Boiler and Storage sizing calcs

[churchillrow]

I realize this is a rehashing of a recent post by E.W. but I've got some specific numbers and I'd like some second opinions on my attempt at rational thinking.
From what I have been able to glean from the forums the boiler can be sized one of two ways: to storage or to load.

The System I am trying to build will be heating two houses, house A @ 54594.3 btu/hr , house B @ 27297.1 btu/hr. and a shop @ 13649 Bth/hr.
House A will have the boiler and storage (1000 gal?) and house B could have it's own storage (500 gal?)or not.  House B is 200 ft from house A and the shop is 60 ft from house A.

So.. Total design heat loss is 95540 Btu/hour.

To size to load

I multiply by 1.1?? and get a 105 094 btu boiler.

To size to storage

If I want to burn for 15 hours a day (7-12 in the morning on one load, 5-10, 10-3 on two loads) I need a boiler with an output of
95540 btu/hour * 24hr. load / 15 hr. burn time

= 152,864btu/hr. boiler

The longest gap between firings is 5 hours so
5hr. * 95540 btu/hr. = 477,700 btu storage capacity or

477700btu / 8.34btu/delta t/gal. / 70 delta t
=818 gal.

Another way of sizing storage I've seen suggested is ensure one full load can be absorbed for DHW in the summer.
Assuming 5 hour burn
5hr. * 150,000 btu/hr. = 750000 btu or

1200 gal.

What I am thinking based on these calcs is that a 150 000 btu boiler is the right choice. I work out of the house so I could conceivably run a 100 000 boiler almost all the time for the short part of the year loads are maxed out. Worth considering?

As to storage 1200 to 1500 gal is making sense to me to ensure that what ever the load we could burn the minimum number of times at max efficiency.

Any thoughts would be greatly appreciated as I'd like to order a boiler shortly.

Thanks to posters past and future who make this forum happen. What a great resource.

 

[Nofossil]

It's worth remembering that your average load is about half your peak load. Your peak looks like 100,000 BTU/hr (remember that the precision of these analyses are +/= 10% at best). In that case, your average load is about 50,000 BTU/hr.

I'd suggest that your calculation for DHW is a bit large - another way of looking at it is 'how often do I want to burn in the summer for DHW?'. I'd suggest that once a week would be a reasonable target. I'd also consider solar hot water for the summer months - go with a low cost collector and don't bother with wood.

As long as you have room for it and can maintain stratification, you'd probably be happier with a bit too much storage rather than a bit too little.

 

[churchillrow]

Nofossil,

Solar hot water is in my future plans but between house reno's and the boiler it might be a ways off. What are the implications of my average load being 50000 btu? I know you are an advocate for smaller boilers but how does this work in practice? We will also have a wood stove and I am still thinking about backup heat for when we are away. Are you suggesting on relying on one of the two for the coldest part of the year or continuous firing of the boiler? What Percentage of peak load would you design for?

Thanks

Augie

 

[Nofossil]

Nofossil,

Solar hot water is in my future plans but between house reno's and the boiler it might be a ways off. What are the implications of my average load being 50000 btu? I know you are an advocate for smaller boilers but how does this work in practice? We will also have a wood stove and I am still thinking about backup heat for when we are away. Are you suggesting on relying on one of the two for the coldest part of the year or continuous firing of the boiler? What Percentage of peak load would you design for?

Thanks

Augie

Oops - I fear that in an attempt to avoid being dogmatic, I may have been unclear. Your load-to-boiler ratio is almost exactly the same as mine. My first comment was intended to suggest that your boiler firing scenario was likely to apply only on really cold days. Most days, you'll get by with a single firing. If you time that for the coldest part of the day, your storage will carry you to the next evening.

I also doubt that you need or would use the amount of storage that you've proposed for summer DHW. That being said, bigger is pretty much better. The only way that's not true is if you don't have good stratification - especially for DHW. A whole 1500 gallon storage tank at 130 is very poor at heating your DHW tank. However, a 500 gallon tank that's half 90 degrees with a layer near the top that's 160 would be great for heating DHW.

 

[killick]

Augie,not to hijack your thread but how did you obtain the heat loss values for your buildings?I have tried three different systems off the net and reached three vastly different conclusions. What I perceive to have been the closest was a "rule of thumb" method.
Thanks;

Earl

 

[churchillrow]

Earl,
My buddy Jordan at Thermalwise in Wolfville did it for me using Hot2000 which can be downloaded for free. Mind you he told me to ignore the numbers in the report and gave me a different set from who knows where. He is hireable though..... www.thermalwise.ca/

Augie

 

[churchillrow]

Oops - I fear that in an attempt to avoid being dogmatic, I may have been unclear. Your load-to-boiler ratio is almost exactly the same as mine. My first comment was intended to suggest that your boiler firing scenario was likely to apply only on really cold days. Most days, you’ll get by with a single firing. If you time that for the coldest part of the day, your storage will carry you to the next evening.

I also doubt that you need or would use the amount of storage that you’ve proposed for summer DHW. That being said, bigger is pretty much better. The only way that’s not true is if you don’t have good stratification - especially for DHW. A whole 1500 gallon storage tank at 130 is very poor at heating your DHW tank. However, a 500 gallon tank that’s half 90 degrees with a layer near the top that’s 160 would be great for heating DHW.

Nofossil,
Taking 150 000 btu boiler as a given storage remains as the question.

The longest gap between firings is 5 hours so
5hr. * 95540 btu/hr. = 477,700 btu storage capacity or

477700btu / 8.34btu/delta t/gal. / 70 delta t
=818 gal.

Assuming that 1000gal. will more than take care of DHW in the summer is there a compelling reason to consolidate the storage close to the boiler or distribute it to where the loads are (I am heating two houses 200 ft apart)?

Thanks,
Augie

 

[Gooserider]

One could probably make a case either way for consolidated or distributed storage, but my gut feel is that you would probably be better off distributing it, if you can figure the best way to balance the storage to match the DHW demands so that both houses will drop to the firing point at about the same time...

My reasoning is that with consolidated storage, you will be pumping a lot of water around on an ongoing basis to heat the remote loads, and from what I've seen one of the larger places people loose heat is in the ground piping. Even if it's well insulated, you can't get around the basic fact that it's a lot of radiating surface. If you have distributed storage, you'd only be pumping water through the ground loops while actually charging the storage or running off the boiler... Presumably this means you would get more DHW out of a burn if the heated water just sat in the respective storage tanks until needed to heat the demand in each house.

Gooserider

 

[killick]

Thanks Augie.

Earl

 

[churchillrow]

My reasoning is that with consolidated storage, you will be pumping a lot of water around on an ongoing basis to heat the remote loads, and from what I’ve seen one of the larger places people loose heat is in the ground piping.  Even if it’s well insulated, you can’t get around the basic fact that it’s a lot of radiating surface.  If you have distributed storage, you’d only be pumping water through the ground loops while actually charging the storage or running off the boiler…  Presumably this means you would get more DHW out of a burn if the heated water just sat in the respective storage tanks until needed to heat the demand in each house.

Gooserider

Makes sense, once the insulation on the pipes are heated up less heat will be lost from the water flowing afterwards. Hopefully with good controls I can get "House B" to satisfy its loads from the storage and then charge storage completely when it is below a useful temp. The other reason I can see to minimize the number heating cycles through the underground pipe is that each time you will be stranding hot water in the pipe although maybe not a significant amount.