Boiler sizing query?

[Ezeeno1]

Hi,

Looking for a bit of advice please, I've had a new boiler installed by a local guy, a good guy who I've used before and I'm just not sure if the boiler has been sized correctly and not being a plumber I just wanted to understand if I'm being OTT.

Previous boiler - 18KW ideal
Tank - 210L megaflo
New boiler - baxi 800 16KW, heat only

no heat loss calcs were done, when they got the boiler out the box I questioned whether 16KW was enough and they said yes, it's what you had before (I since found out I previously had a 18KW)

Again, no calcs where done but for context, the house is;

Detached
5 bed
1805 square feet (c. 168m2)
3 floors
2 showers
1 bath
17 wall rads
1 plinth heater

They had set the boiler to 90 degrees, outside temp was 10-11 degrees, house was 17.5 degrees at 5:15am, heating came on to rise to 22 degrees, by 6:50am it was only 20.2 degrees.

I turned the boiler down to 65 degrees, following morning same outside temp, 5:30am the house was 18.7 degrees, heating came on to rise to 22degrees, by 7:30am it was only 19.9

The above heat rises and the times taken seem way off to me when discussing with others how their heating works

Engineer says he wouldnt recommend boiler at 65 which is a fair comment, I've since set it to 70, but I think for the size of the house, a 16kw boiler and running at 90 degrees just doesn't sound ideal or economical.

He's being very good about it and said he's never had this problem before and he will think of a plan, personally I can't see anything other than doing the heat loss calcs and sizing accordingly, where I suspect a 24KW boiler would be ideal, with possibility of 30KW

Can I please get your thoughts?

Thanks

 

[CountryFan]

Might be slightly undersized, but needs to be based on rad sizes really. Did the Ideal work ok? Why was it changed? Loosely based on 17 rads, at say 1Kw per rad, that’s 17Kw, so already undersized imo. Why do you think you need 24 or 30? That’s just overkill. Setting to 90 degrees is mental, supposed to be going lower, not higher, than as you have stored hot water it needs to be at least 65.

 

[Ezeeno1]

The

Might be slightly undersized, but needs to be based on rad sizes really. Did the Ideal work ok? Why was it changed? Loosely based on 17 rads, at say 1Kw per rad, that’s 17Kw, so already undersized imo. Why do you think you need 24 or 30? That’s just overkill. Setting to 90 degrees is mental, supposed to be going lower, not higher, than as you have stored hot water it needs to be at least 65.

The old boiler broke; PCB, and was more economical to replace boiler as the boiler was 17 years old as is the house

I've seen recommend calcs online that say 1.5kw per rad and then 3-6kw for the megaflo which I was I've got 18 rads x 1.5kw = 27kw, plus 3kw for the megaflo getting to 30kw

I did mention about the temp and he said he wouldn't recommend the boiler in 65 because it wouldn't heat up the tank, but they had previously left it on 60 when they installed my hive, I've since set the boiler to 70 degrees to allow some temp drop whilst getting to the megaflo

 

[drives]

Your house won't have a heat loss of 24-30kW. The average 4/5 bed detached is probably 10kW max.

65 outlet flow temp should be fine for tank if you set it at 55 (which again should be more than hot enough for your DHW demands).

Are all of the rads getting as hot as they should be? 90oC flow temp is ridiculous. 65 should be more than adequate to get your house up to temp unless your radiators are very small or balanced incorrectly/flow limited for some reason.

First steps if it were me would be to check the rads and check the pump is providing adequate flow rate.

 

[Ezeeno1]

Your house won't have a heat loss of 24-30kW. The average 4/5 bed detached is probably 10kW max.

65 outlet flow temp should be fine for tank if you set it at 55 (which again should be more than hot enough for your DHW demands).

Are all of the rads getting as hot as they should be? 90oC flow temp is ridiculous. 65 should be more than adequate to get your house up to temp unless your radiators are very small or balanced incorrectly/flow limited for some reason.

First steps if it were me would be to check the rads and check the pump is providing adequate flow rate.

