Myson Heat Loss Program

[Lunny]

Hi,

Does anyone have a copy of the Myson program? Can't find it anywhere on t'internet.

Reason I need is that I have had a ch guy round and I think he's oversizing everything to avoid comeback. For example about 10,000 BTU for a 4mx2mx2.7m bedroom (2 external walls) Also suggested a 37.9kW boiler for a 3 bed end terrace house (100 yr old). The reasoning is that the boiler (not a combi) would not need to work as hard etc. I understand that oversizing condensing boilers is not as much as an issue as they are modulated but surely this is enough to heat the neighbours house too.

I thought I'd try calculating some rooms last night. Found the formula, and have taken into account wall sizes, windows, doors, U-values, ventilation rate etc but it all seems on the low side the bedroom was coming out at about 1.4Kw. As an experiment I put a 2kw electric heater in there for 30 mins and it just managed to heat the room up.

Also labour for all fitting is =A3800 - resited and new boiler, converting to a sealed S-Plan and 7 rads (one resited). Does this sound reasonable?=20

Cheers!

[Andy Hall]

If he's working in BTUs then he should be shown the door anyway.

If he's mixing BTU and kW units then the above should apply together with a steel capped boot.

If you would like the Myson program, send me an email.

I would be surprised if 38kW were needed for this application. I might believe 25.

[Lunny]

Supposing that the BTU calc for the rads comes to about 50,000 (calculated at -3 degrees outside and with an extra 20% to be on the safe side) and domestic HW is required, would the boiler below be a good choice?

BTU max output, fully modulating etc, etc...

Cheers, Matt

[John Rumm]

It does sound excessive... worst case I could calculate for my place was about 8kW and that included the 2nd floor as well (although compared to the rest of the house that is well insulated).

[The Natural Philosopher]

Sizing boilers and radiators on heat loss is not always such a good idea.

The peak output determines how FAST the place will heat up..

Then, providing you have proper thermostats, the heat loss will only determine the duty cycle of the boiler.

And provided that you have a bit of hysteresis on the stats, that will prevent short cycling.

So efficiency of heating is not really a function of the boiler or radiator SIZE.

They just need to be big enough to heat the place up quickly enough.

If you are at home all day in the winter..you may run the heating more, and need less peak output..OTOH if you leave the house empty in the day, you should go for a bit more radiator size and a bit more of a boiler.

By the sound of it, something around 25KW for the boiler, and about 4KW in the larger rooms and less in the smaller sounds fairly sound.

Pay more attention to thermostats and controls than to rad and boiler sizing.

I think that if I were to do a rad installation again, I'd go for an electrical stat in every room, with a motorized valve for every room, and connect the switches all in series to control the boiler - no master stat.. :-)

[Roger Mills (aka Set Square)]

I presume you mean *parallel*? If wired in series, the boiler will only fire when *all* rooms are cooling for heat simultaneously. Saves on gas, I suppose.

[Roger Mills (aka Set Square)]

Did you take air changes into account? That can account for a substantial extra heating requirement.

[Lunny]

Yes. Sorry, this is what I meant by ventilation rate.

I'm getting the impression that I don't really need a boiler as big as the ch installer suggested....

I'll see what the Myson calc says.

Cheers.

[Lunny]

Andy,

Have PM'ed you with my email address. Let me know if you don't get it.

Cheers, Matt

[Andy Hall]

Sure, but here we are talking about three external walls that are probably just double brick. That makes a huge difference.

[Andy Hall]

Sorry, nothing received.....

[Lunny]

Andy,

Please send to lunny1973@*remove*hotmail.com

Cheers, Matt

[Lunny]

Obviously removing *remove* :)

Cheers, Matt

[The Natural Philosopher]

Ooops. Mea culpa!

yup parallel the room stats and put them in series with the timer..

[John Rumm]

Yup, but that is what I have got: 9" solid wall with 1" render over

[Andy Hall]

I'm surprised that you had so little heatloss.

However, keep in mind that a terraced house is often narrow and deep rather than more square like a detached or semidetached house. It may also have an extension section for kitchen and third bedroom.

The result is a substantial outside wall area.

[Lunny]

I've "Mysoned" the house and it says the boiler size required is 19kW. Allowing about 4kw for hot water would mean I'd need 23Kw. Would getting a 24kw boiler be a good idea? Would it mean the boiler would be overworked at all or does it just mean that the house/HW will not heat up as fast? What would be the advantages/disadvantages of oversizing the boiler?

[Tony Bryer]

if the boiler you are considering is a modulating one then there's no huge problem with it being a bit oversized - when the full output is not required the gas burn rate will be reduced to what is needed. If you oversize and the boiler cannot modulate down then it will run in stop/start mode for most of the heating season.

You can check boiler outputs from our free browser, see sig

[Andy Hall]

That sounds more reasonable.

This is where some interpretation of the results is needed.

The 4kW figure stems from when it was common practice to heat the water in the cylinder using convection circulation from the boiler (aka gravity). This would be about the typical trasnfer rate and would be happening all the time that the heating is running as well as when HW only. Therefore the figure was added.

Nowadays on new installations one can't fit gravity systems (building regulations) and the typical arrangement is to use a diverter valve and fully pump the circuit to the coil of the cylinder. Especially if the cylinder is a Part L type or better still a fast recovery, the heat trasnfer rate can be 20kW or possibly more. The idea is that when HW reheat is needed, the boiler is switched over from heating and all of its output goes to the cylinder. Since this is more, then reheat is fast and then the boiler is switched back. Ergo, you don't have to allow for water heating.

I think that something in the 24-25 or even 28kW range is fine.

If you go for a modulating boiler, and typically will with a condensing model that is normally required nowadays, it will run at lower output when running the heating. In any case, for much of the year you won't need anything like 19kW anyway.

[Steve]

Andy,

Can I ask you to please send me a copy also.

Remove the '.remove' from my hotmail address.

Thanks