Cast Iron Period radiators - efficiency & programmable TRVs?

[Simes123]

Soon to complete on a period Church conversion.  All external walls have been insulated - not sure how well yet, and it has ceilings and a well-insulated attic space.  One feature that has been retained in some rooms are cast iron radiators.  I'm interested in energy efficiency, but rather like these old Rads and would probably like to keep them for the same reason that the current owners have.  Thinking of fitting modern programmable TRVs (though they would detract from the period look) as an efficiency measure, but am wondering if I might be better off fitting modern radiators instead.  What are the pros and cons, and will it make that much difference?  There are modern rads elsewhere on the circuit that are disguised/boxed in with vents for convection.

Re programmable TRVs - like the idea, but think they will just look wrong on a period Rad.  What other options are there if I want to get some kind of per room control (it's a big property, and I can feel the bills looming!).

The property has a modern combi boiler, and the period rads have been thoroughly flushed and pressure tested.  Yet to move in, so not sure how they will perform in practice yet!

Appreciate any thoughts.

[Billy]

I have some rather nice cast iron radiators running off a gravity system.  Cast iron stuff generally has a high water content which means a long heat up time and therefore not a quick system to react.  Modern systems use very low water content, quick heat up and response.  As for your TRVs spoiling the aesthetics, would it be possible to have remote valves and sensors?



billy
 

[Countrypaul]

Could you use a normal thermostat linked to a motorised valve some distance from the raditor? The thermostat could be located whereever in the room is appropriate and the motorise valve could be under the floor/in a cupboard/ in the next room basically wherever was not detracting from the radiator. More expensive than the TRV admitedly, but 1 thermostat might control several raditors in one room.

Paul

[Baz]

A good quality TRV should open progressively not bang/bang so the volume &b thermal mass shouldnot be an issue. Also the energy content of even an old radiator is probably only 1/2 KWh so not going to be that big  an overshoot unless in a small room.
You could fit the TRV but then paint and disguise it in some period or appropriate way that allows air circulation. Something carved and fretted with wood and brass will fit the bill.

[Simes123]

Some good ideas - thanks!  From a bit of digging around, I see that "period" appearance TRVs are available (albeit not programmable ones) - these would satisfy aesthetic considerations as an interim.  I'm hopeful in the end state of being able to control individual rooms to different temperatures at different times of day and/or days of the week.  I like the remote valve idea as that would lend itself to some kind of centralised remote control, but think it might be cost prohibitive given we'll have just bought the house.

I think most typical CH systems are on a loop though (? - not sure - I thought upstairs, downstairs?) - I presume I'd have to somehow segment the system to get more granular control, to include "demand" calls to the heating if I were to use motorised remote valves?  I'm enthusiastic and a competent diyer, but the design of plumbing systems is not my forte, even if I can solder a joint effectively! 

With a TRV mounted on the radiator, I'm still not clear in my mind how that affects the circuit on which the rad sits.  Presumably there is a feed to the rad, which the TRV restricts or allows based on temperature, water flows through the Rad, and exits from the return and continues on the circuit.  What happens if the TRV shuts off completely - is there some kind of bypass for the radiator to allow HW to flow to the other rads in the circuit?   I'm an Electronic Systems engineer by profession, not a Plumber if you hadn't guessed

Simon

[Billy]

Si,

You know all about it already, if the trv is shut then the water bypasses and goes to the next rad or back to the heat source or heat dump.  Just the same as electrics in a way, the heat dump is a zenner or heat sink perhaps and heat source is battery or mains (potential difference).  Just substitute the water for leccy.  Well ish anyway.  If the trvs are all shut then the heat source senses that and shuts down.

Modern systems can be zoned which allows for separate areas to be controlled. So if your bedrooms are not used until the evening/night then they can be kept at a conservative temperature when compared to the living areas.

billy
 

[Simes123]

Billy,

Thanks,

I'm not sure if I've quite got it with the rads yet?

In electrical terms, I'd imagined them as a series circuit.  Rad 1 in - heat - Rad 1 out to Rad 2 in - heat - Rad 2 out to Rad 3 etc.   For each TRV, I suppose I imagined the TRV to be a resistance at the input of the Rad, thus I wasn't clear how if the flow to Rad 1 was restricted, what impact it would have on Rad 2 etc.  If the TRV is shut, is there a bypass for the rad in parallel to take the fluid to the next Rad - ie, is there a branch (under the floor I guess) to bypass the Rad should the Rad valve be shut off? Si

[knighty]

there's 2 pipes, 1 feed, 1 return, all radiators in parallel :-)


that's why the tap on the other end of the radiator (the one you don't normally touch) is supposed to be just cracked open 1/8 of a turn

so you get even flow through all of them :-)

[Simes123]

Ah, the light comes on (doh).  Thank you - all makes sense now

[Simes123]

OK, a further question that springs to mind as a result

I've seen some people suggesting that a Programmable TRV has a limitation in that it cannot demand heat from the boiler.  I understand that entirely from an electrical perspective given that there is no connection to the boiler (I realise there are some programmable TRVs that are exceptions).  However, if I'm now understanding correctly, if the TRV opens up to meet a higher demand temperature from the room, water will circulate through the Rad, lose heat to the room (at a rate governed by temperature differential to the room, the Rads surface area, convection efficiency etc), and thus the return feed temp to the boiler will drop, with the consequence that this drop in temp will be sensed by, and will trigger the boiler in any case (if necessary).  So there wouldn't be anything to gain from having the Programmable TRV contact the boiler electrically to demand heat, even if it could?  I imagine the hystereses loop might be a little large with overshoots occuring, but the system would still be closed loop.  Have I got it about right?

Simon

[knighty]

yes

when people say "they can't demand heat from the boiler"

I think what they mean is... nothing will happen if the boiler is turned off (on a timer etc..)

or if you have a thermostat, which is in a warmer place than your radiator/valve

[Simes123]

Ah perfect - thanks.

I hadn't considered the timer issue - the spousal unit always overrides the timer when she needs heat in our current place, but the rads all have TRVs.  I suppose that could be an issue during a period of absence if the Frost prevention setting were set on the valves, but the timer prevented boiler start up.   The (rented) house we are currently in, is not fitted with a wall stat at all.  It was explained that it wasn't required given all the rads (bar the usual one) are fitted with TRVs.  I presume if a wall stat were fitted, you'd set it higher than the highest temp you are likely to demand from the TRV to avoid them competing.

I was wondering as a consequence if boilers are fitted with a frost prevention override to the timer?

Thanks for helping me understand!