Boiler sizing..

[joshgroeny]

Hello - hope someone can advise me on this one...

I used the Sedbuk Recommended Boiler Size Calculator posted previously on this forum, to calculate the boiler size I need.

It reckons I need a 18kw boiler. This seems very small given that it's a 3 storey Victorian town house, with a 4 storey rear addition.

Also, almost every boiler on the market is capable of mch higher outputs than 18kW. In fact, it's almost impossible to find one this small..

I'm putting in underfloor heating throughout which is supposed to be more efficient and I'm estimating 2kW for DHW so what I'd like to know: Is less than 20kW genuinely big enough and does anyone know of a good small (20kW) boiler?

Thanks in advance...

Josh

 

[BigBurner]

What are you doing for DHW? How many baths? showers?

 

[mogget]

Also, almost every boiler on the market is capable of mch higher outputs than 18kW. In fact, it's almost impossible to find one this small..

I don't know where you're looking, but smaller boilers are readily available. Mine is 15kw and the smallest I've seen for sale is 9kw. I suspect you may be looking at combi boilers in which case you don't need to...

...add 2kW for DHW.

I think before you decide what boiler you need, you need to decide what kind of system you're going to have, ie a combi boiler, or a separate hot water cylinder/thermal store.

 

[joshgroeny]

Thanks for your replies...

Could use some advice on the system to use actually. I've installed a central heating system with a combi boiler in a 3 bed flat before, but this is a much bigger house with 2 bathrooms. Only one bath though, the other a walk in shower...

Any other factors I need to take into account? I prefer a combi boiler as it seems I have less pipe to run and no tank to install anywhere...

Do I understand correctly that I don't need to add 2 kW for DHW with a combi boiler?

Ta,
Josh

 

[D_Hailsham]

I used the Sedbuk Recommended Boiler Size Calculator posted previously on this forum, to calculate the boiler size I need.

It reckons I need a 18kw boiler. This seems very small given that it's a 3 storey Victorian town house, with a 4 storey rear addition.

No reason for that figure to be wrong. Many systems were oversized - mainly because the average installer had no way of calculating what was really needed; also to ensure the installer couldn't be blamed for a cold house!

Is your house mid-terrace? If so it only has two walls loosing heat to the outside walls - one when you realize that the back wall is now an internal wall between the old house and the new extension - unless the houses either side are empty and cold. Did you calculate the requirement for the old house and extension separately, taking into account the correct number of external walls, insulation, windows etc, in each case?

 

[joshgroeny]

Hi - yes I did. I split the house into 2 and did separate calculations. Not dead accurate for the reasons you mention but still well below the 35 kW or mowe that most people seem to reccommend for a 4 bed house with conservatory.

Does a bigger boiler give me a higher flow rate? ie if I wanted to install a monsoon shower would i have to get a bigger boiler? or would i look at a more expensive electric shower instead?

I'd like to be economical and environmentally friendly by going for a smallish boiler but I want it to work well in all situations. as well..

 

[mogget]

Do I understand correctly that I don't need to add 2 kW for DHW with a combi boiler?

That's right. In fact most combis have a much larger heating output than the heat loss of the house dictates. This is because 24kw and upwards is needed to heat water 'instantly'. Generally a combi boiler will modulate down for heating.

You'll probably have trouble filling a bath and having a shower at the same time, unless your mains supply is supremely good and you have a very high output boiler.

One option to consider is to have a combi, but add a hot water cylinder for filling the bath.

 

[D_Hailsham]

Does a bigger boiler give me a higher flow rate?

Yes. The size of boiler is calculated from:

kW = flow (litres/sec) x temp rise (°C) x 4.2

So, if you want a flow rate of 15 litres/minute (0.25 l/sec) and a temperature rise of 35°C, you will need a 36.75kW boiler

I'd like to be economical and environmentally friendly

And you want to install a monsoon shower?!

