Radiator sizing calculators

[overlander matt]

I'm looking at ordering some cast iron radiators shortly to replace some rather miserable 1980s panel radiators. I have fitted cast iron rads in my last two houses and never really got the sizing right using online calculators which tend to vary in the number of variables they consider. Does anyone have a good experience with one that they could recommend for an older property?

Another question - would locating the radiators below sash windows be a sensible option where possible?

Thanks in advance.

Matt

 

[88v8]

I used the Ravensbourne calculator when I installed iron rads in two houses, 1977 and 1982, but it was a book and I've no idea where it is now, sorry. There was a good deal of subjectivity such as the number of air changes per hour, and estimating U values for non-standard constructions.
It worked well, in that we had roughly the target temperatures in each room, all running off one stat in the hall.

I think the important thing is to use the same methodology throughout the house, then the system will be basically balanced. If in doubt, go 5-10% larger on the rads, then you won't have to run the boiler quite so hot to get the rad output.

However, if you're planning to keep the boiler, there is no point in hanging more rads on the system than the boiler can heat, and if the house is adequately heated now, the overall heat requirement vs the boiler size can be a useful yardstick against which to check your estimates.
Ideally, allow an extra 20% on the boiler size for cold snaps.
The hot water cylinder, iirc, was estimated to absorb about 15,000 btu, although now I write that it looks a lot. Hmm, 4-5kw, maybe it's OK.

Again, if the rooms are currently comfortable, you can look up the existing rad outputs with reasonable accuracy.
Although I didn't use them, thermostatic valves can also cover a multitude of calculation sins, I believe one can get 'period' lookalikes nowadays.

In general, rads should always go under windows so as to minimise the thermocline across the room. Sometimes the 'pros' will put them back to back either side of an internal wall to minimise the pipe runs but this is just wrong. The worst place is opposite a window, that will guarantee an orrible draught across the room as the rising warm air from the rad meets the falling cold air from the window.
Where it's impossible to put them under a window - French doors - then either side of the window.

In larger rooms, it can be better to have two small rads rather than one big one. Don't put them behind an ottoman, for instance, as I did in our sitting room, it dunnarf block the heat.

When placing beneath a window, a shelf or window board should be provided above so that the curtains can sit on the board with no gap for the heat to go up (says he ungrammatically).

Wall-mounted rads are best, imho, as they can be rotated down from the wall by undoing the top bracket, and then one can paper behind them, and paint the back of them, and they don't impede flooring work. I fixed mine with 3/8" coach screws in no 24 fibre Rawlplugs.

You know to apply the minimum of paint to iron rads; just a flash, otherwise it will eventually crack.

Ivor

 

[paulc]

wrong.[/i] The worst place is opposite a window, that will guarantee an orrible draught across the room as the rising warm air from the rad meets the falling cold air from the window.

If the property is well insulated and has good double glazed windows, the radiators can be mounted elsewhere in a room. Current recommendations is in the coldest part of the room which traditionally used to be under the window. I've been replacing and moving the radiators here as I renovate each room. Putting a radiator on an internal wall opposite a window hasn't resulted in any cold draughts. Moving the radiator away from the window now means I can hang heavy curtains without worrying about tucking them behind the radiator when drawn.

 

[livias]

I'm not a fan of under the window rads because I like long curtains
I put my rads in the most inconspicuous place possible but I put them in old timber framed farmhouses where they are completely inappropriate. If you have a Victorian house then old style radiators would not be out of place in the same way
In a larger room I put two well spaced rads and I think to some extent that is more important than size because the body notices a temperature gradient

 

[88v8]

If the property is well insulated and has good double glazed windows....

Yes, that could be influential. It perhaps illuatrates a difference between installing in a modern house where one may have more freedom with the radiator layout, and a period property with no dg and little insulation where a traditional layout will produce a more even temperature.
But other than the curtains - and curtains are important decorative features and good insulators in their own right - I think there is no detriment from placing the rads under the windows.

Ivor

 

[Flyfisher]

I'm looking at ordering some cast iron radiators shortly to replace some rather miserable 1980s panel radiators. I have fitted cast iron rads in my last two houses and never really got the sizing right using online calculators which tend to vary in the number of variables they consider. Does anyone have a good experience with one that they could recommend for an older property?

