Heat loss calculator for radiator requirements - where to find one?

[Dave]

I've tried a few but they seem to give wildly differing numbers (and one gave the same answer regardless of the inputs!), can anyone suggest one that works?

[Andy Hall]

The Myson and Barlo ones are OK.

The Myson one is not on their site at present, but I have a copy and will happily send it to you if you drop me a mail.

One of the problems with some of these programs is that they have incorrect U values for given object types - so it's always as well to check the numbers against the list in the Building Regulations Approved Documents.

The other factors that can introduce error are that some programs do not account for floor heat loss correctly. The reasonably correct approach is based on the combination of outside wall lengths and floor area and there are correction tables of modified U values to cope with it. Computer programs should do all of this as well, but if not, there will be an error. Some programs have correction factors for whether the heating is on all day, exposure of the house and so on. The defaults may not be sensible.

Finally, you do need to make sure that heat loss by air exchange is reasonably accounted for. This is a big chunk of heat requirement in most properties.

[Dave]

Thanks Andy, email sent.

Do you happen to know whether the U values in the myson software are correct? Estimating air exchange is tough, any suggestions on how to approach it?

[Andy Hall]

On its way.

The ones that I used were, but I didn't check all of them and had to add a couple.

I used the standard numbers given in the program, which are an industry standard. If the house is exposed or the windows are older sash types or you have gaps in the floorboards then it would be an idea to up the numbers a bit.

You can also look at the various correction factors and adjust to your pattern of use and location.

You don't need to include the typical 3kW or so for the hot water. This is used for older cylinders with basic coil.

Are you going for a condensing boiler? If so, you can design the radiator provision around a 70 degree flow and 50 return. This will keep the boiler at the efficient end of the scale and also give you plenty of head room for very cold weather. It does imply larger radiators of course. Don't forget the correction factors from the manufacturer data tables. These are normally 0.89 for 82/70 degree operation and 0.6 for 70/50.

[Jim Alexander]

I use Myson V1.05, not sure if that is the latest version or not. Noticed recently that there is a design program available at

for £15.

Find the Myson useful for the maths because it takes into account your mean water temperature and the room temperature but then transfer the results to a spreadsheet. There is a "funny" with this version. The required outputs are presented on a Dt60 basis but with a confusing product reference comment to Dt50 and so have to be factored down to Dt50 before they are useable with current rad data.

Jim A

[Dave]

I can size the rads for a 70 flow but can't see how to achieve a 50 return. If the house is heating from cold (and all TRVs are open) the return will presumably be much cooler than when the house is up to temperature and all/most TRVs shut; is it really possible to design for a specific return temperature?

[Dave]

Jim Alexander wrote: ... snipped

Thanks for the warning, where does it say that it's on a Dt60 basis?

[Andy Hall]

That's part of the EN 442 test standard. Normally it is mentioned in the radiator data sheet as well. You can deduce it anyway, because there is an accompanying table of correction factors for different dT values. For 60 degrees it is 1.0

[Andy Hall]

OK.

The radiator calculation is to first of all determine heat loss using

-3 degrees as the outside temperature (considered worst case) inlcuding air change losses etc. and correction factors for exposed position, intermittent use (needs more during start up) etc.

You will have figures in watts for each room.

If you work on 70 degrees flow and 50 return, then the mean water temperature is going to be 60 degrees. Let's say you want 20 degrees in the room. The Mean Water To Air temperature will be 40 degrees. Most radiator manufacturers give a correction factor of 0.6 for that. You could be a purist and use slightly different numbers for temperatures in different rooms but it probably isn't worth it.

From this, you pick your radiators. So for example, if you needed

1200W in a room, then you would need a 2kW nominal radiator.

A condensing boiler, by virtue mainly of its heat exchanger design, can manage a dT of 20, sometimes as much as 25 degrees.

If everything were precisely matched (i.e. radiators and boiler exactly matching heat loss), then the boiler would reach a steady state of 70/50 degrees and would be operating in the condensing range as well.

In practice, as you say, you won't have these levels of heat loss because the system will have a design margin and weather will typically be warmer.

Under those circumstances, when the system is heating from cold, it will go flat out to begin with. Some boilers will allow you to set the maximum flow temperature and you could set that to 70 degrees and keep the return under 50. This is good from the efficiency point of view (lower return temperatures, especially below the condensing dew point are more efficient) or you could set the control at 82 degrees and the system will heat more quickly.

With a modulating boiler, the burn rate will reduce accordingly.

If the heat demand reduces (e.g. because TRVs start to close, then the boiler detects the load reduction by monitoring of the temperatures and again reduces burn rate.

The point of designing around 70/50 is that you can operate the boiler in a more efficient range. Efficiencies of condensing boilers tend to be better than many non condensing types anyway since the heat exchanger is usually larger. The efficiency increases as return temperature reduces, but in the condensing range the *rate* of improvement of efficiency increases with falling temperature.

[Roger]

The message from Dave contains these words:

You are looking at this from the wrong perspective. The 70 flow, 50 return is an element you use in calculating the required radiator sizes. In operation you won't initially get the 70 flow as the boiler has to heat the return from cold but you should get (ignoring any losses in the pipework) a 20 degree rise across the boiler.

If the rise is too low decrease the pump speed, too large increase the pump speed. Unless you have a continuously variable pump (do such animals exist?) you are not going to get this exact and as the TRVs start closing off the circuit resistance will increase mimicking the effect of decreasing the pump speed so I think you should aim low* if you have a condensing boiler and want to benefit by recovering any of the latent heat out of the steam in the flue.

