What size boiler please?

[pghstochaj]

Hi everybody,

I am just getting quotations to replace my exiting boilers in the house I have just moved into. It is a large house which currently has two combis, one per floor. Both are past their best and I would like to replace them with a single system boiler and a 300 litre indirect unvented tank. The boilers are 30 kW each with a heating output of 16 kW each, effectively being zoned per floor. I guess they were sized for hot water flowrates. I don't know if they have struggled on cold days to provide sufficient heating.

The first person that has visited was a commercial fitter and suggested a 80 kW commercial boiler. I was shocked and I couldn't understand his reasoning for this. The second person that visited and suggested (I think) 35 kW and stated that 40 kW would be oversized and inefficient. Both have suggested a twin coil unit.

I have added up my radiators and they total to 30 kW with a few that I am worried might be undersized plus the potential for some underfloor heating in the kitchen longer term.

Based on this, I feel that 40 kW is completely reasonable, what do people think of this opinion please? The cost to get this wrong is significant but the increased cost between boilers is small!

Thanks for your help!

 

[Doggit]

Modern boilers will modulate down quite happily, so I can't understand why a 40Kw would be oversized and innefficient. If you've calculated correctly, and got to 30, then in theory, even with UFH, 35 should be fine, but going to 40 shouldn't be an issue if you want to lay safe. The commercial fitter was just fitting what he knows about, but the other fitters is either being prudent, or doesn't understand modern boilers sufficiently. Get other people in before you go any further.

 

[DP]

Perhaps two boilers again, balancing the load to provide space heating, underfloor heating and heating of cylinder. Twin coil cylinder and a return from furthest tap back to the cylinder (where possible). Combi option could be considered first combi to supply direct HW to kitchen/ utility and other to a shower and cylinder for the rest

Good controls essential

What boilers and models do have at present

 

[D_Hailsham]

Sizing a boiler based on the total output of existing rads is not a good idea. You may have 30kW of rads, but how do you know that is correct?.

Use the Baxi Whole House Boiler Size Calculator (Uses BRE method and data) to calculate the heat-loss for your house. This will tell you what size boiler is needed to meet the heating requirement. This will be much less than is required for hot water, if you have a combi boiler, but may be enough if you have a hot water cylinder.

 

[Razor900]

Sounds like a job for a pair of 24kW boilers on a header.

We need more details of the house really

 

[ericmark]

A modern boiler has a variable output, the idea is the thermostatic radiator valves (TRV) control the flow to each radiator and the by-pass valve opens as all the TRV's start to close, as the return water temperature increases the flame height in the boiler goes down, so instead of the central heating switching on and off, it gradually varies, this reduces the hysteresis in the rooms, the room thermostat if fitted is only to turn system off in the Summer, control is by the TRV.

For this to work it is important that the boiler is correctly sized, too big and it will start turning off/on too soon as the house warms up and increase the hysteresis, too small and the house will not get warm enough.

Wind direction, insulation, and ventilation means no two houses are the same, there is a value on every radiator so if you add up all the kW ratings of the radiators that should be larger than the kW rating of the boiler, in other words with a cold house the boiler should be able to run flat out.

I tried the calculator linked to for my mothers house and it seemed about right, if I wanted to reduce heating costs I would be looking at a heat recovery unit. But we all tend to pump out heat into the garden, I have a tumble drier, this takes heated air and blows it outside, also the bathroom has a fan also taking heated air and blowing it outside, I have replaced every light bulb with LED so no heat from lights any more, so in the last 15 years the heat required from the central heating boiler has gone up.

My house three bedroom designed and built around 1979 had a single gas fire 4.5 kW in the centre of the house with an open plan, it actually did heat the house, it was a bit hit and miss, leave a bedroom door open and the bedroom was roasting hot, close it and it was cold, there was no thermostatic control, it needed us to manually turn the heat up or down, and the kitchen was a bit on the cold side. When I fitted a 20 kW central heating boiler I found this had problems keeping the house warm, and I then got rid of the hot water tank and fitted a boiler solely for hot water I then found the boiler would just about cope but always had the gas fire as stand-by, then I got cavity wall insulation, loft insulation and fitted an extra radiator in the living room and the boiler is now ample. Main problem looking back was I undersized the living room radiator.

However 4.5 kW to 20 kW is a huge jump, and again looking back the gas fire was radiated heat as well as convected so air changes did not cool the house as much. My biggest mistake was miss calculating how much heat would go upstairs, OK open plan house so stairs in centre of house with no door on stair case, but upstairs radiators were far too big, and down stairs far too small. Which resulted in the boiler not being able to give full output as down stairs radiators too small, cure was a 4.5 kW Myson down stairs as well as radiator.

 

[Steelmasons]

The commercial bloke could be oversizing along with the other bloke possibly undersizing...then again how would you expect to receive any meaningful advice from a forum regarding building heat loss when you fail to supply any information.
I appear to have lost my crystal ball.

