Sensible Boiler Sizing

[Luke]

Hi folk, I'm in the process of sizing up central heating requirements for my house. Just to give you some background it is a 3-bedroom semi detached house built in 1957 with upvc double glazing and wooden floors upstairs and down, topped off with 100mm of insulation in the loft.

The plan is to fit a system boiler in the loft, connecting it to the existing HW tank, which is also served by solar panels via a separate solar coil. I have spent a long time deliberating the pros/cons of this approach and this seems to be the best option.

I have carried out heat loss calculations and the CH requirement came out as 13.3kW, based on being pretty generous (rounding up to the nearest half meter, not deducting chimney breast area, adding on 15% for north-facing rooms etc).

If I add on 2kW for the HW requirement, and 10% for overall losses, the total figure is 16.8kW.

My boiler of choice is the Vaillant Ecotec Plus which comes in various outputs including 15, 18 and 24 kW. In my eyes the 18kW version is bang on the money. My only worry with choosing this option is the possibility of future expansion (perhaps adding a single-storey extension to the back of the house which will increase the heating requirement by, say, 3kW), or the fact that I may not have added sufficient "leaway" in my calcs.

Could I ask if you think the 18kW is a good choice given the figures above?

Thanks in advance Luke

[Vortex5]

Sounds about right to me.

I installed a Vaillant Ecotec Plus 428 (heat only version with no internal controls) on a sealed system heating an old 5 bed home with absolutely no problem at all.

It's well made hardware and very easy installation.

[The Natural Philosopher]

Be a bit careful. I did the same. Put 10% on top of my heat loss calcs and yes, its enough, but only just. On cold winter days what counts, especially if you are not running 24x7, is how fast the house will heat up from cold. Or fairly cold. I have to run the UFH 24x7 simply because it takes about 3 DAYS to heat the whole mass of the house and especially its concrete floors from 5C to 19C, and at least three HOURS to get a 1C rise in temperature ..

So if you have very little thermal mass, or if you are at home 24x7, an undersized boiler is fine,. But you may curse it if you have a lot of thermal mass, and you switch your heating off when away for the weekend, or even just overnight. Size the boiler more for the minimum rate of temperature rise that you think acceptable, not for the maximum heat loss.

They may be similar, but usually they are not.

[NT]

sounds right so far

no need to add 2kW, HW doesnt take anywhere near 2kW average. Normally HW will heat during the moments the CH isnt running. In extremis this just means starting the HW a bit before the CH comes on.

normally one would add insulation at the same time as extending, and get no net increase. Cavity walls are pretty poor insulators, its usually easy to cut out a lot of heat loss with CWI, or in some cases other options.

What TNP says about rate of rise is only true if you leave the house empty for more than a day, or its so badly insulated that it drops much in temp overnight. If you're away a lot I'd take his suggestion, but for most of us its not a problem.

NT

[The Natural Philosopher]

Its a feature if for any reason you need to get a part of a house - say the guest room - up to temperature from cold.

Its not a matter of bad insulation either. Even the best insulation can result in a pretty rapid rate of drop through ventilation losses if the house is lightly built inside. If there is no thermal mass, then temperature drop is by definition, instantaneous irrespective of insulation. The time constant of the house is a function of thermal mass AND insulation.

And even triple glazing, is very very far from the best insulation.

a boiler that can just match the heatloss, will take an infinite time to reach the temperature at which that heat loss occurs.

I don't have problems except on days when its sub zero and blowing: then I have the situation where the boiler ruins continuously without an break for several hours. Because the concrete slab needs to go from room temp, up to about 40 degrees to be able to start transferring serious heat to the room..conversely it sits there radiating nicely for hours after the thing is switched off, but that's no help really.

Look at it this way. ideally you want a 'square wave' response on your heating. The moment its on, temperature jumps to the stat setting, and stays there. The moment its off, it drops to whatever the frost stat is set to. That is the most economical heatloss you can get with it being warm when you are there, and cold when you are not.

Now the impact of a low boiler output, is the time before you e.g. get home that you need the thing to come on. every minute of that is wasted heatloss from the house. Which is warmer than it needs to be.

I have thought about this a lot, and have come to the conclusion that two completely different approaches suit and are optimal for two different lifestyles.

If you are only in the place for short periods, a low thermal mass house that heats up quickly, with an oversized boiler, is optimal. It cools down fast, saving heat, and heats up fast, saving heatloss during the warm up period. In short you don't waste heat by it being warm when you are not there

If you are there 24x7 as I am, then 24x7 heating with a smaller boiler and a high thermal mass is actually more efficient. Because it utilizes solar gains in the day to carry the heat into the night.And since you are in it anyway all the time, there are no heating up periods needed.

We tend to run with timed or no heating in the summer and timed in the spring/autumn, but switch to 24x7 for the winter months. Thats because the minor drops at night in the areas we don't use much are easily sorted by the amount of power we have if we decide to use one of those areas. But in winter, unless I keep those areas up to temperature, they never get there, not for many hours.

