Tuning CH boiler

[Kostas Kavoussanakis]

I was browsing Ed Sirett's CH FAQ and followed the link to the "Replacement Boiler Size Calculator" site:

my 95 m2, three-bedroom, one-floor, detached, L-shaped bungalow in Scotland, it calculated 9.5kW, of which 2kW is for DHW (is that sufficient? I have no idea how to calculate it).

I did the L in 2 rectangles and supplied the window sizes manually. Where the smaller second rectangle meets the larger first I did not count the wall (so it appears as a 3-wall room).

My current boiler is a 25-yo Potterton Netaheat. It says "10-16" on it, which I presume is kW and the thermostat goes 1-5.

So, with these in mind, here are my questions:

1. Does the sizing calculator work with non-condensing boilers?

1. I see Ed says that the calculator tends to undersize. So, should I aim to set the thermostat to around 2 or 3 (which, if linear, would be about 12-13 kW, about 30% above the calculation)? Is the boiler efficient at such a low setting?

Any answers will be most appreciated.

Kostas

[Ed Sirett]

IIRC the is no allowance for DHW in the new PART-L calculators. Anyway some leeway is helpful.

Yes the 10-16 is the output range which is a preset by the installer (by adjusting the gas valve over the permitted range of pressures).

Yes, except that the SLIGHT undersizing makes me nervous of taking it too seriously. It does not matter what sort of boiler you have this is a calculation to determine how much heat is needed to heat the dwelling.

The thermostat does not affect the power settings. If the boiler is switching off an on from time to time during normal operation then you can be sure that it is big enough. The fact that you have it at a moderate setting means that you have enough radiator capacity.

So if you are replacing the boiler a unit with 12kW would be quite comfortable. you might even go down a bit, but warm up time will lengthen. Warm up time does not figure in the Calculator!

[TheScullster]

"Kostas Kavoussanakis" wrote

A lot will depend on the construction of the property, level of insulation, outer skin brick/block configuration. Also there should be some account taken of the temperature differential - i.e. what is an acceptable "lowest outside temperature" in the area? Maybe all of this is taken into account in the calculator.

I did a spreadsheet for my place detailing wall construction, insulation and heat loss to adjacent rooms etc. This I then compared to my father-in-law's rule of thumb figure of 1500 Btu per 1000 cubic foot (0.053 Kw per cubic metre) and this was about 10% higher than my calculated figure.

So, to get to the end of the ramblings, for approx boiler sizing your heat requirement would be 0.053 x 95 x 2.4 (assumed ceiling height) = 12Kw.

Phil

[Ed Sirett]

Yes, crudely, by wall type and location.

[Kostas Kavoussanakis]

Thanks Ed and Phil,

I am trying to tune the boiler thermostat for efficiency and learn a few things in the process if I can. I took it down to 3 (a previous thread had suggested that it should be all the way up, so as to heat my cold lounge, but I could hear it short-cycling).

At the moment (~1.5 hours after it started this morning and still trying to get the Honeywell CM907 to 21 deg) the boiler works for ~2 mins before its stat switches it off for ~3.5 mins; the pump is going non-stop. Is that still classed as short-cycling?

Also, what is the effect of lowering the water temp? I presume that the boiler will work for longer periods; does this automatically suggest higher gas consumption?

Thanks, Kostas

[Ed Sirett]

I would say from what you have posted that the radiators have to be hotter in your house to heat the cold lounge. (if that makes the rest of the house too hot then those radiators need to have TRVs and/or have them set lower).

Try a higher setting if the lounge is still cold add another radiator.

[Kostas Kavoussanakis]

All (but the one in the hall, where the room stat is) have a TRV. The idea about a third stat is noted. A previous idea about properly balancing the whole system may need to take precedence. However, with the water temp lower the cold lounge "feels" warmer in the evenings (when it matters on weekdays), maybe because the CH pump works more. Weekends (when the stat tries to keep 21 deg from 07:30 to 22:30) has always been another story.

Comfort aside, is there an answer to my initial question about boiler efficiency? And is "2 mins on 3.5 off" short-cycling, assuming that the pump works while this is observed?

Thanks,

Kostas

[Ed Sirett]

Technically it is short cycling. How much of a problem depends on your boiler. A Netaheat (?) would not be too bad with these sort of times.

You will only get rid of short cycling with proper balancing and controls and a modern boiler.

For the lounge to feel warmer with a cooler boiler setting must mean that the boiler runs for longer. For you the lounge looks like it's the hardest room to heat so either move the thermostat in there and add a TRV to the Hall to stop the hall over heating. or add more radiator(s) to the lounge so that room heats up better relative to the hall.

[Kostas Kavoussanakis]

The (cost) of the latter just made me appreciate short-cycling :-)

Thanks, all noted!

Kostas