$2000 Solar Space + Water Heating — Radiant Floor Design

$2000 Solar Space + Water Heating -- Radiant Floor Design

$2K Solar Space + Water: Radiant Floor Design

This section goes through the design of a DIY heat distribution system that will distribute the heat your solar collectors produce to the house.

Guidelines for Solar Heat Distribution Systems

A couple things to bear in mind for systems to distribute heat from solar systems:

Try to choose a system that will heat with not so hot water: Solar collectors will operate more efficiently if they are producing producing lower temperature hot water. That is, they are better at producing a lot of fairly hot water than they are at producing a little very hot water. This means that a heat distribution system that can make use of water at moderate temperatures is better than one that needs high temperature water. For example, radiant floor heat can use water all the way down to about 80F to produce heat, while traditional radiators want water up at 160F. A solar system that is asked to produce 160F water is not going to be very efficient. We used a simple, staple up radiant floor heating system to distribute the heat for our system. Because our system is relatively small and can produce only a limited amount of heat, the floor system is very simple consisting of only one 200 ft loop of PEX — its also very cheap. Radiant floor heating is not the only choice — the Alternative section below lists several good and simple methods for distributing solar heat. Just concentrate on distributing the solar heat: Another important thing to bear in mind is that in most cases you can just concentrate on a system that is sized to deliver the most solar heat that the collectors can gather in a day to the house over that day. This is usually requires a lot less hardware than a heat distribution system that has to meet the full heat demand of the house on the coldest day of the year.

The reason this works is that most people will retain the existing heating system in addition to the new solar heating system. This means that all the solar heat distribution system has to be able to handle is the most heat that the solar collectors can produce in a day, and because the solar storage tank usually stores a full day of solar heat, the distribution system has all day to distribute this heat to the house.

But, also see the notes below and here for the situation in which the solar system is the only heating system and must provide all of the heat and include a backup heater for non-sunny weather.

Sizing Guide

In sizing the heat distribution system, it is probably fairly safe to say that your solar system on a sunny day can deliver about 800 BTU/sqft of collector area. So, if you have a total collector area of (say) 200 sqft, and you estimate that the domestic hot water uses the output from about 50 sqft of that, then then the maximum solar heat output of the system for a sunny day going to space heating would be about (150 sqft)(800 BTU/sqft) = 120,000 BTU for the day. The heat distribution system should be able to distribute this much heat to the house over a full day. How much heat your floor loop can transfer to the house depends on the length of the loop, the temperature of the water you are circulating, and on the R value of the floor and floor covering between the PEX floor loop tube and the room, and also on whether you are using heat spreader plates. This is a handy plot form the Warmboard Website that shows the relationship between heat output needed, the temperature of the water you circulate to the floor, and the R value of the floor covering. The plot lets you estimate how much heat a given size floor can distribute to the house based on the R value of your floor covering and the average water temperature you want to circulate to the floor. For example: Assume: — We need to distribute 120,000 BTU from the storage tank to the house (as per the example above). — 900 sqft of radiant floor heated area. — R value for your floor covering is R1 — Target an average of 85F for the circulated water (to keep the solar collectors operating efficiently) — That the system can operate up to 20 hours a day to distribute the 120,000 BTU in the storage tank. Reading off the plot where 85F water supply and R1 floors lines cross gives a heat output of about 10 BTU per hour per sqft of floor. So, the Heat Output of the floor for the day = (10 BTU/sqft-hr)(900 sqft)(20 hours) = 180,000 BTU per day So, the system makes the 120,000 BTU per day with quite a bit of margin in hand. Given that this is a Warmboard plot, and Warmboard is very much on the efficient end of radiant floor heat delivery systems, it is good to have some extra margin over the needed 120,000 BTU per day if you are using a less efficient system. Bear in mind that this is just a rough way of estimating the amount of floor loop needed — you can probably find methods that more exactly match the actual type of radiant heat loops you plan to put in.

$2000 Solar Space + Water Heating -- Radiant Floor Design

Alternative Heat Distribution

There are other ways to distribute heat to the house from the solar system.

Larger radiant floor If you have a larger solar collector for space heating than we do (a good idea), then you may need more than one radiant floor loop to distribute the heat to the house. Our Solar Shed project, which has 240 sqft of collector has 5 floor loops that are fed from a manifold — it is described here. — look in the section on heat distribution to the house.

Hydronic baseboard Regular hydronic baseboard units can be used to distribute solar heat to the house. Although these units are generally intended for higher temperature water that solar systems typically deliver, they still deliver significant heat at lower temperatures.

As an example, 20 ft of the SlantFin hydronic baseboard in the table operating on 110F solar heated water could deliver (160 BTU/ft)(20ft)(24 hr) = 77,000 BTU over a full day.

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