CH Energy Requirements Estimate

[JohnW2]

The discussions in this thread, and my associated one (click here), have been very interesting and thought-provoking but have not yet really answered the question I posed in my OP, basically asking how reasonable/credible people felt my heat-loss calculations were. In particular, the one response above which was addressing that question, was somewhat vague, namely ...

11kw sounds massively out

I would be very interested to know in which direction gas112 felt that my calculations were 'massively out', and hence in what ballpark he would have expected the answer to be.

To remind you all, I am talking about a fairly large late Victorian 3-storey mid-terraced house, with a total floor area of about 110 m²). Other than for the pitched roof, it’s fairly poorly insulated, with solid 9” brick external walls, single-glazed wooden-framed windows and a suspended wooden ground floor. It is a 'narrow but deep' house - some 5m x 17m overall, hence with relatively little in the way of outside walls, and the two adjacent properties are occupied and heated.

My initial heat-loss calculations suggested a total heat energy requirement (for space heating alone) of around 11 kW, with most rooms requiring a radiator of around 1.5 kW. I have subsequently re-visited those calculations with various alterations to the assumptions, and the 'worst case' of these suggests a total of 13-14 kW, with rooms most commonly needing radiators in the ballpark of 1.5-2.0 kW.

The question of how I translate estimated radiator heat outputs into radiator size (depending upon flow temp and delta T) has been (and continues to be) discussed at length in the other thread.

So, retuning to my initial question, I wonder if people feel that (given the nature of the property) my heat-loss calculations for this house are reasonable/credible - and, if not, roughly what they would have expected the total heat-loss (hence radiator requirements) to be?

Kind Regards, John

 

[Mister Banks]

I have been going through this myself and I have used Heatpunk to calculate my heat loss. It is very comprehensive.

 

[JohnW2]

I have been going through this myself and I have used Heatpunk to calculate my heat loss. It is very comprehensive.

There are umpteen calculators out there, varying from the pretty simplistic (essentially just looking at floor area or room volume) to the much ore sophisticated, and I've tried many of them. However, the variation between the results they give is so great that quite frankly, I'm more inclined to trust my own, pretty 'comprehensive' calculations 'from first principles - at least I know how those calculation have been done

However, as we have been discussing in the other thread, this whole exercise takes on a different light when one realised that the radiators that would be needed to satisfy the calculations for an 'optimally efficient' heating system (delta T of 30, or maybe even 20 degrees) are usually 'impractically large' to be installed in a domestic property!

Kind Regards, John

 

[vulcancontinental]

As long as your ceilings are between 7'6" and 8'6" 11kW sounds fine in traditional calculations.

If you're running a heat pump 24/7 maybe a little lower.

 

[JohnW2]

As long as your ceilings are between 7'6" and 8'6" 11kW sounds fine in traditional calculations.

Many thanks. The ceilings in the upper two floors are below 8'8", but those on the ground floor are slightly over 9' - but I p[resume that will not make a very big difference to what you are saying, particularly given that I will definitely 'add a margin' onto the calculated heat output requirements.

If you're running a heat pump 24/7 maybe a little lower.

I'm definitely not contemplating a heat pump. However, even if I were I don't think that would alter the amount of heat output required from the radiators - but, because of the lower temps, would have a massive effect on the size of radiator required to achieve those heat outputs - as I've said, seemingly to the extent of requiring radiators which would probably usually/often be 'impractically large' in a domestic setting. I'm therefore not sure what those wanting to use heat pump[s are meant to do!

Kind Regards, John

 

[JohnW2]

Even this isn't the full picture.
Nest may be calling for heat, but it doesn't mean the boiler is constantly firing; if the boiler return temperature was above the boiler thermostat setting, the pump would continue to run without the boiler firing.

Having 'reflected' (dangerous!), I think that I may have 'bought into' what you were saying a little too quickly!

Other than for the inevitable imperfections in my estimates of U-values, the only thing 'wrong' (in terms of the real-world situation) with my ('steady state') heat-loss calculations was the assumption that the outdoor temperature would remain constant.

In terms of the physics, if the heat-loss calculations say that, in a steady-state with 24 degrees across the external walls/roof etc., the (continuous) heat loss is, say, 12 kW, then the boiler would have to produce heat at an average rate of 12 kW throughout the 'steady state' period. If the outside temp remained unchanged for, say, 16 hours of 'heating on', then the boiler would have to provide 192 kWh of heat energy output - something like £12 at current prices.

Even given 'perfect' heat-loss calculations. what actually happens in practice is obviously crucially dependent upon how outside temperature actually behaves (and how long per day one heats the house).

Kind Regards, John

 

[jj4091]

The room I'm sitting in in the only one remotely warm at present - about 21 °C, but that's thanks to the 2kW fan heater which has been on for most of the past 16 hours! The outside temp is currently hovering around zero. The temp in the kitchen (which has easily the most 'outside wall' of any room) is about 5-6 °C (similar to the cavity of the fridge ), but the rest of the house around 9-10 °C, I presume largely thanks to the neighbours (and partially the fan heater). You can see why I'm designing a heating system

So if you have 8 rooms all of a similar size a rough guide would be you need 16kW in total to heat the house to a comfortabel temp. A 24kW like Random grinch's would be more than adequate and probably being the most common size, the cheapest. You could then decide to zone it off how ever you decide.

 

[JohnW2]

So if you have 8 rooms all of a similar size a rough guide would be you need 16kW in total to heat the house to a comfortabel temp.

That would be an upper bound, but the reality would probably be a fair bit less. As I said, when I did that experiment two of the four walls of the room had large temp differences across them (since what was on the other side was totally unheated),, such that heat losses from the heated room were quite a lot higher than they would be when all rooms/areas in the house were heated.

A 24kW like Random grinch's would be more than adequate and probably being the most common size, the cheapest. You could then decide to zone it off how ever you decide.

Indeed. I'd already come to the conclusion that a 24 kW boiler would probably be the way to go.

Kind Regards, John