CH boiler replacement

[stewamax]

I am making tentative plans to replace my ancient Thorn M56/76 boiler - very reliable but (due to house extensions) way under-powered.
I have measured the frontal area of all 22 radiators, and noted whether double, finned and so on, whether they are on an internal or external wall and in what direction (N/S/E/W) the external walls face. And I have estimated the maximum desired temperature of each room.
What more information do I need to size a boiler and how do I go about it?

Although not prejudging the result, I am looking at such boilers as the Valliant ecoTEC Plus 35 - I would like a stainless steel heatex but Valliant apparently don't use them on boilers for open systems like mine. Perhaps I should 'close' the system...

[sg31]

I would suggest you calculate the amount of BTU required in each room and see if the existing radiators are of adequate size. This BTU calculator should help

https://www.mrcentralheating.co.uk/btu- ... gLNFvD_BwE

There are others online if you use Google. You can then size the boiler accurately for the heat needed. I'm not suggesting you change the radiators at this stage but it seems a good idea to have a boiler that can cope if you decide any room might be a little under heated and would benefit from a new rad.

[stewamax]

sg31 is quite right: I will check that the existing radiators were sized correctly first and only then tot up the BTUs needed.

[Dod101]

Unless you are proposing to install the CH boiler yourself I would then ask your installer for his recommendation re size. After all they are doing that sort of thing all the time and should be able to give you a good idea. You can then check the one calc against the other. I would go for a bigger boiler than the minimum you think you can get away with.

Dod

[staffordian]

...I would go for a bigger boiler than the minimum you think you can get away with.

Dod

An interesting point, IMHO.

I don't know enough about condensing boilers to have an opinion, but I wonder if a smaller boiler running flat out (or close to it) is more efficient than a larger boiler running at less than full capacity.

I do know it's not necessarily straightforward because modern boilers can "modulate", which, as I understand it means they are not just on or off, but can regulate their output, presumably by reducing the gas flow. Does this mean a larger boiler will adjust to the required output, or is it nothing to do with this?

Also, I understand that condensing boilers are only fully efficient over a limited temperature range; (something to do with the amount of condensing?). Does this mean a boiler matched to the required output will be more efficient than a larger one which is "just ticking over"?

Separately, but related, is it better to have the boiler thermostat relating to the heating circuit set high or low, and does the output of the boiler influence which might be the best option?

Again, different factors might influence one's decision. I have mine quite low (around 2 on a scale of 5) because it feels better when the rooms heat up more slowly, and this tends to give a more even heat as the temperature tends to overshoot the target set on the room thermostat less than if the radiators are hotter. However this may or may not be the best way to run the boiler from an efficiency point of view.

I realise these musings may confuse rather than help the OP but they might be points which should be considered, and I for one would appreciate more enlightenment on this subject.

[stewamax]

I do know it's not necessarily straightforward because modern boilers can "modulate", which, as I understand it means they are not just on or off, but can regulate their output, presumably by reducing the gas flow. Does this mean a larger boiler will adjust to the required output, or is it nothing to do with this?

Such boilers have their own (internal) controllers that adjust the flame size according to the difference between the actual and the desired outgoing water temperatures. The latter can be set by the user on the boiler itself or by an external controller. The 'difference' is not quite the whole story because any controller worth its salt also measures the rate at which the difference is changing and (crudely...) for how long the difference has existed; these are the traditional 3-term (proportional, derivative and integral) corrections. The controller can then adjust the flame size not just in line with the gap between desired and actual outgoing temperatures but, in order to minimise overshoot (hysteresis), how fast the gap is being narrowed.

Where life gets more interesting is when there is also an external controller such as an intelligent room thermostat that works out similar (3-term) corrections according to how the room temperature changes. Such thermostats may then simply send ON/OFF commands to the boiler or may send it a desired outgoing flow temperature based on its knowledge of the boiler's current outgoing flow temperature.

Furthermore, the thermostat may also take into account the outside temperature (if such a sensor is fitted) and calculate a desired outgoing flow temperature based not just on the room temperature and how it is changing but also the outside temperature and how this is changing.

Add to this the fact that TRVs on the room radiators are also trying to maintain the rooms at set temperatures, that the building may have more than one heating zone (e.g. more than one room thermostat) and that the boiler has to supply heat for domestic hot water, the process of telling the boiler what the outgoing water temperature set-point should be - with the microprocessor in the boiler actually modulating the gas supply (hence flame size) - is not trivial!

