What size boiler

[D_Hailsham]

sizing a boiler to match possible undersized rads is quite literally the stupidest thing I have ever heard.

Even worse is sizing the boiler to match possible oversized rads. This is easily possible if the insulation of the house has been improved.

size the boiler for the heatloss of the house plus any extra you think you may need for hot water or any future extension.

That's what I said earlier.

also find a boiler that has a good modulation range that can meet your needs.

for example if the heatloss comes out at 15kw then a boiler that can go from 4kw-30kw is better then one that can only range of 9-16kw...

Very sensible advice, but few installers thinks about it. Considering that a boiler runs for most of the time below the calculated output, the lower a boiler can modulate, the better. It's a pity other manufacturers don't follow the example of Geminox, who produce boilers which can modulate between 1kW-10kW or 2kW-17kW.

To the OP

You can find the output of your radiators by using the Stelrad Elite Catalogue.

If the rads add up to less than the calculated boiler size, you have a choice: Install more/larger rads or put jumpers on when it gets very cold. If, on the other hand, the rads add up to more than the calculated size, you dont' have to do a thing. It just means that you will be able to run your boiler at lower temperatures.

 

[bengasman]

sizing a boiler to match possible undersized rads is quite literally the stupidest thing I have ever heard.

Hence my remark about allowing for possible upgrades and/or additions.
As the op made no remarks along the line of: "2 of the rooms are often too cold....", or even "usually ok, but in the really cold periods we had last year... "it is fair to presume he is quite happy with the temperatures in the house and thus empirical data prove the rads to be sized correctly and do not need any upgrades.

$64,000 question:
What is more stupid: installing a bigger boiler than needed because some theory says it's too small but the user is quiet happy with the way things work, or installing a boiler based on the size of the rads plus allowance for upgrades/additions and ignoring a general theory?

 

[bengasman]

sizing a boiler to match possible undersized rads is quite literally the stupidest thing I have ever heard.

Even worse is sizing the boiler to match possible oversized rads. This is easily possible if the insulation of the house has been improved....

Another clever contribution from our resident householder/google hero.

Oversized rads an even worse problem
All rads ( unless they are categorically too small ) are oversized during 10-12 months per year. That is why professionals install trv's where needed and make sure there is an accurately working roomthermostat.

 

[GeorgeBainbridge]

The Intergas has a standard pump. Do you know what that is?

Didn't Standard become Triumph in the 60s?

Yes, and they used the same water pumps.

 

[ajrobb]

Lifestyles vary. Undersized radiators may be adequate for the occupants.

Oversized radiators are a gift for condensing boilers with weather compensation. They are not worth paying for, but worth keeping if you have them.

But I see that sticking with Y-plan is not going to give the condensing efficiencies of W-plan with separate flow temperature controls for CH and DHW (or even a combi with no HWC).

 

[D_Hailsham]

Oversized rads an even worse problem.

I did not say that; read what I said:

Even worse is sizing the boiler to match possible oversized rads. This is easily possible if the insulation of the house has been improved

To take a simple example:

A house had central heating installed with a 15kW boiler and 15kW of rads. (I am ignoring hot water for moment.) Since then, the insulation has been improved so the heating requirement is now only 10kW. Do you still install a 15kW boiler or would you now install a 10kW model?

The only advantage of a 15kW boiler is that it would allow the house to be heated to 20°C when the outside temperature dropped to -11°C!

As ajrobb rightly said:

Oversized radiators are a gift for condensing boilers with weather compensation.

All rads ( unless they are categorically too small ) are oversized during 10-12 months per year.

I assume you mean 10-11 months? However, you are just stating the obvious. In any case heating systems are sized for the expected "worst case". But, if recent winters are any indication, the -1°C outside temperature used may have to be reduced to -3°C or even lower.

 

[ajrobb]

With W-plan, must one choose DHW priority or CH priority or is there an alternative? For example, CH priority until DHW drops below 45°C and then switch to DHW priority. Except in the most extreme weather, there should be plenty of time available to heat the HWC to 60°C between CH demands. As long as the HWC reaches 60°C for at least one hour a week, there should be no legionella issue.