Not having a condensing boiler myself I haven't had to deal with the practicalities of this but ISTR from long ago that to get any condensing effect the return must be below 53C and that it is not an on/off effect but develops over a temperature range.

*This may not necessarily be correct but keeping the return temperature low is critical in getting the best out of a condensing boiler. No doubt someone more knowledgeable will be along in a minute to pull my contribution apart but don't take any notice of dIMM unless he is backed up by a good majority.

[Jim Alexander]

It doesn't, you need to check the data sheets to find out.

Jim A

[Dave]

This is one of those aha! moments - light has dawned and, as usual, the answer's pretty obvious once you look at it from the right direction.

Many Thanks to Andy and Roger (below)

[Andy Hall]

There are a few approaches here.

a) There are pumps like the Grundfos Alpha, which will detect increased flow resistance from TRVs starting to close, and will back off accordingly. These are outside of boiler control though and are responding mechanically.

b) There are boilers with integral pump where the typical three pump settings are controlled by the boiler electronics. The Keston Celsius

25 has this, for example, and is able to roughly match pump oomph with boiler heat output.

c) Version of b) where the boiler electronics control the pump on a continuously varying basis. The MAN Micromat does this and can vary the pump oomph between 20 and 100% of full power.

Yes. There isn't a sudden condensing efficiency orgasm that happens at the dew point. Efficiency increases with falling return temperature and exhibits a greater *rate* of this below the dew point. Ergo, the objective is to get the temperature as low as possible for as much of the time as possible.

[Jim Alexander]

Hmmm. Preparing for "compulsory" replacement condensing boilers in May 2005 (two jags bombshell No2) had a holiday project trying to find some hard data on this. Failed miserably. Any links to actual data over the full operating temperature ranges?

Yep, unfortunately my radiator system is designed for 70deg mean water temperature. Works well but has little margin for condensing operation.

Jim A

[Roger]

The message from "Jim Alexander" contains these words:

ISTR that this was raised on the ng several years ago, possibly during one of the interminable arguments with dIMM and maybe even back in the days when he was pretending to be Adam. However finding that item is beyond me.

Think yourself lucky. I am not sure mine was designed at all but when it falls to 10 below outside I have to run mine continuously to have any chance of the temperature approaching 19C at the times I want it to. I have a round tuit buried quite a long way down the pile that had the materials necessary for insulating the walls (without robbing me of too much room space) priced at £1000 with the cost of plastering throughout on top of that. Not sure if the payback point would be anywhere short of infinity.

[Tony Bryer]

Does the graph on

help?

[Andy Hall]

Several things here.

1) There are to be some exemptions which will basically operate if replacement of an existing conventional boiler with a condensing one will be too disruptive. This will be cases related to flue arrangements, condensate drain etc., and a points scoring system is used. Thus if you have a backboiler with conventional flue in the middle of the house, then you may score sufficient points not to need to replace with a condensing model. However, if it's a wall mount in the kitchen, you would be pretty unlikely to meet exemption criteria. Details are at 2) There are some typical curves for condensing boilers on the Keston web site. 3) It doesn't matter if you have an existing radiator design for the more typical 82/70 boiler arrangement. When the weather is really cold, the boiler will wind up to these temperatures. When it's warmer, less heat will be needed and it will modulate down and try to operate at as low a temperature as possible. So for much of the year, you would still be operating in the more efficient range below the dew point. Obviously if you take the situation over the whole year it won't be *as* good, but still worthwhile.

[Jim Alexander]

Guess we have both read the rules but for the education of anyone who hasn't my gripe is that although the guidance claims to avoid situations which are "too costly" the cost is NOT taken into account, only theoretical disruption. Not only that but the acceptability of the lowest cost assessed position to the householder and existing furniture and fittings are totally ignored in the assessment. Not a gripe about the installer but he will get his money. Absolutely a stealth tax on the hard-up. An election looms does it not?

Thanks for the Keston link. Seems to me that the main efficency advantages acrue from the larger heat exchanger.

How exactly does it do that? Maybe I have got this wrong but I thought the modulation worked by reducing the gas rate to control the flow temperature to its setting. I would have to manually adjust the temperature according to the weather to achieve lower flow temperatures?

OK, I've turned the temperature down, doesn't the water heating take an age now and give me legionnaires disease?

Not if its a condensing combi but I'm not going there.

Jim A

[Andy Hall]

A stealth tax on everybody - just like all the rest.

Yes indeed. Whether all of these intrusions into people's lives and the stealth taxes will be sufficient to influence how they vote is another thing though.

Until you get down below the dew point.....

There are a couple of ways.

If you have TRVs, they will respond to the rooms becoming warm and reduce the flow. The boiler is monitoring the resulting changes in temperature of the water and will respond to it by adjusting output. Since the heat required to be put into the room to maintain a given temperature varies with outside temperature (plus a lag), there is going to be a correlation between burn rate and outside temperature.

Some boilers have outside temperature monitoring and weather compensation. The effect here is that the boiler now knows directly what the heat requirement is going to need to be.

You can always adjust the water temperature down. Obviously, the upper limit needs to be enough to provide adequate heat under cold conditions. Other than that, cooler water means longer warm up time.

It depends on the boiler and its connection arrangements. If it has a way of knowing that the heat demand is for a water heating cycle, then it can go to a higher temperature for that. If not, even if the flow temperature is reduced to 70 degrees, it will be enough to maintain a cylinder at 60 degrees.

Quite. The point is though that the boiler does then know the difference between HW and CH.

[Phil Addison]

Interesting, but 100% efficiency at 20C could do with some explanation.

Phil The uk.d-i-y FAQ is at

NOSPAM from address to email me