[Loir]

Two years ago we had our gas boiler replaced. We decided on a Vaillant system boiler (we have solar water heating so have a cylinder with two coils).

All modern boilers are condensing and, as far as I know, modulate the power output to what is required. The replacement of an ancient boiler has saved us a lot of money!

Steve

[Dod101]

All modern boilers are condensing and, as far as I know, modulate the power output to what is required.

Is it true that all modern boilers are condensing? I would not have thought so but stand to be corrected. I have a rather old oil fired boiler which does the business for heating and water but is probably not very efficient. It is not condensing.
Dod

[staffordian]

All modern boilers are condensing and, as far as I know, modulate the power output to what is required.

Is it true that all modern boilers are condensing? I would not have thought so but stand to be corrected. I have a rather old oil fired boiler which does the business for heating and water but is probably not very efficient. It is not condensing.
Dod

As I understand it, all boilers, at least all gas boilers, have to be condensing now.

Not sure about oil fired ones though.

[Hardgrafter]

Do1 et al

The following web site has lots of info. Your old boiler would be SEDBUK G rated. https://www.boiler-systems.co.uk/sedbuk-rating/

Regarding the existing CH system, not only do the radiator sizes need to be checked, but the piping sizes also. You may have to change from 15mm to 22mm or larger near the boiler, in order to push out all the heat required.

Most modern boilers (e.g. WB Greenstars) include a water pump, so you could dispense with that on your old system.

There's little difference in cost between the smaller and larger capacity boilers, as a lot of the costs are in the control system.

[PrincessB]

I have some thoughts on your 22 radiators...

Twenty Two radiators! I have eight, or nine if you count the electric one in the bathroom so on this round of the radiator off, you've won by a County Mile

Back to the topic,

Radiators heat output is calculated on the temperature differential between the radiator and the room air being 50ºC higher in the radiator. So a 2kW rated rad running at 70ºC in a room which is 20ºC would output the full 2kW.

If the same radiator was only heated to 45ºC in the same 20ºC room, the heat output would only be 1kW.

I'm using kW as a unit of measure of heat output rather than BTU as I find it easier, I'm not suggesting you've got electric heating.

As a project.

With the aid of an infra red thermometer (eveyone should have one, they cost about £15) you can scan the working temperature of your radiators.

You could get a reasonable idea of their heat output by measuring the sizes and then finding the specifications of a similar sized and specced radiator by using a site such as screwfix who have a pretty good search tool on their website.

You can then calculate how much heat each radiator is actually outputting.

HTH,

B.

PS,

I learned this the hard way, my kitchen/diner extension has radiators rated at 4.5kW (or 4500watts) which were sized on the heating requirement for a new build structure which would require 2kW of heat to maintain an indoor temperature of 22ºC when it was 0ºC outside.

Thankfully, I took that number and doubled it as I was unaware at the time of the +50ºC rule.

The new room has three radiators a very long way from the boiler fed by pipes which are arguably too thin running in series.

It's 20ºC in here at present and the radiators are at 55ºC, 50ºC and one is at 30ºC (for some reason).

Basically I'm not getting anything like the 4.5kW I expected. Thankfully I've still got sufficient heating to keep the room warm regardless of the outside temperature if the heating is running. If we go away for a few days and come back to a very cold house, it takes an absolute age until the room is habitably warm again.

[stewamax]

If we go away for a few days and come back to a very cold house, it takes an absolute age until the room is habitabley warm again.

I think I mentioned on the old Fool site Jeff Howell*'s interesting contention that keeping the walls of all rooms above dew point by leaving heating on low but constantly should save money. This was because when walls drop below dew point the damp in the block-work lowers its insulation properties significantly (as well as encouraging mould).
For example, relative (external) humidity today is around 75%. If, for example, I am away and I let my my (Winter) room temperature tick over at 10 deg C, dew point is around 7 deg C. Howell's contention would then be that the walls (or at least the inner wall) should also be kept above 7 deg C. If I then switch the heating off entirely, relative humidity rises as the room temperature falls and as it approaches 100% will condense on the walls and (more importantly) within the walls - whose thermal conductivity then rises and heat is sucked from the room (which drops the air temperature further etc)
How valid this idea is in practice I don't know ... but to PrincessB point, the time needed to get the house to an acceptable temperature may not be just the time needed to heat the air and internal fabric but also the time to dry our the inner skin wall and thus raise its insulating properties.


* - the Sunday Telegraph's former Ask Jeff 'property doctor' columnist