December 18, 2008

Abstract: 

The combination of tankless hot water heating and solar hot water heating creates some challenges that we have been researching with our builder partner, Coastal Habitats/Coastal Green Building Solutions, Hilton Head Island, SC. Sending solar preheated water into a tankless water heater can cause wide temperature fluctuations at the domestic taps. The purpose of this research project is to design and install a solar hot water system that features a tankless water heater integrated with a solar hot water panel such that consistent temperature control can be achieved at the domestic taps. This report is an excerpt from the 2008 Building Science Corporation Industry Team Building America Annual Report.

1. Executive Summary

1.1 Solar Hot Water Research Overview

The combination of tankless hot water heating and solar hot water heating creates some challenges that we have been researching with our builder partner, Coastal Habitats/Coastal Green Building Solutions, Hilton Head Island, SC. Sending solar preheated water into a tankless water heater can cause wide temperature fluctuations at the domestic taps. The purpose of this research project is to design and install a solar hot water system that features a tankless water heater integrated with a solar hot water panel such that consistent temperature control can be achieved at the domestic taps.

1.2 Key Results

BSC worked directly with the builder and plumbing contractor at the Building America Prototype house (Bryant Park Cottages, Molly, Lot #8) to install a prototype system which integrated a solar hot water heater with a tankless water heater plus small storage tank and circulator. Temperature sensors, flow meters, valves, and bypasses were also installed accommodate monitoring of the system performance during short-term tests and long-term monitoring.

Short-term testing confirmed that the solar/tankless hot water heating system, without the small storage tank and circulator modifications suggested by BSC, would not reliably provide the expected hot water temperature to the domestic taps at low flow rates. Addition of the small storage tank successfully resulted in hot water reaching the taps at very low flow rates, and when the water temperature supplied to the tankless heater from the solar collector was elevated but still less than 130oF. However, other unexpected temperature instability issues with the prototype design were observed and we are continuing to work toward the best resolution.

At a different test house, water quality issues, causing too frequent clogging of the tankless hot water heater inlet filter and mineral scaling of pipe fittings, were found to be the only problems in a prototype tankless/combination space and domestic hot water heating system. Electronic water conditioning and pre-filtering are solutions that were investigated and seem to be working well.

1.3 Gate Status

1.3.1. Source Energy Savings and Whole Building Benefits ("must meet")

This project meets the Gate 1B “must meet” requirement for source energy savings. The prototype system which couples a solar hot water heater with a tankless water heater plus small storage tank and circulator, is expected to save energy relative to the same solar hot water heater coupled with a standard storage type water heater. The prototype system is also expected to reduce inefficient tankless hot water heater cycling and reduce hot water use while improving temperature control at the domestic taps.

1.3.2. Performance-Based Code Approval ("must meet")

This project meets the Gate 1B “must meet” requirement for performance-based and
safety, health and building code requirements for new homes. The solar hot water/tankless water heater integrated installation will be fully compliant with all
relevant performance-based codes.

1.3.3. Prescriptive-Based Code Approval (“should meet”)

This project meets the Gate 1B “should meet” requirement for prescriptive-based safety, health and building code requirements for new homes. The solar hot water/tankless water heater integrated installation will be fully compliant with all relevant prescriptivebased codes.

1.3.4. Cost Advantage (“should meet”)

This project is expected to meet the Gate 1B “should meet” requirement for strong potential to provide cost benefits relative to current systems. The prototype system which couples a solar hot water heater with a tankless water heater plus small storage tank and circulator, is expected to provide energy savings, water savings, and increased temperature control at the domestic taps at a small incremental cost relative to the same solar hot water heater coupled with only a tankless water heater.

1.3.5. Reliability Advantage (“should meet”)

This integrated product should meet the Gate 1B “should meet” requirement to meet reliability, durability, ease of operation and net added value requirements for use in new homes. Sensitivity to water quality resulting in too frequent clogging of the tankless heater inlet filter, and scaling inside pipe fittings, presents some reliability concerns that would be common to similar systems with or without the prototype modifications of a small storage tank and circulator.

1.3.6. Manufacturer/Supplier/Builder Commitment (“should meet”)

BSC has discussed these system issues with technical staff at a major tankless water heater manufacturer (Rinnai America Corp.). They were more willing to discuss the water quality issues than the solar integration or the combination space and domestic hot water system integration (they said we were on our own with those systems). BSC will continue to conduct more research on this advanced subsystem in partnership with our builder partner and NREL before approaching a manufacturer about collaboration toward wider market solutions.

