What is oversized?

Reynz_2

Continuing my saga of the Mad Installer. How much over is considered oversized? I recently talked with a contractor about installing a Unico System. He prepared a Heat Loss and Gain Calculation for my house. His calculation, including an upstairs attic room that does not have baseboard hot water, came to 90,346 BTUH.

1. Does that mean the next larger size Weil McLain Ultra gas boiler 105 should have been used rather than going to the 155?

Leadpipe, you thought you were going to get yelled out, I can just imagine what is going to happen with this question.

2. If I should have had the 105, rather than the 155, how much of a problem if any, would that cause if I went with constant circulation and set the target temp no higher than 148?

Reynz

R. Kalia

??

> He prepared a Heat Loss and Gain

> Calculation for my house. His calculation,

> including an upstairs attic room that does not

> have baseboard hot water, came to 90,346 BTUH.

> 1. Does that mean the next larger size Weil

> McLain Ultra gas boiler 105 should have been used

> rather than going to the 155?

>

I m not sure what previous disussion you are implicitly referring to, but the answer to the above is yes.

> Leadpipe, you

> thought you were going to get yelled out, I can

> just imagine what is going to happen with this

> question.

>

I don't think the answer is particularly controversial. Heat loss calculations are quite conservative. If you have past usage data, you might be able to determine that you could use an even smaller boiler, perhaps because the house i s better insulated than the calculations assume.

> 2. If I should have had the 105,

> rather than the 155, how much of a problem if

> any, would that cause if I went with constant

> circulation and set the target temp no higher

> than 148?


I don't follow the question. If that water temperature will heat your house through your existing radiation on a design day, it will work whatever the boiler size may be. The boiler size makes a difference only because it takes a bigger boiler less time to heat the water and so it short-cycles, which is bad. I believe the WM Ultra has modulating burner (i.e. variable output), but still, the bigger boiler will have a larger minimum output and so will cycle more often.

So there are two links in the chain: boiler to water, and water to house. Let's take a design day. The water temperature for that day should be the minimum temperature needed to heat the house through the radiation when the circulator runs all the time. Then, the boiler size should be the minimum size needed to keep the water at that temperature (by supplying the actual heat loss to the water, which then transfers it to the house).

Ted_9

I would like to add that if your also cooling the house, oversizing is not good for a regular ducted system. But, when using the Energy Saving Products high velocity air handler, they recommend going slightly larger. I dont know if this is true with other high velocity units.

Firedragon_4

Most codes only allow for a MAXIMUM

of 25% overloading, 5% undersizing, what do you think that tells ya?

Reynz_2

There are two types

There are two types of HVAC companies; One who installs equipment, could not care less how, why or if it works as it is suppose to, and one who is concerned with the customer's comfort and energy consumption. I have met both kinds.

eric_2

Variation to the theme

I constantly get the comment "Well I'm planing on an addition in the near future and want to have the unit sized to handle the extra load" Of course, most poeple get to the new project as timely as it has taken me to get to my radiant heat project (2 years and counting). The upsizing usually exceeds the 25% over barrier. In the mean time, they have a less than desirable, over-sized system. How do you guys handle this situation?

Constantin

Depends on the equipment... no?

Note, I'm a mere homeowner, but I have picked up atidbit here and there over the years. IMHO, much of your question hinges on what the consumers are willing to pay for.

For example, if you're installing a modulating boiler, the modulation will partially take care of the "oversized" nature of the rig. However, an appliance running 100% on its design day ought to be more efficient than one that does not (cycling losses and all that).

Similarly, many oil heating appliances will allow a wide range of nozzles to be installed, allowing once more for the boiler to be "oversized" in terms of potential hating capacity while using a smaller nozzle in the interim for the current heating requirements. Interestingly, using a small nozzle (within the allowable range) increases AFUE energy efficiency as well.

Similarly, for HVAC-equipment you could install a variable-speed air handler in the basement, along wiht a multiple-speed condenser outside. Then handle the addition like any additional zone. Here, the two-speed compressor will give you some very rudimentary modulation control as well.

However, modulation of any kind is going to cost $$$ compared to the low-cost builder-grade options like swapping nozzles. Here, the interaction with the client becomes very important, as you don't want to price yourself out of the market. Thus, I would put together a estimate matrix w/4 options:

One collumn for the house as is, the second pre-planning for the house with the expected addition. The rows offering basic versus premium equipment. Then explain what they mean in terms of energy efficiency, noise, etc. Then leave it up to the consumer to decide what he or she wants to live with. Be explicit about what kind of an expansion the oversized system can handle and how permanently oversizing is a bad thing...

By informing them about their options, you are allowing them to make good decisions. Naturally, you want to be somewhat careful about what you reveal, considering how easily a customer could take a detailed quote to a hack...