 

[Agile]

You need to calculate the heat loss of the building and use that as a guide for the boiler size needed.

Normally it will need to be at least sized to provide an output equal to the existing rads.

Combis are sized to the hot water output.

In a larger house a cylinder is often a better solution. But it depends on the house layout and how many people live there and their usage pattern for hot water.

Tony

 

[ianmcd]

You need to calculate the heat loss of the building and use that as a guide for the boiler size needed.

Normally it will need to be at least sized to provide an output equal to the existing rads.

Combis are sized to the hot water output.

In a larger house a cylinder is often a better solution. But it depends on the house layout and how many people live there and their usage pattern for hot water.

Tony

tony read the post he has already told you that he has a 300L unvented cylinder

 

[Agile]

tony read the post he has already told you that he has a 300L unvented cylinder

I think that I have read it very well!

He said that he has two combis!

Only that he would rather like an unvented. NOT that he has one now!

Hi everybody,

I am just getting quotations to replace my exiting boilers in the house I have just moved into. It is a large house which currently has two combis, one per floor. Both are past their best and I would like to replace them with a single system boiler and a 300 litre indirect unvented tank. The boilers are 30 kW each with a heating output of 16 kW each, effectively being zoned per floor. I guess they were sized for hot water flowrates. I don't know if they have struggled on cold days to provide sufficient heating.

 

[PullerGas]

The commercial bloke could be oversizing along with the other bloke possibly undersizing...then again how would you expect to receive any meaningful advice from a forum regarding building heat loss when you fail to supply any information.
I appear to have lost my crystal ball.

Oh there's plenty on here Steely that'll pinpoint the exact size of boiler & system required, by knowing f.ck all about the building structure, heat loss or the occupants lifestyle.
It's frankly an amazing place & the insight of some is breathtaking!! There's one individual, that'll even give you a fixed price for the job, whilst sitting on the Tube!!

 

[pghstochaj]

Wow, didn't intend to cause such a number of disputes, sorry!

I will fill in the heat calculator from the link above and see what it comes up with. However, the house was built in 2002 so you would hope that this would have been designed using the same type of calculator. I work in the construction of power stations and review both the administration building design and the process plant designs, so am very familiar with this type of process. Perhaps I was assuming too much in the original design and build of the house and that the rads were appropriately sized. Nevertheless, the heating cannot output more than 30 kW with the current radiator design.

My question was therefore more about "if the radiators are 30 kW and assumed to be correctly sized, and I want a 300 litre unvented tank with reasonable reheat time, what, as a rule of thumb, would you suggest the boiler should be?". It wasn't about heat loss as such.

I will let you know when I have done the calculator, or at least checked several of the bigger rooms to see whether the rads are appropriately sized.

Current boilers are Halstead ACE HE units to answer one of the questions above.

 

[pghstochaj]

Actually, the whole house calculator will not be sufficient, the house isn't shaped as a rectangular block with four walls and two stories all with equal height ceilings. I will need to size up heat loss for a few rooms and use that method.

 

[pghstochaj]

I have checked several of the rooms using a radiator size calculator. The radiators appear to be appropriately sized. For example:

Living room: 4 kW of radiators (installed) versus design of 3968 W.
Family room: 1.83 kW (installed) versus design of 1641 W (one of the rooms I think is undersized due to the amount of floor to ceiling glass).
One the bedrooms: 1.83kW (installed) versus design of 1582 W.
Another bedroom: 1.5 kW (installed) versus a design of 1386 W.

I am therefore reasonably confident that 30 kW of heating is not unreasonable, so return to my original question of what allowance to make for the unvented hot water tank. I spoke to the commercial fitter and he stated that on reflection, 60 kW would be more appropriate and he wants to go this large to ensure hot water supply is consistent.

This is not a normal house in size or design so it is making it difficult to get a consistent response from suppliers. A third one will visit later but I don't want to waste everybody's time by getting 10 opinions and quotes.

 

[Doggit]

The house design shouldn't affect the calculations Pgh, as it's the contruction of the walls, the amount of glass, the contruction of the floor, as well as the size of the room that will help you work out the size of the rad required. You've already got a system installed, and you've worked out the size of the existing rads, and it's pretty clear that the system works, or you'd be complaining about cold rooms, so you're on the right track. Puller and Steel are better at this than me, and they'll advise you of how many KW to add for the unvented cylinder, and that should tell you whether the guys quoting you are in the ball park or not But even I know that you don't need 25KW for for a 300litre unvented cylinder, so he's still not sizing the system correctly, but I think it's the cylinder that will determine the recovery time, not the boiler.

I think you'll know when you've got a couple of sensible quotes, so come back to us then, but IMHO dump the first 2 guys.