short cycle. And if it does short cycle not as detrimental to efficiency like with a ASHP.

 

[a twig]

Worth pointing out that a lot of these “low and slow” studies that say constantly heating the house is cheaper consider 19’C as a thermostat temperature to be “low”

 

[paulc]

Gas modulates down well so is less of an issue. Less likely to short cycle.

Whilst there are a few gas boilers that can modulate down to around 3.2kW, they tend to be top end (expensive) models. The cheaper ones, you might be lucky to get it down to 6kW, so will likely short cycle in most average size homes (see comment below).

Worth pointing out that a lot of these “low and slow” studies that say constantly heating the house is cheaper consider 19’C as a thermostat temperature to be “low”

I tried the "low & slow" one day during the recent cold spell. Commanded a set flow temperature of ~40°C over a period of about 5 hours and left the boiler to short cycle when it needed to. Ended up shutting down for around 10 minutes, and then running for 20-40 minutes, producing an average 3kWh of heat - In total, 16kWh of heat for 17kWh of gas (I'd already used 7kWh first thing to get up to temperature). Normally, I'd just have the boiler firing up for a couple of hours in the morning, and the same again in the evening, generating 15-20kWh of heat. I'm still on the fence about "low & slow" or just have fixed AM/PM slots. Too many factors outside my control to say for sure which is better - When the sun is out, I get significant solar gain, and heat loss is governed by just how cold it is outside.
For reference, I have a fairly new Viessmann 050 30kW combi that can modulate down to 3.2kW (depending on flow/return temperature). Also have ESPHome acting as a thermostat (via OpenTherm) and Home Assistant to handle the scheduling.

 

[Flyfisher]

The problem with zoning off rooms, particularly with an oil boiler is they don't modulate down.
So a boiler sized for the whole house is going to short cycle if only heating a couple of rooms.
This makes them less efficient.
Add on top of that the rooms you are trying to heat are losing heat to the cold rooms you may find you struggle to get those rooms up to temperature.

It depends a lot on the shape of the house but heating the whole house means the boiler can run more efficiently and the heat loss Vs the volume you are trying to heat is a lot less.

So the savings may not be as large as you might expect when heating fewer rooms.

This is amplified massively with ASHP.

Gas modulates down well so is less of an issue. Less likely to short cycle. And if it does short cycle not as detrimental to efficiency like with a ASHP.

All good points, though we're fortunate to have mains gas so I think boiler modulation is less of an issue here. Not only because it's gas but also because it's undersized for the house so it's mostly running full-on rather than short cycling. Even then, it struggles to get rooms much above 18c and less if all zones are on, which is where the wood stoves come in to their own (one of which has a back boiler that feeds into the thermal store, which also helps).

But I entirely agree with your basic premise that heating systems can be quite a complex mix of options these days and even more so when applied to a typical period property.

 

[LadyArowana]

But I entirely agree with your basic premise that heating systems can be quite a complex mix of options these days and even more so when applied to a typical period property.

That comment had me musing as to what even is a “typical period property” our lovely old girl is Georgian, mid terrace, ashlar stone, 5 levels with the lowest a half basement. We are zoned floor by floor for heating, gas boiler, radiators. With shutters and doors to the stairwell closed a room heats up quickly while on a different level it will be rather chilly. For us a low and slow approach probably wouldn’t be sensible, low is not my preferred temperature

 

[Flyfisher]

A very fair point Milady. I'm not sure what came over me when describing properties built from very different materials over a span of 500 years as 'typical'

I shall attempt the weak defence that I was thinking about their generally poor thermal characteristics at the time of writing, though even that will also vary considerably. Hmm. Probably the only 'typical' thing about period properties is that they are 'old'. At which point I'm going to stop digging . . .

 

[LadyArowana]

A very fair point Milady. I'm not sure what came over me when describing properties built from very different materials over a span of 500 years as 'typical'

I shall attempt the weak defence that I was thinking about their generally poor thermal characteristics at the time of writing, though even that will also vary considerably. Hmm. Probably the only 'typical' thing about period properties is that they are 'old'. At which point I'm going to stop digging . . .

I think we could add “expensive to care for” to “old” I wonder if there’s anyone here brave enough to admit theirs has been completely trouble free and no more expensive to keep in good repair than a well built modern place……. silly me of course I am joking, they wouldn’t be here if they were in that happy situation now would they

 

[paulc]

I wonder if there’s anyone here brave enough to admit theirs has been completely trouble free and no more expensive to keep in good repair than a well built modern place…….

Demolition starts on 83 faulty new-build Cambridge houses

Bulldozers move on to a new development near Cambridge due to faulty foundations.

www.bbc.co.uk

And https://www.bbc.co.uk/news/business-46454844
Sometimes, "modern" and "well built" should not be used in the same sentence....

 

[Kearn]

and no more expensive to keep in good repair than a well built modern place…….

a what?!!?

There’s a new build estate nearby, called something Meadows… clue is in the name.

Cracks were seen in the slabs as they went in, no big deal.

First wave of lucky new owners move in.

6 weeks later they’re all living in a hotel as their lovely new builds are sinking…

I’ll still be putting our 500+ year old girl back together after many of the 10 year warrantied homes have come and gone!