[Andrew Gabriel]

I put central heating in in 2002 (timed to beat Part L, but actually it conformed with Part L, and I believe still does even though Part L's requirements have been raised over the years).

End terrace 1900 house, calculated total heat loss as 11kW using a beta version of the Myson Java heatloss calculator (not sure that heatloss calculator ever got past being a beta).

I wanted a condensing boiler, but they were still quite rare at the time. Smallest I could get of any quality was 25kW, modulating down to 7kW. IIRC, I sized the radiators for 55C operation.

In subsequent operation, I would say the heatloss calculation was some way off - fortunately an over estimate. I didn't investigate where the error was, but Andy Hall looked at his workings with it and decided some of the values it had in its heatloss database for various building materials was too high. I had also been generous with the radiator sizing.

Anyway, the net result was that I could maintain the house up to temperature with a boiler flow rate of 45C at -3C outside, which was frankly much better than I had planned on the installation being able to achieve. This also gets me higher boiler efficiency than even normal condensing operation at 70/50C achieves, so in hindsight, I'm rather pleased with it. I can also crank the boiler up to max (82C) to get a very rapid heating up from cold, where the oversized radiators can dissipate the boiler's full 25kW. (Actually, they never make it up to 82C, because their power output reaches 25kW slightly below this.) This means I can run the central heating purely on house occupancy, rather than on timers. At that time, I was working for Sun in a role where I often flew off somewhere for days or weeks, and even when in the UK, my working hours varied day-to-day, and this form of heating matched that perfectly.

Actually, the house also had remote control of the heating (dating back to about 1999), so I could call up the house on the way back from the airport, and have the house already warmed up by the time I reached the front door, but the very rapid heatup from cold which the new central heating system provided kind of reduced the need for that to some extent.

One thing is that the Myson calculator got the relative radiator sizes between the rooms absolutely spot-on. The positioning of the house means I don't get differential thermal gains between morning and evening in different rooms. The net result was that the TRV's were actually not required at all to make local room adjustments, and I can simply leave them all on max (effectively disabled), and all the rooms on the same floor heat up at exactly the same rate. The system is separately zoned upstairs and downstairs. That was partly so if I'm coming back late and going straight to bed, I can just heat up the upstairs, or if I'm working at home during the day, I can just heat the downstairs. It's also important when heating a house from cold, since a lot of the upstairs heat actually comes through the ceiling from downstairs, but that doesn't happen when heating up the whole house from cold, so you need extra power injected upstairs until downstairs is up to temperature. TRV's could handle that in the case of a single zone though.

[The Natural Philosopher]

Interesting case study. Thanks for posting that.

I take it this setup never short-cycles then.

[ransley]

Just curious, here in the US 100mm of fiberglass is about R 15. I live where it gets to -20f and the mimimum code requirements are R35 with optimal being around near R 60, I have about R 90. Insulation settles and fiberglass batts loose effectiveness the colder it gets. So code mimimum here is about 225mm and we find optimal near 400mm. Dow, Owens Corning, Johns Mansville all have Zone calculators we use to determine how much insulation we need. It must be very warm over there to consider only R15 as effective. About 60-65% of heat loss is up the attic. I had rooms noticalbly warmer in winter by going from R30- R

90, why are your codes so lax on insulating. On the boiler you need recovery, true you dont want oversizing but how do you cope and figure in the lower temp the condensing boiler runs at. Cast iron radiators on old systems ran hotter, and some radiators by design like copper baseboard need hotter temps but you want to keep about 140 f - Farenhite maximum on the condensing units or efficency drops off dramaticly. I will say insulate alot more, its the cheapest thing you can do and can keep sizing down.

[Andy Burns]

If most houses here had 400mm of insulation internally there wouldn't be enough room left to fit a cat into the rooms, let alone swing it, and if

400mm was fitted externally then detached houses would become terraced houses.

[Andrew Gabriel]

That's because 100% efficient was defined in the UK in terms of what a non-condensing boiler could achieve, and not as energy usage and waste from the boiler. They aren't really greater than 100% efficient in absolute terms, just greater than the theoretical max efficiency than you could get from a non-condensing boiler.

Actually it can get damn close, because in the UK, the maximum heating output is required for such a small proportion of the time the heating is in use, and condensing mode can be used much of the time even with a system not originally designed for such water temperatures. Main problem is that neither heating installers nor home owners generally have any idea how to run their systems efficiently.

[Andrew Gabriel]

It does if just the upstairs zone is calling for heat, as the power output of that zone at 45C flow is less than 7kW, the minimum boiler modulation level. It takes quite a while though, and if the upstairs was actually up to temperature and just calling for heat occasionally to compensate for thermal losses, then the calling for heat signal isn't on for long enough for the boiler to start cycling. It doesn't "short"-cycle anyway, as it seems to let the water temperature rise to about 50C before cutting off, and then requires it to drop to below 40C before it will fire up again, so there's rather a lot of hysteresis.

