Desuperheater for 2 systems, one load loop

Discussion in 'Geothermal Heat Pump Applications' started by OBS, May 5, 2012.

  1. OBS

    OBS New Member

    I'm specifying 10 ton w2w system for a large home with 130k design heat load (radiant floor). One equipment option would have two 5 ton heat pumps connected to a single load loop.

    I'm considering DSH for one of the heat pumps with 50 gal buffer supporting natural gas primary water heater. It makes no sense to add DSH to both heat pumps, but in this configuration, I won't be able to sequence (alternate) the heat pumps, which is standard practice when I have two systems supporting a single load loop.

    The thing about sequencing is that the benefits cannot be calculated. It's a risk mitigation play to avoid dominant use of one system.

    Does anyone think I should lose the DSH in order to keep the sequencing?

    Also, in general, does a DSH have better payback with a closed loop vs an open loop system?
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I have several systems like you describe

    I am in DC about to call it a day after designing a job in Owings and spending the day hiking in Mount Vernon with our daughter. So this will be short.

    Your can still share the tank with two desuperheaters if piped correctly and still give your machines equal exercise.

    Call me or PM me here.

  3. OBS

    OBS New Member

    Thanks for your reply, Mark. I understand how two could be used, but I don't see the marginal benefit being worth the added cost since there would be relatively few hours when both would be producing hot water at the same time.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    A couple things here. First, assuming a 130kbtu load, I would guess you are in a northern, heat dominated climate. Now, are you sure 2 x 5 ton HPs are giving you enough BTUs to carry 100% of the load requirements in a hydronic application? An open system might help here. Second, do you plan to run the a/c off the same HPs? Third, once you have Water-Water HPs installed, why don't you run a larger hot water tank as a priority zone, and skip the desuperheaters and gas tank, and make all your domestic hot water via the geo system?

    Here is an example, one heatpump, but same concept.
    Temperature and Energy logging by: Web Energy Logger

    Here is a multiple heat pump scenario with A/C, this time with a buffer tank in which 5 heat pumps desuperheaters feed into. Still on full time priority on the DHW.
    Temperature and Energy logging by: Web Energy Logger
  5. OBS

    OBS New Member

    Yes, the project is in heating dominated climate. AC will be chilled water, design load is roughly 90k. We're considering open loop pending well test results and treatment options. Depending on w2w brand/model, heat output for 10 tons nominal at design conditions is 118k to 135k with open loop. There will be a small supplemental boiler. If we go closed loop, we'd need to weigh cost-benefit of adding more geo capacity vs increasing the supplemental fraction. The site has natural gas connection.

    I'm intrigued by the idea of using an indirect tank for dhw. Even though COP would be less with higher EWT from the DHW loop, it would still be cheaper than ng (utility has metered space heating rate of about 7 cents/kWh!). Even on design days, peak loads only last a few hours so there's always going to be excess capacity on a daily basis. However, with no way to model this, I'm hesitant not to provide backup capacity. Are indirect exchange tanks available with electric elements? Doing backup with gas would hardly be worth the additional first cost, since backup fraction should be very low if controls are set up properly.
    Last edited: May 7, 2012
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Two options to consider:

    Throw in another 5 ton heatpump, have it prioritize on the DHW, and supplement the heating system the rest of the time. Skip the gas boiler, the venting and exhaust pipe, the gas supply line. The loopfield might need some tweaking for peak flow, but the total loopfield capacity should not change that much. Loopfield capacity is more determined by total seasonal load, and instead of putting the additional load of the DSHs on it, you might as well add some heat pump capacity and call it a day. Plus, I don't know how much supplement capacity you were running, but supplement (peak) capacity usually does not increase the overall load on the loop much.

    Second, even with a gas supplement boiler, you can have one of the heatpumps prioritize on the DHW through an indirect, and have the boiler being the 3rd stage feeding the buffer tank. You would satisfy your entire hot water needs with one heatpump, have it switch over as a second stage if not needed for DHW, and in case your total capacity suffers, the 3rd stage gas boiler turns on. No need for any heat elements in the DHW tank. A tekmark 264 should control everything, together with 3 way valves and a Tekmark 150 for the DHW tank.
  7. OBS

    OBS New Member

    No plans for gas boiler. The backup boiler is a low cost 3kW electric, appropriate for the small supplemental fraction. The DHW load is less than 3000 Btu/hr (averaged) in mid-winter based on historical gas bills, so adding a 3rd heat pump of any size, let alone 5 tons, is out of the question. Even with two 5-ton units as currently specified, the second unit only operates 10% to 12% of total system operating hours (per GeoDesigner).