Forgive me ignorance, I am not a plumber but if 4 and 5 bed houses are classed as large houses in the UK and the baxi 800 is a resi boiler, why are they made up to 30kw?

There is No visible control on the heatrae sadia megaflo. All rads get hot, they balanced the system before they left, unsure about the pump and I don't know how to check it but they don't raise it as an issue.

Am I right in saying though, with the boiler set to 65 degree, and without you knowing specifics of my house, you would still expect more than 2.2 degree heat increase in 2 hours when the outside temp is a mild 10-11 degrees?

 

[drives]

Forgive me ignorance, I am not a plumber but if 4 and 5 bed houses are classed as large houses in the UK and the baxi 800 is a resi boiler, why are they made up to 30kw?

There is No visible control on the heatrae sadia megaflo. All rads get hot, they balanced the system before they left, unsure about the pump and I don't know how to check it but they don't raise it as an issue.

Am I right in saying though, with the boiler set to 65 degree, and without you knowing specifics of my house, you would still expect more than 2.2 degree heat increase in 2 hours when the outside temp is a mild 10-11 degrees?

Outdated thinking when it comes to sizing boilers, general lack of proper heat loss calcs, habits when it comes to heating homes in the UK with blasts of high heat rather than slow and constant perhaps.

If all the rads are getting hot I can't really see how a more oversized boiler would help matters anyway. Regarding reheat times they don't sound outrageous to me if you are reheating from set back. What outlet flow temp did you have the Ideal set at? A few degrees can make a significant difference. Is the boiler cycling on and off excessively or does that side of things seem ok?

Worth reading:

https://www.heatgeek.com/articles/heat-loss-calculations

You can also check the outlet and inlet flow temps when your boiler is running which would be worth looking at, to see if all running as expected and generally consistent outlet at whatever you've set it as.

In summary I'd be amazed if the sizing of your boiler is a (or the) problem.

 

[camerart]

Hi E,
I read that it is ore efficient to have a smaller boiler working hard, than a large boiler cruising? C

 

[Exedon]

Should have been a full room by room heat loss calculation done, only accurate way to size boiler.
Also with a heat only boiler I'd use one that can do hot water priority . Boiler is sized to rad heat load only (no dhw allowance).
Then when heating on boiler can run at lower temperature to keep it it in condensing mode longer.

 

[drives]

The OP is right though that the vast majority of online sizing guides direct 1 to 1.5kW per radiator, but no real rationale as to why. Does this assume it’ll be one rad per room and each room has an average heat loss of 1.5kW? The heating hub and heatgeek seem to be the only online resources that recommend things should be done differently. Interestingly they are the only reference material to back up their recommendations with actual heat loss and a bit of science, rather than a ‘rule of thumb’.

I presume 28/30kW heat only boilers are sold, because that is what people want to buy even though they are hugely oversized in most scenarios.

 

[ericmark]

My house also 5 bedroom, I never heat all rooms at the same time, bedrooms heated at night, living room heated in the day, so the radiator size needs to be a lot larger than the boiler size.

Also, one thermostat to switch boiler on/off is not going to work, not saying need a thermostat in every room, but when some rooms heated in the day and others at night, you need at least two thermostats which can turn the boiler on.

There is a big difference to maintaining the temperature and recovering the temperature. To get rooms warm fast, I heat them in sequence, but also I found the TRV heads had anti-hysteresis software which was OTT. So would set the room to 22°C at 7 am, then 20°C at 8 am, to get around the anti-hysteresis software.

This varies make to make, with Wiser it is claimed to work out how long it will take.

 

[Londoner2]

I have a 5 bed mid terraced house, rooms are not that big, during buiolding works in 2019 i installed new combi 30kw boiler without sizing n calcs, replaced some older rads and it worked out ok.

 

[drives]

I have a 5 bed mid terraced house, rooms are not that big, during buiolding works in 2019 i installed new combi 30kw boiler without sizing n calcs, replaced some older rads and it worked out ok.

Combi completely different as would have to be sized for hot water demand.