1.3.7. Gaps Analysis (“should meet”)

Outstanding issues that our research aims to more fully determine include: 1) prototype system performance and occupant acceptance regarding temperature stability at the domestic water taps over a year of occupied monitoring; 2) the frequency of temperature instability at the taps if the small storage tank and circulator modifications were not used; and 3) annual energy, water, and cost savings.

1.4. Conclusions

BSC was successful in creating a prototype water heating system that merged an Integral Collector Storage (ICS) solar hot water heater with a tankless water heater plus small storage tank and circulator. The system achieved the goal of providing temperature control at the domestic taps regardless of very low flow, frequent on/off flow, or solar pre-heated water supply. The system is expected to provide energy savings, water savings, and increased temperature control at the domestic taps. An important but easy to implement improvement was already made. Another proposed improvement is scheduled to be implemented and retested. Long-term monitoring under normal
operating conditions will begin once the house is sold and occupied.

2 Introduction

Tankless domestic hot water heaters are becoming standard practice for many high-performance homes. Domestic hot water heating amounts to 10 to 15 percent of total energy use in these homes. Upgrading from a standard tank/storage type gas water heater with an Energy Factor (EF) of 0.56 to a tankless hot water heater with EF between 0.82 (natural gas) and 0.85 (propane) can save 40 to 50 percent on hot water heating energy use, for a total energy use savings of around 5 percent.

The combination of tankless hot water heating and solar hot water heating creates some challenges that we have been researching with our builder partner, Coastal Habitats/Coastal Green Building Solutions, Hilton Head Island, SC. Sending solar pre-heated water into a tankless water heater can cause wide temperature fluctuations at the domestic taps. All gas-fired tankless hot water heaters have a minimum firing rate_usually not less than 15 kBtu/h. If the entering water temperature is too close to the outlet temperature setpoint, then the unit may not fire, causing wide temperature fluctuations at the domestic taps. One major manufacturer recommends that the water going into the unit be no warmer than 75°F because 15 kBtu/h will give a 50°F temperature rise at 0.5 gpm (the lowest flow rate to activate the heater) and 75+50=125, which is the highest recommended domestic hot water temperature.

Referring to Figure 5.1, at the minimum hot water draw rate of 0.5 gpm, the minimum firing rate will produce a 50oF temperature rise. That means that if the water heater outlet set-point is 125°F, and the inlet water temperature is greater than 75°F, then the water heater will shut off, delivering water to the tap as low as 75°F. If the flow rate increases, or if the inlet water temperature falls, then the heater will fire again and start delivering 125°F water to the tap. Referring again to Figure 5.1, depending on the water flow rate (between the minimum and maximum allowed by the unit) this type of temperature fluctuation can occur with solar preheated water anywhere between 75°F (at 0.5 gpm) and 121°F (at 8.5 gpm). It is very likely that the solar preheated water will quite often fall within that range.

Figure 5.1: Water temperature rise versus flow rate for a typical gas tankless hot water heater

Tankless water heaters can cause annoying temperature fluctuations at the taps for other reasons as well:

  1. Tankless hot water heaters are activated when water is drawn through the unit. If the water flow rate is below the typical 0.5 to 0.9 gpm minimum to activate the unit, the unit will not fire, delivering water to the taps at the water main temperature. That can cause occupants to increase the flow rate beyond what was needed in order to get hot water. Energy and water savings could be realized if that limitation was eliminated.
  2. With frequent on/off type water use, hot and cold water will be intermittently delivered to the tap due to water heating delay times as the burner cycles on and off. It can take 10 seconds after the beginning of a hot water draw for a tankless heater to prove proper draft and fire the gas burner. Rinnai now keeps the vent blower running for about one minute after the end of each hot water draw in order to reduce the re-firing time to a few seconds. So, delivery of cold water between on/off hot water demand can typically last from a few seconds to 10 seconds. Not only is frequent equipment cycling inefficient, but hot water waste could be significant as occupants leave the water on when it is not needed in order to avoid the temperature fluctuations.

Field research is being conducted to evaluate whether these situations are actuality a significant threat to the success of tankless hot water heating systems in high-performance homes, and whether or not there are cost effective solutions. In 2007, the systems shown in Figure 5.8 and Figure 5.9 were proposed by BSC as solutions, with an initial focus on the integration of solar hot water heating with tankless hot water heaters.

The system of Figure 5.8 adds a small, well-insulated storage tank that is backed up with electric heat to a setpoint approximately 10°F below the tankless hot water heater outlet setpoint. In that way, the temperature fluctuation delivery problem should be resolved for conditions when:

  1. The hot water demand is below the minimum flow rate to fire the gas heater: and
  2. The inlet water is warm enough that the water heating capacity needed is below the minimum firing capacity of the gas heater. The frequency and duration of this condition, and the heat loss from the tank will impact how often the electric heating element will need to come on.