In short, no. For basically the same reasons you’ve already discovered regarding the many variables. We’ve been gradually rolling out radiators in our house but we also have wood burners or open fires in the main rooms so we really use the rads as a means to keep the chill off while not using the room and then use the fire to bring it up to ‘working’ temperature when it’s occupied - which is quickly achieved if not starting from a freezing room. In this way the sizing calcs are less critical.

But I appreciate this approach is not for everyone and if I was doing things differently then I’d probably just install a much larger radiator than even the calculators suggest (maybe multiple rads) because the effective capacity can easily be ‘dialled down’ using thermostatic valves but cannot be ‘dialled up’ if the system is undersized. I say ‘probably’ because cast iron rads are certainly not cheap!

 

[overlander matt]

Thank you for the suggestions. Perhaps I should have mentioned that the house is neither well insulated nor double glazed. It has rather large Georgian sash windows at the front (two in each room) which on the ground floor has a space of only 40cm below the glass. This might well dictate the radiators go elsewhere... The first floor bedrooms have around 80cm below the glass and could well accommodate the radiators. There is of course the consideration of curtains and how they interact with the radiators... The windows all have their original shutters which might help on the draught front. Unfortunately, as we have only just bought the house I don't know how the old heating system performed. All I know is the previous owners used to go to Tenerife each winter!

It looks like oversizing the radiators will be the best course of action. That does however get a bit costly... The pipework for the old heating system is rather hefty - at least 30mm diameter feed and return pipes on the ground floor and without insulation below the floor voids! I think I will look at a zoned system in the house which will hopefully be a bit more controllable. Like many others here, there will be supplementary wood burners around the house that will provide additional heat.

Thanks again.

Matt

 

[Abi Cott]

I've gone for vertical column radiators between the windows on my front wall, 500 x 1800mm seemed to supply a more than adequate btu, allows curtains to the floor on the adjacent windows and tackles the issue of a thermocline? Admittedly mine are modern column radiators, the budget didn't stretch to Victorian style, and it's a Georgian house anyway.

My room is much much smaller than yours, but tall and thin might be worth looking at, for at least one heat source.

I spent a lot of time looking at TRVs there are some traditionally styled ones, but as with the radiators I opted for modern with a traditional twist rather than traditionally styled.

 

[Flyfisher]

We've done the same as Abi, a radiator between two large sash windows that have insufficient room below them for radiators.

We could squeeze an 1100mm radiator between the windows but were limited in height as we couldn't find anything taller than a metre in cast iron (probably just as well as we'd have to strengthen the floor!).

 

[fernicarry]

I think I will look at a zoned system in the house

I'd highly recommend this. Zone the system, down to individual rooms if possible, then you don't need to worry about balancing the heat gain across rooms, just go as large as you have space for/can afford in each room. The conventional wisdom of balancing the system so that each room heats up at the same rate just means that the whole house has to go at the speed of the slowest room. Which is kind of mad in a traditional house where you are almost never as warm as you would like to be.

With a thermostat per room each can call for heat as and when needed and being programmable you can put unoccupied rooms on a lower temperature. I work from home so the bedroom I use as an office just runs 9-5 on weekdays. When the kids are away their rooms are turned off which holds them at a set-back temperature.

The heating is supplied from a thermal store so it will happily supply small loads like just one room calling for heat. Most boilers won't be able to modulate this low so bear that in mind if you do go for zones. A thermal store is a good idea for lots of reasons. Use a modulating pump that will balance flow rate against demand. This save electricity but also reduces the circulation noise when not all zones are open.

We plumbed all our ground floor rads individually back to a manifold (the sort used with UFH) hidden in the kitchen so all that's on show are plain brass valves.

 

[overlander matt]

Thanks for your replies. I am very pleased to hear from you Fernicarry having read a little about your system elsewhere. The idea of a thermal store is an interesting one - I considered it briefly at our last house before eventually just installing a gas boiler and unvented hot water cylinder. This time I think it might be worth further consideration... It is a larger house and has the potential for multiple heat sources although I suspect it will probably just be a boiler and wood burners.

We did take a brief look at ground source heat pumps but the upfront cost has proved prohibitive - £50k for the system and £35k for 7 boreholes! Whether this becomes a more cost effective option in time, who knows? I feel the thermal store might well provide some level of futureproofing if compatible with new technology. I shall aim to oversize the radiators to function with a lower supply temperature with this in mind too.