The boiler (Keston Celcius 25) only has manual control of the flow temperature. The room temperature monitoring and checking against the temperature set point is all done by a computer program I wrote, which then generates the appropriate calling for heat signals for the upstairs and downstairs zones. I did also replace the manual boiler control with computer control of the flow temperature, but

a) I was getting slightly cold feet about having replaced one of the boiler circuit boards with one of my own design, in case the boiler had some sort of catastrophic failure (as some other Kestons did) and I got held liable even if I wasn't to blame, and

b) the control board I built interfaced to the PC's built-in games port, and I had to swap out the PC (a Pentium 166MHz) for something more recent (a Pentium III ;-), and lost the games port.

So, for the moment I have reverted to manual control of the flow temperature (effectively lost the weather compensation part of my program), although the computer still controls the calling for heat signals (which doesn't require any internal modifications in the boiler).

One other thing about using a low flow temperature is that I get virtually no overshoot of the room temperature. On another installation I computerised which uses a non-condensing Potterton Profile boiler, the higher radiator temperature means that if you cut the calling for heat signal when the room gets to the setpoint, the higher energy stored in the hotter radiators causes overshoot of the room temperature, which you can regard as excess waste heating.

[Luke]

Thank you to everyone who took the time to reply. Reading the responses has been very thought-provoking. I admit I hadn't thought of the benefits of an over-specified boiler when you want to get the house up to temperature quickly. Our usage patterns (that of my wife and I and a baby in October) will probably not support the 24/7 operation that a optimised boiler output will provide. Therefore, perhaps I should over-specify. However it has dawned on me that there is no point whatsoever in overspecifying the boiler if I don't also massively increase the size of the radiators, otherwise I'll be making no use of all that spare capacity.

The house currently has an electric boiler with standard wet radiators. As I've only lived here a week I have no idea how the system performs in the depths of winter, although I do know the radiators here at present are way way smaller in terms of heat output compared to the radiators I have specified using the heat loss calculator. Bearing in mind we also plan to have a gas fire in the living room, there's a coal fire in the dining room should we need it, and we're highly unlikely to want ALL radiators blasting out heat ALL the time, as well as heating up a stone cold tankful of hot water, even in the middle of winter, I feel inclined to go for my original choice, that of the 18kW model.

Thanks again. Luke

[Andrew Gabriel]

What's the power output of the electric boiler?

Given you don't know how adequate the existing system is, you could ignore it and work out what you need from scratch. Then look back at the existing system, and see how much of that it provides. You might need to change the radiators in every room, but that might not mean buying all new radiators. For example, you might chuck out a couple of the smallest, move all the others so each room has bigger ones, and then just buy the two biggest you need.

Where you have supplementary heating in some rooms, that requires some careful planning of the room stats for the central heating, so you don't end up with the gas fire prematurely heating the living room, causing the room stat to drop the call for heat, and the rest of the house goes cold.

[The Natural Philosopher]

Good approach. Often big rads can be swapped with small rads, and huge ones put in, and the small ones chucked..

yup.

Put any master stat in corridor or landing, and TRV everything else. Dont put it near the front door if epole WILL leave it open.

FWIW we run heating pretty cool - 16-17 - to save dosh, and put fires on in the rooms we need warmer.

[David]

I admit I have only skimmed through your post but what I will say is that Vaillant boilers can all be easily down rated via the function 0 (d.0) in diagnostic mode, we have a combi boiler with a 28kw output but have it rated at around 17kw. The larger boilers will not modulate so low though. Our models only goes down to 12kw so a lot of the time its cycling because the heat required is even less than 12kw.

[ransley]

Nothing is greater than 100% efficient, electric heat is but with even a condensing boiler flue gas is about 75-80f by AFUE ratings which we use 97 to maybe 98% is as high as they go. True radiant or underfloor heating is the way to go with condensing units. But from what ive read output temp over 140-145f is where efficiency will start to drop.. Cooler is better for sure.

[ransley]

Non condensing boilers her never get a rating of more tha 85% AFUE or total efficency, Afue is just a rating system.

[ransley]

If a boiler is properly sized and maximum water heating temp is at its design peak , which is about 145f average, than worrying about turn down ratio is a waste of time, its not going to make a difference to the average home owner.

[The Natural Philosopher]

run.http://en.wikipedia.org/wiki/Turndown_ratio_(boilers)- Hide quoted text -

It is relevant if its the difference between long cycling at low output and fast cycling at high output, since the ignition process wastes gas (or oil)

Or if its a pilot light, that in itself is also a waste. I am not familiar with gas boilers, so I have no idea if they use pilots or spark ignition these days.

I do know there is a huge waft of unburnt kerosene every time my oil boiler fires up.

[geoff]

In message , harry writes

There are still a lot of p pilot boilers out there (Potterton Puma for example)