How do I work out what size a replacement boiler should be ?

[D_Hailsham]

And,if you do decide on a combi ,unless the house is massive,there is no need to even consider heating output

So you buy a WBGreenstar 42CDi for the hot water capability. But the heating requirement for your well insulated house is only 10kw and the boiler's heating output is 9.8-30kW. Therefore the boiler will be running in on/off mode virtually all the time. Not very efficient.

 

[tonybhoy]

Ehr,no. Boiler is fully modulating on heating and hot water plus you can,if you want,have that particular boiler start on a lower output for the heating.

Boiler leaves the factory on 100% but than be adjusted down. Never had to do that though cause most of the time when I'm installing that boiler the heating load will be near the 100,000 BTU heating output.

Never read MI for a combi in recent years that required you to do anything other than check the gas rate/WP at the boiler whilst commisoning said appliance. There is no adjustment required according to the MIs.

Years ago you had to set the heating load manually on the PCB but not now.

 

[Agile]

Those who know a lot about boilers and heating systems do set the heating output to match the load as this complies with the energy efficiency advice and ensures more efficient operation of the boiler.

The manufacturers do mostly say that its not "necessary" to adjust the CH output but thats not the same as saying that its not beneficial.

As Hailsham says, a powerful combi can only "modulate" on an on/off basis which is less efficient. Its an unfortunate byproduct of having a powerful boiler to give good hot water potput.

Tony

 

[tonybhoy]

Well that's all moot isn't it? More important to fit a room thermostat and control the boiler that way rather than tinker about with sevice settings which fankly installers should steer well clear of.

And, the WB 42 CDi isn't a 42KW output boiler for heating ,it's 42KW only for hot water and of course it condenses on both heating and hot water.

MIs are the most important instructions re the working of the boiler and I'm sure if manuacturer's wanted us to adjust the service settings then they would provide us with that info in the MIs,surely?

 

[Agile]

Manufacturer's get a lot of problems from installers fiddling with the boiler settings when they dont really know what they are doing and they get an easier life if people dont fiddle!

So they dont say its imperative that the heating setting is matched. Any competent engineer will know how much better it is when it is matched as well as increasing the efficienncy.

Just yesterday, I went to a newly fitted boiler where the installer had "adjusted" the min gas pressure wrongly causing it to lockout on minimum gas pressure because the setting was less than zero.

That cost the installer £84 for me to sort out!

Tony

 

[D_Hailsham]

the WB 42 CDi isn't a 42KW output boiler for heating ,it's 42KW only for hot water and of course it condenses on both heating and hot water.

I did not say that the boiler gives 42kW for heating; I said:

boiler's heating output is 9.8-30kW

That means the lowest it can modulate down to is 9.8kW. So, if you have a house which only needs 10kW max - i.e to provide a room temperature of 20°C with an outside temperature of -1°C - when the outside temperature is higher than -1°C the boiler will not be able to modulate low enough and will revert to on/off mode.

 

[David937]

I agree with the the comments regarding inefficient boiler operation if the boiler has been sized purely for the DHW requirements.

We have a boiler which has a modulation range of 12kw-28kw and virtually all the time it is cycling.

 

[tonybhoy]

[

That means the lowest it can modulate down to is 9.8kW. So, if you have a house which only needs 10kW max - i.e to provide a room temperature of 20°C with an outside temperature of -1°C - when the outside temperature is higher than -1°C the boiler will not be able to modulate low enough and will revert to on/off mode.

Well you had better tell the boiler manufacturers to amend their MIs.

And,frankly,I install per the boiler manufacturer's instruction not some post on an internet forum. This point re the MIs is drummed into us when we go for assessment BTW.

I doubt wether the vast majority of boiler installers even know there is a service menu you can enter to adjust the heating load simply becuase there is no mention of it anywhere in the MIs. Only way you find out is by going on a boiler manufacturer's service course which is where I learned there was a service menu for the Greenstar CDi range.

And,BTW you implied the 42 CDi was the total output of the boiler by using that figure first

 

[snb]

there is a service booklet for engineers with the greenstar boilers which includes all this info. Comes with the boilers or you can download it

 

[D_Hailsham]

BTW you implied the 42 CDi was the total output of the boiler by using that figure first

Read what I said!

So you buy a WB Greenstar 42CDi for the hot water capability. But the heating requirement for your well insulated house is only 10kw and the boiler's heating output is 9.8-30kW.

"42CDi" is the boiler model number, it does not imply that the output is 42kW; and I was careful to say that the heating output is 9.8-30kW.

 

[tonybhoy]

Anyway,chances of fitting a 42KW combi boiler(17 ltires hot water per/min) in a small flat with a heating load of only 9.8KW is not high is it?

 

[Agile]

Three of the worst cases of over sizing boilers that I have seen were small three bed semis with a heat load at a max of 10-12 kW but they had been fitted with heat only 32 kW boilers.

One of them by a CORGI and one bought by a woman who thought the biggest would be the best. She had serious problems which were not helped by her unreg builder who fitted it.

Although everyong is talking of a heat load of about 10 kW its important to remember that is the MAXIMUM when the temp outside is -1°C outside.

For most of the heating year its 5-17°C outside and that makes the heating load more like 3-6 kW which gives an on/off ratio for the boiler of about 50:50 or 60:40.

Tony

 

[D_Hailsham]

Anyway,chances of fitting a 42KW combi boiler(17 ltires hot water per/min) in a small flat with a heating load of only 9.8KW is not high is it?

It was only an example.

But take David937's comment:

We have a boiler which has a modulation range of 12kw-28kw and virtually all the time it is cycling.

He has a Vaillant 937, chosen specifically for the HW performance. The HW output is 37kW and CH output is 12-28kW.

As houses get better insulated, the heating requirement will reduce.

I have a 4-bed detached, built in the late 1980's. It met the minimal insulation standards of the day, so single glazed wooden windows, no cavity insulation and about 2 inches in the loft. The boiler installed was 15kW open vented. The house is now loft and cavity insulated with double glazing everywhere. The boiler output needed is now 8-9kW. So, when I change the boiler (still going after 20 years), I would need a combi which modulates down to about 3kW with a HW output of about 30kW. You can't get one! The lowest any WB combi goes down to is 7.2kW and Vaillants's is 6.7kW.

 

[David937]

The walls of our house a solid and the loft is insulated, however the loft which is my room get's very cold because the insulation is for when the loft is not used as a room i.e a cold roof. That's what you get if you use roofers who don't know what they are doing.

It must be said though that most of the cycling is caused by the Honeywell CM927.



You don't have to get a combi do you D_Hailsham?

 

[gas4you]

The walls of our house a solid and the loft is insulated, however the loft which is my room get's very cold because the insulation is for when the loft is not used as a room i.e a cold roof. That's what you get if you use roofers who don't know what they are doing.

Or ch installers who do not know how to size the rad properly

It must be said though that most of the cycling is caused by the Honeywell CM927

Personally I wouldn't class that as cycling as the post intimates. Actual cycling of a boiler because it is too big (can't modulate down), is a different kettle of fish than a room stat clicking on an off