30kw or 21kw boiler.

housebuyer143

I had a quote for a new boiler and the guy is really nice, seems to know his stuff with good reviews. 
He gave me the option of a 21kw boiler with 10yr warranty or the 30kw with 12yr. 
He says that the house doesn't need 30kw but it does future proof if I wanted to get new bigger rads (which I might) and they boiler is modulating so it won't necessarily pull 30kw all the time. Also said in the settings there was an option to manually reduce it? I suppose I don't want to have really high bills. 

Just checking what people think - should I go for the 21kw or 30kw? The price is £200 different. 

Rodders53

What do the other two or three Quotations recommend and how do the prices compare.

Brand/model of boiler?
Combi or System?

Have you considered Heat Pump and Grant from Govt?

housebuyer143

Rodders53 said:

What do the other two or three Quotations recommend and how do the prices compare.

Brand/model of boiler?
Combi or System?

Have you considered Heat Pump and Grant from Govt?

The other one went with 24kw 🙄 so who knows! This guy is fairly priced I think, and a bit cheaper than the others I had - he also has the qualifications to work on cylinders which I think is useful.

The recommended ones are Worcester Greenstar 4000 21kw, or Greenstar 8000 30kw. System boilers. 

Not really thinking of a heat pump at the moment - we don't really have the right sized rads or insulation and currently need a new boiler as ours isn't working. 

Albermarle

For some reason, heating engineers tend to overspec the power of the boiler needed.
A typical 3 bed semi could probably manage with a 12 KW system boiler.
For example I have a bit larger than average 4 bed older semi, with 13 radiators of various sizes and my 18Kw boiler heats the house up with no problems.
Also you should look at the minimum KW the boiler can modulate down to. The lower the better. 

FreeBear

housebuyer143 said: He says that the house doesn't need 30kw but it does future proof if I wanted to get new bigger rads (which I might) and they boiler is modulating so it won't necessarily pull 30kw all the time. Also said in the settings there was an option to manually reduce it?

Crunch the numbers for each room using this calculator - https://www.stelrad.com/basic-heat-loss-calculator/ - That will tell you what size radiators you need. I'd suggest using a Delta T (ΔT) of 40°C (or 30°C if you plan on getting a heat pump at some point). A ΔT 40°C gives you a flow temperature of 60°C which will give you better boiler efficiency - Lower is better as it pushes the boiler further in to condensing mode.

Once you know how much heat you really need, you can then decide on boiler rating.

I recently fitted a heat meter to my CH system so that I had a measure of just how much energy was actually needed. Bearing in mind, a typical 3 bed property needs around 7-8kW, and I have around 10kW of radiator, the boiler only runs at 3-5kW (I need a cold spell to get more data). It is just as well that the boiler I have (rated at 25kW for heating) will modulate down to 3.2kW.

If I recall, you have a Megaflow tank for DHW - The specs should tell you what the reheat times are for various boiler outputs. No point in going bigger if the specs say 18kW gives you an acceptable reheat time, unless the Stelrad calculator is saying you need a lot more for heating. The other factor to consider is the minimum output of the boiler. If you need 5kW of heat for the CH and the boiler will only modulate down to 7kW, it will spend a lot of time short cycling. That is bad for long term reliability and gas consumption.

Below is the relative modulation curve for my boiler (Viessmann 050 30kW). As you can see, apart from an initial 20 minute burst at 32% (11kW), it runs at 7% (5kW) for ~40 mins, before dropping to minimum (3.2kW) for the last 50 mins. If it couldn't go below 5kW, it would have been short cycling.

Y axis is a relative %, not actual output in kW.

Another plot, this time of actual power output in kW.. Since fitting the heat meter, the CH hasn't been running very often.

housebuyer143

Albermarle said:

For some reason, heating engineers tend to overspec the power of the boiler needed.
A typical 3 bed semi could probably manage with a 12 KW system boiler.
For example I have a bit larger than average 4 bed older semi, with 13 radiators of various sizes and my 18Kw boiler heats the house up with no problems.
Also you should look at the minimum KW the boiler can modulate down to. The lower the better. 

Yeah, I have the same sized house and the 18kw one isnt that bad, but it can struggle (probably age though) - so to get 30kw to me it just sounds insane. But I think because you can manually adjust the KW (range rating it's called), he has suggested it, to get the longest warranty. 

I like him, but not sure I'm sold on the 30kw boiler! 

Looks like they both modulate down to 1:10, but I found the 21kw model stated as going down to 3kw.

FreeBear

housebuyer143 said: Looks like they both modulate down to 1:10, but I found the 21kw model stated as going down to 3kw.

What make/model of boiler is he recommending ?

There are not many that will modulate down to 3kW. Some WB and Viessmann models do, but they tend to be the exception.

housebuyer143

FreeBear said:

housebuyer143 said: Looks like they both modulate down to 1:10, but I found the 21kw model stated as going down to 3kw.

What make/model of boiler is he recommending ?

There are not many that will modulate down to 3kW. Some WB and Viessmann models do, but they tend to be the exception.

This is the Greenstar 4000 21kw

daveyjp

Unless you live in a wing of Windsor Castle a 30kw boiler for a heating circuit is OTT, whether it modulates of not.  
 

The 30kw may also be much larger.

FreeBear

housebuyer143 said:

FreeBear said:

housebuyer143 said: Looks like they both modulate down to 1:10, but I found the 21kw model stated as going down to 3kw.

What make/model of boiler is he recommending ?

There are not many that will modulate down to 3kW. Some WB and Viessmann models do, but they tend to be the exception.

This is the Greenstar 4000 21kw

Not quite a 10:1 modulation range, but being able to go down to 3.2kW is certainly good. Plenty of capacity if you fit larger radiators - Even then, I suspect it would spend much of its time ticking over at 6-8kW. And it should give you a rapid reheat time for the Megaflow.

It is vital that you fit a smarter thermostat that can call for reduced heat and also up the flow temp when heating the tank (often referred to as Priority Domestic Hot Water). With a smart(er) programmable thermostat, you can set different temperatures throughout the day. That could potentially save ~10% on gas, although the saving is likely to be around 5%.

Now that you've had quotes for a gas boiler, get one for a heat pump and see how it compares - https://octopus.energy/get-a-heat-pump/ (you don't need to be an Octopus customer).

housebuyer143

FreeBear said:

housebuyer143 said:

FreeBear said:

housebuyer143 said: Looks like they both modulate down to 1:10, but I found the 21kw model stated as going down to 3kw.

What make/model of boiler is he recommending ?

There are not many that will modulate down to 3kW. Some WB and Viessmann models do, but they tend to be the exception.

This is the Greenstar 4000 21kw

Not quite a 10:1 modulation range, but being able to go down to 3.2kW is certainly good. Plenty of capacity if you fit larger radiators - Even then, I suspect it would spend much of its time ticking over at 6-8kW. And it should give you a rapid reheat time for the Megaflow.

It is vital that you fit a smarter thermostat that can call for reduced heat and also up the flow temp when heating the tank (often referred to as Priority Domestic Hot Water). With a smart(er) programmable thermostat, you can set different temperatures throughout the day. That could potentially save ~10% on gas, although the saving is likely to be around 5%.

Now that you've had quotes for a gas boiler, get one for a heat pump and see how it compares - https://octopus.energy/get-a-heat-pump/ (you don't need to be an Octopus customer).

Thanks, so 21kw should be ample? The extra warranty is good, but it just seems crazy to get a boiler that's so large (in dimensions also!).

I did the heat pump and they don't install in my area 😞