 

[Justin Passing]

Your rads are reasonably modern so see if you can find the right sort of range. Eg they might be Barlo or Ideal Stelrad..
Measure them all, make a table up and see what they're capable of dissipating. Not that hard, you'll have several the same size probably.
The data will say what the output(dissipation) is for a given rad temp.
It'll be a lot less than the max, when your boiler is working efficiently (ie condensing), at about 56° or below, so that'll give you a different number from the old standard, of 80° on the flow at max..

The coil in your Megaflow is capable of using a lot more than 3kW. Iirc more like 20kW, = the whole boiler. If you put the water on before the heating in the a.m., it'll heat up pretty quickly and leave you all that boiler output for the heating.

Then I'd do a heat loss calculation for a couple of rooms. Your house at that age should be quite well insulated. Even so, check that your rads' outputs are enough, with the boiler condensing. Don't use a rule of thumb, they tend to overestimate, so check what your walls are made of, etc.

That should give you and idea whether the original specifier, over or under did the rad outputs.

When the house is heating up, you aren't just balancing the heat losses, you're changing the temperature, so you need excess so it gets there in reasonable time. I don't remember seeing a figure for how much more, ot a time. I may have forgotten. A changeable parameter here is how early you put the heating on. Also if you're prepared to wear another layer, etc! There isn't/wasn't much agreement. 'Everyone' is likely to say you're wrong. I prefer to be more inefficient and get the house up to temp fairly quickly, than have it take half the day. Boiler manufacurers are biased the other way so they can claim good numbers.

Opinions vary, but you might decide that the rads and boiler should be outputty enough for whatever designed static temperature difference was chosen compared with outside, often 20°C, when the boiler is condensing, but you can let it go hotter can go hotter when the system is heating up. My boiler temp goes up to 70+ when it's the start of a cool day, but it's not there long at all. It might sit at 40 - 45 for the rest of the day.

Depending on the boiler and its controls, settable curves, self learning or whatever, and everyone's understanding of same, you should hopefully be able to get somewhere near a reasonable approximation of what you want. Precision may be elusive!

FWIW, there used to be a rough figure used of about 40 watts per cubic metre, when insulation was bad. That's under 7kW. Yes really! You'll still find it online. Everyone "feels" they want a bigger boiler .
Three floors gives extra problems. You'll need to keep upstairs doors shut, etc. You could get a cheap surface temperature thermometer ( tenner on ebay) to check your balancing. Again, if the boilers at the bottom, it's hard.

 

[Notch7]

I did mention about the temp and he said he wouldn't recommend the boiler in 65 because it wouldn't heat up the tank, but they had previously left it on 60 when they installed my hive, I've since set the boiler to 70 degrees to allow some temp drop whilst getting to the megaflo

Unfortunately this is outdated thinking.

It would have been better to have had a hot water priority system layout and use weather compensation to set the flow temperature for the heating.

It’s madness to set the flow temp to 90deg my boiler runs at a flow temp between around 35 to 50 majority of the time




This is up to date thinking which really should’ve been discussed with you as an option

Also with a heat only boiler I'd use one that can do hot water priority . Boiler is sized to rad heat load only (no dhw allowance).
Then when heating on boiler can run at lower temperature to keep it it in condensing mode longer.

The Baxi doesn’t come with that set up but I believe it is possible to configure your current configuration to hot water priority and use controls like EPH

 

[Notch7]

He's being very good about it and said he's never had this problem before and he will think of a plan, personally I can't see anything other than doing the heat loss calcs and sizing accordingly, where I suspect a 24KW boiler would be ideal, with possibility of 30KW

Can I please get your thoughts?

16kw probably is big enough.


Do you know what you flow and return temperatures are?

Did you get the system water checked before and after?
Was a power flush done?

What is the system pipework - hopefully not microbore.


If the temp difference between them is not very much then the problem is there is not enough heat being dumped into the house, in which case you need bigger rads.

What are your TRV settings - try to have them all as open as possible.

How old are your radiators? - do they heat up evenly without cool spots, are they clogged?
What temp differential are you getting across each one


Basically you’ve had a new boiler fitted without sufficient design work done, sizing a boiler without heat loss calcs is wrong,