I have a lot to learn about thermal store systems and wonder how complicated it really is? Unfortunately, I don't know anyone who has such a system to ask some basic questions. To start with, is there a brand or company you would recommend and do they supply all of the ancillary items you needed to build your system? Do you have a photo of your system?

It's interesting that you ran individual pipework to all rooms on the ground floor - was this in copper or plastic piping? How many rooms was this? How have you configured the upstairs rooms? Sorry for lots of questions.

Our house has three floors and I was considering two zones on the ground floor (one being underfloor heating in an extension), two zones on the first floor (the front part of the house is Georgian with big windows and the back is Victorian with smaller windows and better insulation) and one on the top floor. I had thought this might be too complicated before learning about your system...

Thanks again for your reply! Food for thought.

MAtt

 

[Flyfisher]

We also went down the thermal store route and have used it to connect a gas boiler and a multi fuel ‘boiler stove’ into the heating system. The store tank has spare coils for possible future expansion (I keep thinking about solar thermal) and the heating outputs are divided into three zones, one of which is UFH, though we may add others later - so not quite the ‘one zone per rad’ system fernicarry has described but similar.

We also looked into a gshp but, like you, found the cost to be prohibitive - and we didn’t even need boreholes! In fact, worse than that, since we got connected to mains gas there was little or no prospect of breaking even on the investment in our lifetime, never mind actually save money. Yes, heat pumps are around 300% efficient but since they run on electricity that’s around 3x the cost of mains gas there is effectively no cost saving to be had over mains gas, at least for the foreseeable future. No doubt this will change eventually and if we’re still here when it does then a heat pump could be easily plumbed into our existing system.

One other point. While we’re pleased with our very effective boiler stove, we are fortunate to have plenty of wood available for free (albeit with lots of physical effort!). If we were buying our firewood then I’m pretty sure we wouldn’t use it everyday in winter, as we do, as it would be cheaper to heat the house solely on mains gas. In fact, as far as I can tell, on a pound-per-kilowatt basis, it’s cheaper to burn smokeless coal than purchased firewood!

 

[88v8]

..... has a space of only 40cm below the glass. This might well dictate the radiators go elsewhere...

These chunky chappies are 13" high, designed for use under windows.
https://www.carron.uk.net/carron-radiator-range/victorian-9-column-cast-iron-radiator

Probably not cheap, but one only buys them once.

Btw, for those without central heating but who wish the look of iron rads, I see they do heating elements.
https://www.carron.uk.net/radiator-heating-elements

Your uninsulated piping, inch and a quarter, might have been for a thermo-syphon system. Yes, all piping should be lagged!

Ivor

 

[fernicarry]

I have a lot to learn about thermal store systems and wonder how complicated it really is? Unfortunately, I don't know anyone who has such a system to ask some basic questions. To start with, is there a brand or company you would recommend and do they supply all of the ancillary items you needed to build your system? Do you have a photo of your system?

The principle is very simple really, its pretty much basic plumbing, heat exchangers, pumps, thermostats and the like. Its all the same stuff thats inside any boiler but exposed as individual components. Any plumber ought to be able to understand it but not all plumbers are created equal... I was lucky to find someone who was willing to try something different. Lots of tradespeople just stick to what they know they can make work regardless of whether its the best outcome for the customer.

There are a few companies that make copper cylinders that offer some variation on the theme. You can usually customise them at the time you order in terms of size and number/type of connection points (tappings or coils). Mine came from Newark and a friend, who was so impressed with mine, got his from McDonald. The rolls royce solution is Akvatherm. As flyfisher says you want a few spare tappings for future tinkering. We just did a major rearrangement when we changed the boiler a couple of months ago so the spare tappings were definitely beneficial. You will need to add pumps, valves, thermostats yourself to make a complete system. There is a company floating around that supplies fully finished systems but I found them to be a lot more expensive than buying components and I was still stuck needing someone local to plumb it in anyway (I wasn't quite as hands on then as I am now).

I'll add a pic or two if I can figure out somewhere to host it.

Re: zoning it really depends on your pattern of occupation. For our use case where I'm upstairs during the day then a conventional upstairs/downstairs split doesn't work because I'd want upstairs to be on and my wife would want downstairs to be on and then we'd basically be heating the whole house. Again a tradeperson almost certainly won't ask this question, they'll just do what they usually do, big house == upstairs/downstairs split.