    Solar indirect tanks usually have an electric element for backup. The thermostat for the element would need to be set slightly lower than the aquastat so it wouldn't try to compete with the HP.

  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Gas back up should cost less to run than electric. it also takes far less generator if you have frequent poser failures.
  9. zacmobile

    zacmobile Guest

    2 desuperheaters

    Whenever I install 2 heat pumps I always get them both with desuperheaters, then you can rotate them to give equal run times. Just pipe them in parallel to your storage tank with check valves near the desuper out on each of the heat pumps. The higher system performance & simplicity outweighs the higher initial cost of 2 desuperheaters imo.

    Also, if your staging control is set up right you will get nice long single stage run times & high desuperheater percentages.
    Last edited by a moderator: May 7, 2012
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You mentioned that the site has natural gas connection right after you mention the supplement heat, so this is where my assumption came from. You using Geodesigner for this assumes that you are trying to design this with Climatemaster heatpumps. To keep the TMW happy, supply temperatures should not be above 130F. In order to transfer enough heat from the HP to an indirect tank with to get the tank over 120F (10F delta T) you need an oversized heat exchanger. To my knowledge, only the Stiebel Eltron SBB plus tank has enough surface area, and only if you pipe both heat exchangers together, so you can't use the second one for supplement heat.

    But non of that should be an issue if you can truly satisfy 88-90% of the load with the first 5 ton heatpump. Then it is hard to imagine that you need indeed a backup after the second 5 ton kicks in.

    This begs the question what your design temperatures are, and where the job is located.
  11. OBS

    OBS New Member

    It would be helpful to know how much the DSH option adds to cost to builder for a single 5-ton heat pump with standard 50 gallon water heater (unconnected) as buffer. Can someone give me a rough idea? And would that be much different for a 10-ton heat pump? I've been assuming around $1k.

    There's gas on site for cooking, and dhw if necessary. If we have to switch to closed loop, I'm guessing a gas boiler will be cost justified. The spec includes 4 equipment options, 3 brands. I used GeoDesigner to get a rough idea of 1st stage fraction since it was already installed on my PC.

    It's in Detroit area. Surprised me as well, but supplemental is less than 1% and is cheap insurance. We're looking at incorporating a conventional electric tank as buffer for the floor system (as opposed to electric boiler), so a 4.5 or 5.5kW tank element would provide more than enough backup capacity for any of the geo equipment scenarios on the spec. This is not worth focusing on.

    Great point. The solar tanks have small hx since the incoming water temperature is so high. I'll look at the SBB you mentioned.
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The retail pricelist discounts the TMW 5 ton by $595.00 if ordered without the DSH.

    In addition, you need 2 tanks, one as a buffer and one as final conditioning tank. Then you need to pipe everything. I think you are looking anywhere between $2500-$3000, we usually charge roughly $4000 for the SBB option off the heatpump. So it is about $1000 more, minus tax credits. But now you have 24/7 hot water via the geosystem.

    I would always go with (2) 5 ton units instead of one 10 ton. Better control, same price, more redundancy.

    Have a look at the hydron module heatpumps (or geocomfort, same thing), their 5 ton units put out about 6500 BTU/h more heat out. That gains you 13000 BTUs/H compared to the modeling with the Climatemasters. Keep in mind that

    Used supplement heat element in the buffer tank as supplement stage before. Works well in combination with multistage boiler control. We have it on a Tekmar 264 4 stage boiler control with outdoor reset, and don't allow it to engage as 3rd stage (with 2 HPs) unless the outside temp drops below 0 degrees F.

    The problem might be a flow issue, since the ports for regular hot water tanks might be too small for 10 tons of geo capacity. Waterfurnace makes a buffer tank with 1.5" ports and an electric heat element. Otherwise we use Bock tanks wit 2" ports for multi HP applications.
  13. OBS

    OBS New Member

    The Climatemaster TMW120 actually has two 5-ton scroll compressors. It should provide same benefits as separate units, presumably with internal sequencing controls, simpler plumbing and power, and I hope a slightly lower price than two TMW60's.