The mixing valve shown in Figure 5.8 and Figure 5.9 is required for when the solar pre-heated water is too hot for delivery to the domestic taps, and when the storage tank temperature needs to be high for space heating applications.

The system of Figure 5.9 goes further than the system of Figure 5.8 by eliminating the short-cycling inefficiency of the gas heater and by eliminating the need for any electric heating. The system adds a pumped circulation loop from the side of the small, well-insulated storage tank to the tankless water heater, much like a typical boiler plus indirect water heater system. Whenever the thermostat switch on the storage tank closes, the circulator will be energized, moving water through the gas heater. Tankless hot water heaters have a relatively high flow resistance; the pump needed to force water through the unit will draw about 90 W—not an insignificant amount of pump energy. Therefore, while this system will provide the best operation in terms of supplying regulated temperature water to the taps, and it does not require an electric heating element, there will be circulator energy and storage tank losses that need to be better understood.

The system of Figure 5.9 also has the advantage of being easily adapted to combination space and domestic hot water heating applications as shown in Figure 5.2 In that configuration, the storage tank needs to have 2 additional side ports available for the pumped loop to the hot water coil in the space heating process air stream. Small, 12 gallon tanks of with 4 side ports and 2 top ports are readily available (such as Whirlpool md# E1F12US015V made by US Craftmaster, sold at Lowe’s for about $210).

3 Prototype Testing Results for Integration Of Tankless Hot Water Heating With Solar Hot Water Heating

BSC worked directly with the builder and plumber at the Building America Prototype house (Bryant Park Cottages, Molly, Lot #8) to install the tankless domestic hot water heating integrated with solar hot water heating system shown in Figure 5.9. Temperature sensors, flow meters, valves, and bypasses were also installed according to Figure 5.2 to accommodate monitoring of the system performance during short-term tests and long-term monitoring.

Figure 5.2: Locations of flow meters, temperatures sensors, bypass loops, and hose bib connections for short-term testing

NREL researchers later installed the data monitoring system and conducted short-term tests, according to the test plan as described in Appendix A. Observations from the short-term testing were as follows:

  1. The solar collector appears to be working well based on preliminary testing. The collector efficiency was 42% based on a test where the collector was drained at the beginning and end of the day, which is very consistent with our expectations given the system design and the weather conditions during the test. There is a section of PEX piping near the outlet of the solar hot water system inside the house that could see temperatures in excess of the PEX rating (peak temperature measured right after long-term stagnation of collector was 176°F). PEX is rated to 180°F, so that section of pipe should be replaced with copper to be sure that there will be no pipe failure if the collector should exceed 180°F due to no domestic hot water use for long periods of sunshine.
  2. The following issues were confirmed during short-term tests where the small
    storage tank was bypassed:
    • The tankless heater doesn’t fire at low flow rates (<0.5 gpm). The Rinnai tankless water heater does not turn on when its internal logic determines that the combination of flow rate and inlet temperature would result in the outlet temperature exceeding the set-point even at the lowest burner setting.
    • The tankless water heater doesn’t fire when there is an elevated supply temperature from the solar collector combined with slightly higher flow rates (0.5-0.8 gpm).
    • With solar pre-heated water, the tankless heater sometimes shuts off after turning on for a few minutes, probably caused by the tempering valve gradually decreasing the hot water flow rate, causing the maximum outlet temperature of the tankless heater to be exceeded.
  3. Addition of the small storage tank successfully results in hot water reaching the taps at very low flow rates, and when the water temperature supplied to the tankless heater is too high.
  4. The tempering/mixing valve seems to have a minimum cold water flow. It cannot maintain set point at 120°F when water entering the valve is below about 130°F . A different tempering valve without a minimum cold water flow or raising the storage tank thermostat setpoint should resolve that issue.
  5. The hot water system exhibited some temperature stability issues when using the small buffer tank at higher hot water flow rates. With the storage tank thermostat set at 125°F and the tankless heater outlet temperature set at the maximum of 140°F for the residential heater, a temperature droop was observed at the domestic taps a minute or two after the water draw began. After the circulator started moving water through the tankless heater, the water temperature delivered to the taps increased more slowly than expected. Moving the storage tank thermostat set-point closer to the heater output set-point helped but not enough to resolve the problem. Unless the lower limit of the temperature droop response is above at least 105°F, this would likely result in comfort problems when the occupants take showers at high total hot water flow rates.

To resolve these temperature instability issues, BSC is working with the builder to replace the residential unit with the same size commercial unit which can be set for a higher outlet . . .

Download complete report here.