It's interesting that you ran individual pipework to all rooms on the ground floor - was this in copper or plastic piping? How many rooms was this? How have you configured the upstairs rooms? Sorry for lots of questions.

This was more out of aesthetics than anything so I could hide the gubbins away and leave my new cast iron rads in their pure form. We were also relocating the radiators (to the correct position under the windows) so were replumbing most of downstairs anyway. Downstairs we have 5 zones, 6 if you include the UFH in the kitchen but thats a separate circuit, upstairs I think is 7. Upstairs is pretty conventional, we didn't touch the plumbing at all. Just unscrew the TRV head from the radiator valve and screw on an electric actuator. The actuators are all cabled back to a wiring centre under the stairs as are the room stats (all Heatmiser). A few radiators at one end of upstairs still have TRV's on them but I'll get to them in due course.

Wireless systems are available but I have an aversion to anything that requires a button cell battery to be replaced every few months. If you are renovating anyway then a few extra cables under the floor won't make a difference you just need to plan in advance. No doubt you'll be putting in TV, audio, network and alarm cables anyway as I did(!!). All our plumbing is in copper on account of the meeces.

Happy to spew forth about this all day, PM me if interested. I really think its the single most transformative thing we have done to the house. We have gone from shivering underneath blankets in the winter and an electric shower that was just a trickle to being really quite comfortable (with thick socks and a jumper of course) and endless hot water.

 

[george2019]

I need a completely new system put in... I was thinking along the lines of a standard boiler + HWC with Honeywell TRVs to do the zoning. I take the point about batteries, but they should last two years.

Is the end result basically the same as what you've had put in?

 

[fernicarry]

If you mean Honeywell EvoHome then certainly looks like its attempting to do the same thing but not familiar enough with it to compare. I'm more of a home-brew guy when it comes to technology.

I've read that the battery powered radiator controls can make an annoying noise since they use a tiny motor to wind the pin up and down. The mains powered ones I use have no moving parts, instead they heat up and the expansion of wax inside moves the pin (ok, technically thats still a moving part...).

Bear in mind that if the load is very low due to only one or two rads being open then the boiler is still likely to short cycle. For example ours only goes down to a fifth of its max load thats 35/5 or 7kw, which is enough for 2 or 3 rooms. Some only go down to a quarter. The boiler will detect the load is too low and turn off for a while meaning you get no heat despite what the controls are telling it. Putting the thermal store between the boiler and the radiators solves this problem. On its own that doesn't justify a thermal store but its one of several reasons to consider one.

 

[Flyfisher]

The principle is very simple really, its pretty much basic plumbing, heat exchangers, pumps, thermostats and the like. Its all the same stuff thats inside any boiler but exposed as individual components. Any plumber ought to be able to understand it but not all plumbers are created equal... I was lucky to find someone who was willing to try something different. Lots of tradespeople just stick to what they know they can make work regardless of whether its the best outcome for the customer.

Amen to that!

I knew basically what I wanted to achieve but wasn't at all sure about the best way to go about it. I was also fortunate to find someone who could design a system from scratch rather than just plumb in a standard configuration and who actually seemed to relish doing something a little different.

I wouldn't say I was 'lucky' to find the right person because I spoke to quite a few people and it was fairly easy to detect the ones who either didn't understand what I wanted, or didn't know how to achieve it or simply weren't interested - which is often the same sort of thing when talking to anyone about old houses! But you'll know when you've found the right person because they'll have the necessary open mind, will be genuinely interested, and will be able to offer sensible answers to all the inevitable questions.

And what a joy it is when you do finally find the right person!

 

[overlander matt]

It sounds like we are aiming for a similar system to you Flyfisher with a log burner and gas boiler as heat sources. Luckily the room we plan to locate the boiler backs on to the chimney with the log burner so at least plumbing this in should be fairly simple. I have seen a couple of suitable burners with 12kW ratings - split roughly 5:7 room:boiler. Is this powerful enough to contribute significantly to the thermal store? With small log burners in previous homes, I managed to get away without installing additional ventilation, I guess a 12kW fire needs a separate air supply...

