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Thread: Oversize / Undersize Rads

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    Default Oversize / Undersize Rads

    As I've got some valves on my evohome based system that need to be replaced, I'm also thinking about replacing some radiators, a like for like replacement is the obvious thing to do, but are there any advantages to either under or oversizing replacements on an Evohome controlled system ?

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    Moderator Kevin's Avatar
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    I’m thinking that undersizing creates problems when using optimised start causing earlier and longer runtime. It could also result in near constant running or cycling of a system if a zone cannot reach its setpoint.

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    This is something I have looked at as at some time in the future my 21 year old boiler will need to be replaced. In a number of places I have come across advice that if your radiators are more than 15 years old, such has been the advance in their design and efficiency, and also because of simply their age, it is better to also replace all radiators. I then started to research what might be needed to find that the efficiency of today’s radiators is such that smaller ones will suffice. That gives me a problem as I want to avoid pipe replacement and many of the radiators are not as long as older designs. There are some though where the length is compatible; the height is less.

    My son has recently bought a new house and I was amazed to see how small his radiators are for the rooms they are to heat. Partly due no doubt to greater insulation to comply with today’s standards in a new build but also it reflects the increase in efficiency that today’s radiators provide.

    I would start by measuring each room and there are sites on the internet that calculate the BTHU needed for each room. From that you can then choose modern replacement radiators.

    We do have some central heating installers in this forum so hopefully one or two might chip in with their more professional thoughts.

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    Quote Originally Posted by Kevin View Post
    Iím thinking that undersizing creates problems when using optimised start causing earlier and longer runtime. It could also result in near constant running or cycling of a system if a zone cannot reach its setpoint.
    I think it's probably not a good idea to have a mix of oversized and undersized radiators (relative to their individual room heat requirements) around the house.

    Otherwise when it gets cold it only takes one undersized radiator which is struggling to meet it's set point to call for maximum heat (100% duty cycle from the BDR91 or max flow temp from the Opentherm bridge) and then your boiler is outputting maximum flow temperature more or less continuously while many other rooms may have a much lower demand. So those other HR92's end up having to close themselves right down to a small flow, but it can result in temperature overshoots and oscillations in those other rooms that have normal or oversized radiators as they are getting a high flow temperature when their heat demands don't require it, causing difficulty for the HR92 to smoothly regulate the room temperature in those rooms.

    For the same reason an individual radiator that is really oversized relative to other radiators in the house may cause more tendency to oscillate or overshoot in that room. That oversized radiator would work best if the maximum flow temperature was reduced, however then other rooms without oversized radiators would not be able to reach their set points.

    If you set every room to 20C with the doors closed and let the system stabilise for a few hours ideally you'd like to see roughly similar heat demands from the different zones on the controller. You wouldn't want to see 100% demand from an under-performing zone or two and say 10% or even 0% from other zones as this would indicate a significant imbalance in radiator sizing between rooms.

    I see these kind of issues in my house - despite a relatively large panel the hallway radiator struggles to get the hallway to 20C in the coldest parts of winter, probably due to under door draughts including a slightly draughty front door, (can't do much about it short of replacing the entire door frame and door, which is on the to-do list...) and unless I turn it down a bit it will sit there calling for 100% heat demand from the boiler while other rooms may be at 10-30%.

    Conversely, both our bedroom radiators are massively over-sized large double panel convectors, they can tend to overshoot and oscillate if there is a high heat demand somewhere else (like say, the struggling hallway) causing a high flow temperature as they'd ideally prefer a lower flow temperature for optimal control.

    Just my opinion anyway. Over-sizing radiators is fine as long as they're all oversized proportionally and there aren't any laggards.
    Last edited by DBMandrake; 13th December 2018 at 09:06 AM.

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    Quote Originally Posted by G4RHL View Post
    ...... if your radiators are more than 15 years old, such has been the advance in their design and efficiency............ to find that the efficiency of todayís radiators is such that smaller ones will suffice. ...... it reflects the increase in efficiency that todayís radiators provide.
    Please forgive a note of pedantry here, but all radiators are 100% efficient. All of the heat lost by the water goes into heating the air around the radiator. There are no other losses. So changing your radiators will make no difference to your system efficiency. Bigger radiators or radiators that provide more convection fins may extract more heat from the water, but they will still be 100% efficient. You will not be getting any free heat!