    The other 120 hp on the spec is WF NDW, which is two-stage off-loading scroll IIRC (and has lowest heat output at design conditions), but we also have 2 x NSW60's on the spec. The other model on the list is the GeoComfort GWT60 (2), which as you mentioned, has higher output at design conditions (wouldn't require any supplemental). I don't want to limit dealers at this point.

    Appreciate the heads up on this. I passed your comment and tank suggestions to the team who is dealing with the radiant floor subsystem. I'm not a wethead, but I'm learning.
  14. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Been there, done that. All the 10 tons have (2) 5 ton compressors. List-Price for example is $1200 more for the TMW 120 than for (2) TMW 060. You cannot alternate the run time, nor can yo turn only one compressor on, nor can you control the flow in to only one heat exchanger. However, this what an external boiler control can do much more elegant with (2) 5-tons.. Most importantly, have you ever tried to install a 726 pound heatpump, meaning having it moved inside a utility room? A 5 ton is not only less than half as heavy, but also half as bulky. The last 10 ton took 4 guys 3 hours to move inside the an old basement, that is without breaking through a wall and building the wall back up, which is what we had to do. Same basement, it took 2 guys 1/2 hour for each 5 ton to be placed.
    Once you put a 10 ton in once, you always opt for 2 smaller units...
    Last edited: May 8, 2012
  15. OBS

    OBS New Member

    Thanks. I saw something in the WF NDW docs that made me think it was a 2 stage, but I've now confirmed it has two compressors. So I've deleted that model from my spec and replaced the ClimateMaster 120 with two 60's. The client can live with the installation challenge of a 120, but no sense in paying extra for the privilege and lose control flexibility on top of that! Great call.

    I was looking at the SBB 300 Plus literature and doesn't look like there's an option for an electric element. This may be a deal killer for direct geo DHW. I've decided must have backup since client may have several guests in cold weather (Christmas), significantly exceeding the normal dhw demand. I think adding another $500 water heater or boiler would push this option out of consideration given the client's low average demand.

    Is SBB the only indirect with enough HX capacity to work, given the lower geo water temperatures?
  16. OBS

    OBS New Member

    I just spoke to Chris at Stiebel Eltron and he said they have an accessory electric element kit (3kW) for the SBB tanks, fits in the spare port. It's not cheap (perhaps $200 dealer cost), but serves the purpose.
  17. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I did not know they had those. Sounds great.
    But I still don't understand your rationale. Why you need one? If you prioritize one of the 5-tons on the DHW, you have roughly 50 kBTU/h capacity to cover all your peak DHW demand. In the meantime, you said you have an electric boiler for supplement, which would cover you while this 5-ton is not available for space heat. Or you don't have a supplement heat for space conditioning, have the second 5-ton prioritize on space heat, and have the Stiebel heat element kick in in case the second 5-ton is not available.
    I also would consider the SBB400plus, 23 more gallons and less likely to run out of hot water, for a couple hundred $$$ more.
    Last edited: May 9, 2012
  18. OBS

    OBS New Member

    It's new. Chris said they're waiting on a shipment of gaskets. They added this to better service the solar application since backup is nearly always required and having to add a separate heater adds both cost and complexity. I recommended they offer a large wattage element.

    Good question. It really depends on which brand of heat pump ends up getting installed. At least one model (geocomfort) doesn't require supplemental space heat, and adding backup for space heat would be more expensive (since you shot down my idea of using a standard water heater for the radiant floor buffer tank, due to pipe diameters). We're just getting soft bids at this point. Once we select a mechanical contractor and move to final design, I just want to to make sure my dhw recommendation has legs without adding substantial cost. If we end up with an electric backup boiler for space heat, then I agree, we shouldn't need backup for the indirect tank since dhw calls will take priority.
  19. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    No brand has to have back-up. it really depends on design and control.
  20. OBS

    OBS New Member

    AMI, you missed the point of my comment, which related to the differences in performance among the different models on the preliminary bid spec. For example, one of the models has 118k btu capacity at design conditions and will therefore require backup, while another model of a different brand has 135k btu capacity and will not require backup capacity. It's just how the numbers fall.

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