The chunky radiators look good Ivor. I've been running through my calculations tonight and now have a large spreadsheet with various size rads for each room in the house! It will be decision time soon. Ultimately, it looks like we will try to spec the radiators to fit the available spaces, surpassing the minimum room requirements by 20-50%. That said, the price of the chunky rads is approximately twice that of the 4 or 5 column rads with an equivalent output... For the two rooms in question, the rads will be a bit pricey.

I will PM you Fernicarry at some point soon to find out more about your system. It is definitely of interest and I am just getting to grips with the preliminaries of what we can do at the new house. Finding a plumber/heating engineer is proving a bit tough around the area. Everyone seems to want to work in plastic. I did wonder if there was some merit to it if running pipes to individual radiators from a manifold. Even if it were possible to keep the mice at bay I can appreciate it is frowned upon in the heritage property world. I guess I need to start another spreadsheet to estimate the copper costs...

A further question on the thermal store - is there any option to have additional tanks for hot water closer to the point of use? The boiler room might be convenient for the woodburner connection but it will be a long way to the kitchen and bathrooms.

Thanks once again.

Matt

 

[fernicarry]

A further question on the thermal store - is there any option to have additional tanks for hot water closer to the point of use? The boiler room might be convenient for the woodburner connection but it will be a long way to the kitchen and bathrooms.

Usual way to deal with this is something called a secondary return. You bring a pipe back to the heat source from the point of use and add a circulating pump and various check valves. When the pump runs its like opening a tap, water flows through the heat source, up to the point of use but then comes back through the secondary return. Its like a mini-radiator loop.

It can be wasteful in terms of energy use if the water isn't consumed immediately because the pipes will eventually cool down and the pump will run again to heat them up. Typically you would put it on a timer to just run at peak times of day. I've put the thermostat for mine under the upstairs hall so the pump should cut out as soon as hot water reaches the furthest point. Mine is designed in but not yet operational as we haven't tackled the front bathroom yet. Very much a convenience thing but actually mandatory in some parts of the world where water is more scarce than energy.

That said, if you change from a gravity system to a mains pressure one you should find the time taken for hot water to come through goes down quite significantly. Especially if you also change from 22mm to 15mm for the feeds to each point of use as there is then less water in the pipe to draw off before the hot comes through (but 15mm may not be enough if you are planning some sort of mega-shower).

One thing about integrating the wood stove is the controls. Our old gas boiler would start to struggle to keep up once outdoor temps dropped below about 5' so we'd light the stove to make up the difference and they generally got on fine together. The new gas boiler on the other hand can easily keep the store temperature sufficiently high all on its own, at least it has this winter. This makes the stove turn off its air supply because it thinks its own boiler is hot enough (its seeing hot water that the gas boiler heated up) and it dies down and goes all smokey. Not good for the stove, not good for the environment, PM's and all that. To counter this I've taken to turning the gas off when I light the stove but that does mean that eventually the radiators cool down so then you have to turn the gas back on. I have a few ideas to automate this by holding off the gas boiler based on the temperature of the wood stove return so that the gas boiler runs part time. I think Flyfisher has mentioned something similar as well.

Btw, its also possible to pump from the wood stove if you get one suitable for sealed systems. There aren't too many of these available but means you can site the stove anywhere you want. This is what we had to do. Predictably the first stove man who came to look swore that where we wanted the stove was physically impossible and we never heard from him again...

 

[88v8]

....surpassing the minimum room requirements by 20-50%.

All this complexification is well beyond my pay grade, but just a couple of points:

Larger rads = more metal and more water, more mass to heat. Responsiveness is not a strong point with iron rads, all that extra mass will not help.
The way that Ravensbourne recommended to deal with cold snaps was to oversize the boiler by 20%. Their point is that the rate of heat loss increases as the external temperature reduces and so when it is cold, radiators are able to provide more than their rated output. With a bigger boiler one has spare capacity when an unusual amount of heat is required.
It's also cheaper than oversizing all the rads.
I suggested a 5-10% oversize, but only to take care of the uncertainties in calculation.

Of course, if you are planning to run the system at lower flow temperatures than usual, then you are right about the oversizing.

A minor point... arrange for an 'always on ' loop in the bathroom, meaning it will be on in all year when hot water is being produced. Take it to a towel rail with a TRV. That way you will always have nice warm, dry, towels.
Could also be useful in a laundry room, if you have one.

Ivor