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    Radiator specification has gone through a number of changes which is still catching a few people out. The key is to understand about delta-T's (ie dT or ΔT), which radiator water temp relative to room temp and the BTU/kW (heat loss/requirement) of each room.

    Radiators used to be rated at a 60dT, but EU regs have forced them to be sold at 50dT. The higher the dT, the smaller the radiator needs to be for the equivalent BTU/kW output. In effect, the reduction of 10dT makes radiators bigger to compensate for the BTU/kW output for a room, and it's for this reason people 'think' that radiators are more efficient. They do have slightly better fin design to make them 1 or 2% more efficient, but really, they're just bigger for the equivalent BTU.

    Heatpumps and such like generally advise to use underfloor heating because it's very difficult for a heatpump to raise the temperature of water to 50dT efficiently. However, while it's the 'nicest' option, you don't have to use UFH. Heatpumps work most efficiently with temperatures of 35-40 degrees, meaning they need 35-40dT heat emitters. However while a 35dT radiator is possible, the reality is it will be about 3x the size of a normal rad. And hence why UFH or forced convectors (ie fan coils/aircon-style units) are generally used instead. The point being is that the lower the dT the 'better', but you should keep in mind the practicality of the physical size and space of radiators. And generally try to design your entire heating system to the same dT.

    The key to radiator sizing is to first find out the heat loss characteristics (ie the BTU/kW requirement) of each room. You will then know the BTUs. Then you need to decide how hot you want your radiator water to be. For obvious reasons, the lower the water temperature, the more efficient, but conversely the bigger your rads are. I would advise to at least 'see' what a 40dT setup would look like and if the rad size is impractical, scale back to 45dT and then 50dT. In my experience 35dT with rads is highly impractical as you'll probably need massive K3 rads in all rooms.

    Now in terms of evohome, there are 2 factors to consider:

    1. Although evohome makes it much less important, good old fashioned radiator balancing will still help, which means fiddling with the lock shield (ie the other valve) on the radiator, which will ensure that all radiators heat up roughly the same if all TRVs are open (balancing should be done with no TRV installed). This should avoid most of the issues DBM is talking about.

    2. With more potent radiators, evohome will call for less heat, meaning your boiler will spend more time condensing and/or run at lower temps. If you're also modulating, it's a double win for using considerably less fuel. evohome's learning features mean it will assess how long the room needs to get up to temp, regardless of the water temperature, but so long as the water temperature is consistent.

    So with all that background, the best heat calculator I've found that's not charged for and used by a full MCS-accredited engineer, is this one: https://www.ncm-pcdb.org.uk/sap/file...ms---V1.2.xlsm

    This is far more precise than most BTU calculators and is the best for figuring out the heat characteristics of your house. Once you've plugged in all the dimensions, materials, windows, doors and neighbouring spaces on the other side of walls, ceilings, floors on the 'Room Inputs' tab, then you can go to the 'Emitter Specifications' tab, and see what the BTU requirement is for any given dT.

    With this in hand, then figure out the maximum physical wall space you can have in a room where you want to put a radiator. In general it should be under a window or near-ish a door. For example, I wanted a radiator in my north facing bay window, but that meant the radiator could only be 500mm by 1200mm max.

    With your room heat requirement BTU/kW's and the maximum physical size requirement in mind, it's finally time to choose the radiators! Have a look around to see if you want conventional, flat panel, or a designer radiator, and select based on both it's aesthetic and it's BTU/kW ratings. While all radiators will have a BTU/kW rating at 50dT, to find their ratings at different dT's, you will need a bit of Google-fu. For example, I'm using Ultraheat rads, which have their BTU requirement on this page: https://www.ultraheat.co.uk/premier-...-1457w-wh.html, and to convert to 35-40dT, you need to refer to the 'delta factors in C other than 50dT' in this PDF: http://www.ultraheat.co.uk/media/tec...-technical.pdf (ie 62.2% of the 50dT for 35dT or 74.3% for 40dT).

    Rinse and repeat for all rooms, and you'll have your rads sized to each room for your house, with wife understanding why it needs to be a ridiculous thing in some rooms and compromising with a nice designer contraption in another.

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    Thank you jvallis. Helpful.

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