 

[GeorgeBainbridge]

Lifestyles vary. Undersized radiators may be adequate for the occupants.

Oversized radiators are a gift for condensing boilers with weather compensation. They are not worth paying for, but worth keeping if you have them.

Weather compensation control does not gain by deliberately oversized rads.

Weather compensation control and:

1. A condensing boiler
2. Rads oversized to operate at 60C temp maximum with a 20C delta T across the rads.

The above then improves matters in boiler efficiency greatly.

The method of sizing rads in present systems was devised in non-condensing systems to avoid the boiler condensing - keep it all hot. Running temp of the rads is 80C. Even the coils in the cylinders.

The rads are sized for when it is -3C outside and wanting 20C inside. The boiler is sized to suit to meet this criteria. That is, when -3C outside the boiler is always on and will not cycle. It is working flat out.

If the temperature is higher than -3C then the rads are oversized. That means that in 95% of run time the rads are oversized. If the boiler supplies CH only and sized accurately for the criteria then when the temperature outside is higher than -3C it also is oversized. If the boiler has extra capacity for DHW then it will clearly be oversized at all times.

Weather compensation with room temperature influence will reduce the flow temperature to rads to maintain the room setpoint. Say the outsides temp dictates a boiler flow temp of say 50C. When the temperature approaches the room temp setpoint the flow temp will reduce to quite a low temp until setpoint is reached and then off, until just below setpoint and then the boiler temp will come in low and rise up. So, when the room is up to temp, the boiler flow/rad temps will be very low indeed promoting great efficiencies. The boiler will be very low coming in and out at a low temp just below the room temp setpoint. If someone opens a door and the room temp drops quickly to say 18C the boiler will come in at the full 50C to get back to setpoint ASAP.

 

[GeorgeBainbridge]

With W-plan, must one choose DHW priority or CH priority or is there an alternative? For example, CH priority until DHW drops below 45°C and then switch to DHW priority. Except in the most extreme weather, there should be plenty of time available to heat the HWC to 60°C between CH demands. As long as the HWC reaches 60°C for at least one hour a week, there should be no legionella issue.

Good point. The Gledhill thermal store did this. They held off the DHW reheat until the store temp was below 60C. If the store temp was below 60C it was DHW priority (reheating the store), if above 60C both CH and DHW were on.

The Avantaplus range of boilers allow direct cylinder stat connection to the boiler and direct connection of a 3-way diverter valve, giving DHW priority. These are dual-temp boilers, able to have separate CH and DHW temps. It can go to full power on DHW reheat and drop to the CH setting when on CH. Or drop to weather compensation control on CH, if WC connected.

Having the temperature differential that you state and Gledhill did/do, improve efficiency as the all the boiler power is directed to reheating the cylinder.

The temp differential switch can be obtained by using two cylinder stats and small £5 relay from Maplin. This can be used on Avantaplus.

A combi heating a direct cylinder and using a brass pump, does the same as the W plan, but better and enabling a downsizing of the cylinder. It can be improved using the two cylinder stat setup maintaining a cylinder temp differential.

To clarify, the W Plan is a Honeywell term for a priority system using a 3-way diverter valve.

 

[GeorgeBainbridge]

A house had central heating installed with a 15kW boiler and 15kW of rads. (I am ignoring hot water for moment.) Since then, the insulation has been improved so the heating requirement is now only 10kW. Do you still install a 15kW boiler or would you now install a 10kW model?

The only advantage of a 15kW boiler is that it would allow the house to be heated to 20°C when the outside temperature dropped to -11°C!

It will also heat up the house much faster. I assume a fixed rate burner here. That is where modulating burners come in. If the needs are 10kW and a modulating 5kW to 20kW boiler is fitted the reheat will be quicker and boiler cycling far less. All dependent on the control of course.

 

[D_Hailsham]

The Avanta[plus] range of boilers ... are dual-temp boilers, able to have separate CH and DHW temps. It can go to full power on DHW reheat and drop to the CH setting when on CH. Or drop to weather compensation control on CH, if WC connected.

You only get dual-temp working on these boilers when it has has weather compensation control, which can be as simple as an external sensor.