How to size boiler

You have not described space heating of a house/commercial building/industrial application.

If commercial building/industrial application - 2 boilers will eventually be required so that 1 is standby. Personally, I'd then select one boiler only for water heating demand in summer & 1 boiler to cater for both water heating demand & space heating in winter.

I would suggest a system similar to what I have in my house. It is an oil fired boiler (gas is just as good depending on the price of fuel at any given time) that has an indirect water heater shunted to it; meaning that the boiler water enters the water heater unit via circulating pump through separate lines and coil to heat the potable water within the vessel. The space heating is on one zone (very big old radiators) and the water heater is on the other. The water heater is the priority zone so that if there is a demand for hot water, it will be taken care of first and the house demand after it is satisfied.

I don't know the efficiency of this system versus other configurations but I am quite happy with it as we are never wanting for hot water. It surly takes up less space and is much less an investment than two separate boilers.

For a home I'd agree on this system.

I've had Honeywell System W in several houses and it's great. Fully pumped, after the pump is a diverter valve going to either rads or HW cylinder. Indirect HW cylinder. Room stat and cylinder stat fitted. If either stat calls, the valve goes to right position and the boiler fires. (the valve cannot block flow to both outlets so no safety issue requiring boiler firing to be via contacts on the valve). If both stats call, HW has precedence.

A later version is System Y which has a mid-position valve, allowing flow to both rads and cylinder when both stats call, but I don't think there's much advantage in practice and it's a more complicated valve.

I'm not particularly keen on combi boilers, but that's another option.

Codey

Hello Guest,

Go to the boiler manufacturer's site and they will give good simple advice, on what boiler you need for the size of house, but, also lots of 'tit-bits' that will or may be helpful.

Search site I found

http://www.google.co.uk/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLJ_enGB294GB294&q=Can+I+have+the+same+boiler+for+water+and+space+heating

• Water Heaters With Space Heating Capability Instead of a boiler, a tankless water heater can provide hot water as ... space and water heating on a localized scale, whereas a boiler can be sized to ...
  www.toolbase.org/Technology-Inventory/HVAC/space-heating-water-heaters - 41k - Cached - Similar pages
• EERE Consumer's Guide: Furnaces and Boilers You can estimate the annual savings from heating system replacements by using Table 1. The table assumes that both furnaces have the same heat output. ...
  apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12530 - 42k - Cached - Similar pages
• [PDF] Combination Water & Space Heating Systems File Format: PDF/Adobe Acrobat - View as HTML
  boiler to supply heating energy to the space. heating system while still meeting the needs for. domestic hot water. The space-heating. component can be ...
  www.builditgreen.org/webfm_send/59 - Similar pages
• [PDF] Combination Water & Space Heating Systems File Format: PDF/Adobe Acrobat - View as HTML
  central boilers can operate at efficiencies of 90. percent or higher, making gas much more efficient .... space and water heating system will be the same as ...
  frontierassoc.net/greenaffordablehousing/FactSheets/GAHCfactsheets/15%20ComboSystems%20final.pdf - Similar pages
  Good luck............

You have had some good advice already but I would just like to mention a couple of points that may help you further:-

1) Do not "store" hot water, it is a simple waste of energy. Most modern installations today try and avoid that. If you MUST do it, get a wellinsulated tank and add some more!!!! reduce hot water piping size to reduce the amount of water left in the pipes after usage. Insulate all water piping hot and cold.

2) We have a small house, it only needs an 18KW boiler for heating, but these have too low a thruput for hot water, so that a bath fills slowly, a shower is a dribble. So I bought a 28KW system, it was around 100 more and it gives masses of fast hot water at water mains pressure and a shower is a dream!! I combined it with oversize radiators in every room so that when we want heat, we get it pronto. I have not found any drawbacks yet after more than 10 years....a plumber would never install oversize radiators (too set in their stodgy ways!!), but they have also never tried it, it works great!!!

Gas is best for such units, they are cheaper and burn cleaner. Good house insulation and computer controlled thermostats on every radiator keeps the comfort up and $bills$ down!!