 

[gas4you]

Confuses me that way round

Industry standard accepted by boiler manufacturers is boiler Kw / temp rise x 14.33 = l/min flow

 

[joshgroeny]

Hi mogget -

[/quote]
One option to consider is to have a combi, but add a hot water cylinder for filling the bath.[/quote]

Thanks for the comment. The cylinder and boiler would be linked up? Sounds complicated. Is it? I've hung and connected up a boiler before but that's the extent of my experience.

I imagine the two would have to work together or do I simply have two pipe networks for DHW - one for the bath and shower and one for the kitchen and basins?

Is having a cylinder efficient - ie keeping a tank of water permanently hot just for the morning or evening shower..?

Cheers,
Josh

 

[joshgroeny]

Confuses me that way round

Industry standard accepted by boiler manufacturers is boiler Kw / temp rise x 14.33 = l/min flow

Hi, I think there's a meter in the street which ciould tell me what my flow rate was, else I could fill a bucket and see how long it takes, but I was just wondering what would be considered a good flow rate?

How much would i need to fill the bath in the downstairs bathroom at an acceptable rate, whilst someone else is having an environmentally unfriendly monsoon shower upstairs?

Cheers,
Josh

 

[joshgroeny]

Does a bigger boiler give me a higher flow rate?

Yes. The size of boiler is calculated from:

kW = flow (litres/sec) x temp rise (°C) x 4.2

So, if you want a flow rate of 15 litres/minute (0.25 l/sec) and a temperature rise of 35°C, you will need a 36.75kW boiler

I'd like to be economical and environmentally friendly

And you want to install a monsoon shower?!

Good point there. I was speaking hypothetically!

Your post does raise another question though - the boiler flow rate can be higher than the mains rate? Common sense tells me it can't unless there's some built in tank from which the pump could draw extra volume?

So if I could find my mains flow rate, I'd know what max flow rate to expect from any combi boiler?

Cheers,
Josh

 

[mogget]

Hi mogget -

One option to consider is to have a combi, but add a hot water cylinder for filling the bath.

Thanks for the comment. The cylinder and boiler would be linked up? Sounds complicated. Is it?

It's no more complicated than a traditional fully pumped system. The same methods of getting the heat where it's needed would apply, so you'd have an ordinary S- or Y-plan setup. Effectively you'd be using the combi as a system boiler, with the additional capability of heating mains pressure water 'instantly'.

Do I simply have two pipe networks for DHW - one for the bath and shower and one for the kitchen and basins?

That's right. I would suggest the shower and kitchen on the combi, bath on the cylinder, and basins on whichever is more convenient. This way you get high pressure water to the shower, and a large quantity of low-pressure hot water which will fill the bath very quickly if plumbed in 22mm.

Is having a cylinder efficient - ie keeping a tank of water permanently hot just for the morning or evening shower..?

If the cylinder and pipework are well insulated then little heat will be lost. Assuming the cylinder is in a heated area, it will contribute to the heating of that area during the cooler months, and the heat will not be wasted.

I feel I should add that I have little experience of underfloor heating. I do know that it can can co-exist with radiators, and see no reason it would not co-exist with a hot water cylinder.

If you're wanting a pumped shower, it may be worth ditching the combi all together and installing a system boiler and large hot water cylinder(s).

 

[D_Hailsham]

Your post does raise another question though - the boiler flow rate can be higher than the mains rate? Common sense tells me it can't unless there's some built in tank from which the pump could draw extra volume?

Yes, you are correct. The limiting factor is really the mains flow rate. If you know that and the temperature rise you want to achieve - usually taken as 35°C - you can work out what size boiler you need. The equation I quoted can be rearranged many ways. So, if you reduce the flow, by turning the tap down, the temperature difference will increase until the safety devices in the boiler shut down.

So if I could find my mains flow rate, I'd know what max flow rate to expect from any combi boiler?

Yes. But if the flow rate is more than the boiler maximum, all that will happen is the water will not get so hot. It's the equation again!!

 

[joshgroeny]

Gents - thanks a lot for all your comments. That's really helped my understanding of boilers in general and to focus my search on the right one for this house..

Many thanks,
Josh