Forgive me if this has been discussed, but I did a search and didn't see anything.

Taco is about to ship their new 00-VDT circulator and it seems like an interesting idea. Size for the maximum flow, set the delta T and let the pump throttle itself back to whatever gpm is required.

Since the 00-VDT is basically a simplified 00-VS I'm wondering if anyone has installed a system that works this way and how it turned out?

I like the idea of only one circulator doing it all and using the minimum energy required, but I'd feel a lot better if I had an idea of what the downsides might be.

well, I wonder how it handles very low temps.

say you design for a 20 dT. but it's october 5th, and your water temperature (reset of course) is only 80 degrees... what does it do?

I'm interested to know as well, that's an interesting control method. I would also like to know how fast it ramps down.. I doubt it is fast enough to drop pressure bypass valves, like the pressure sensitive pumps can... but if this is or could be combined with pressure sensitivity, it sure would be great! its literature http://www.taco-hvac.com/uploa...eLibrary/102-359.pdf says PBV can be eliminated.. but I don't see how it can deal with a zone closing when it's on high speed.... it wouldn't see the temperature difference until after the "slam". I guess if there were a problem, it would be a short window, but still..

pretty cool though, if you're careful!

That's a real good question. I'll give Taco a call and see what they say. Anything else you'd like me to ask?

Hopefully they'll come up with an answer we buy why it isn't a problem, but if not I like the idea of pressure/temperature. The former would be relatively short acting whereas the latter would have a long time constant perhaps?

I have to make a comment on outdoor resets because I just don't get it. It seems to me these are just a bad proxy for estimating the load (10F and sunny could be a lot less load then 20F on a clear night with wind). Why use that to vary the water temp when you can measure the load directly?

There are a number of ways to do this for example a thermostat can tell you (although I don't know if any thermostats incorporate the feature yet). Think about duty cycle. If the thermostat is calling cycling on a 10% duty cycle well then that's a clear signal the system could be running with cooler water. Likewise if it is running at 90% then it's running out of headroom. So all one needs to do is watch the duty cycle and pick a number as the centerpoint and vary the water temperature based on that.

This is a direct measurement of the load. Whether its cold and sunny (with lots of solar gain) or clear and warm the system just doesn't care. It simply provides the right temperature water to keep the system running.

The control might be as simple as watching the duty cycle on the pump(s). Assume a setpoint of say 75% on. If it drops to less, lower the temp, above that raise it (you can even adjust the slope or make it non-linear). This would adjust the system to handle the worst case zone.

So, what you think Rob? Its not that outdoor resets are bad, they're just the best they could do before we had dirt cheap microprocessors. We can a lot better now.

You're right that reading load directly is best, and a couple of control systems use the method you talk about, I believe Exquisite heat does, and if I'm not mistaken the new Ray Mestek boiler might do something similar as well.

Then there is Tekmar that actually senses in each zone, even better, that can take you darn close to 100% on times.. and react more or less aggressively based on how far from current room setpoint you are, something duty cycle alone cannot tell you.

Outdoor temperature is a pretty good proxy, actually. It can't deal with solar gain very well, but if you have a lot of solar gain.. the outdoor temperature is usually rising. Also, the solar gain is rarely affecting all areas of the house and areas without it still have the higher load. And, if you have mass walls or the like, I could make the arguement that outdoor sensing might be more useful than sensing "on lag". Mass emitters should be another headache for such "wait and see" calculation methods.

However, not having the reaction of water temperature connected to the inside of the building is certainly not the best we can do these days, you're right. Outdoor reset has issues, for sure. I guess you can pick your poison if you do anything other than the best

All very good points. What it says to me is that radiant systems are in general very forgiving, particularly if you can run with the water temperature on the high side.

Unfortunately I don't have the luxury. The hot water comes from air sourced heat pumps and reverse cycle chillers. The efficiency of the system falls as a strong function of the water temperature I demand. 105F is easy, 115F cost a bit and beyond that it costs a lot. So there is a strong incentive to set up the system to run low water temperatures.

This was planned for. The house is extremely well insulated and air tight. The radiant is a dry mount system with plates on sleepers right underneath the hardwood flooring. And the climate is generally mild (Portland, OR).

However we do get the occasional very cold day and since the efficiency curves for the heat pumps don't cross the gas hot water heater until 24 F I'd like to run off the heat pumps as much as possible. But I'd also like to avoid using more power than is required and that would indicate the use of a variable speed pump and some way to control it.

This is the attraction of the Taco design, whether the parameter is pressure or temperature or both. I can size the pump for those few days when I am going to have to move a bunch of water (minimum temperature before the system switches to gas) yet run it at a fraction of that the other 99% of the time. Since ECM motors are more efficient at any speed and much more efficient at low speeds (compared to an oversized pump and often to a smaller single speed pump due to impeller losses) this seems like a very nice simple solution especially if it takes care of controlling itself.

The way I envision it the thermostats control the valves (and are smart, such as the Tekmars) and the pump controls itself. Everyone's happy (and no fancy central controller to buy, program and maintain).

Let me know what you think?

PS One other note. Unlike the chillers since it doesn't cost much extra energy to run the water hotter when the heat source is the gas hot water heater that is how the system is set up. This allows me to handle the extra load without excessive flow rates for those extremely rare times when it gets really cold without losing the efficiency I get running the chillers at a lower temperature for the rest of the time. Since code requires a backup to a heat pump anyway and I wanted a tank to prevent short cycling I used a run of the mill hot water heater making the back up close to free.

you're really overestimating the importance of flow rates I think. Pick the highest water temperature area in the home, run that at minimum delta-T, and the rest of the house could easily halve the flow rates or better.

Heat pump people are obsessed with flow I'm noticing (on the dirt side, I can see why). In radiant, I'll tell you most loops out there I'm involved with are running at 0.2 GPM. Not all of course, but most. without raising the SYSTEM water temperature. in the "temp setter' area, I'll run higher rates. So in the end you're really probably talking about an 87 watt pump for a system pump in most homes (possible, at least) vs your variable speed unit.

that said, saving pump energy is good. I just don't find it critical or anything. Just good. In most markets you can cut down a roughly $5/month expense from that pump switch.

A recent radiant living magazine showed a unit that was charging up a buffer tank during the day when it was warmer out, to coast through the night. that seemed pretty cool to me, but you'd have to size the tank carefully and it would probably outstrip water heater range pretty quick.

Fair observation. I need to go run my numbers and see if the difference between worst case flow (just before the system switches to the water heater with its higher water temp) and typical (a 40F day) is that much. If not then your right I'm overdoing it.

In which case if I go with a single speed pump then I'm back to a bypass valve for those time its only a couple of zones open.

I thought about a buffer tank and it really isn't worth it. Perhaps if I'd put one in as part of the concrete pour, say just a big rectangular indentation in the crawl, it might have been fun to play with but only if it was close to free.

BTW thanks for all the thought you've put into this. I really appreciate your time.

And I promise to let you know what Taco says about the issue you raised.

Losing the bypass helps, but the only variable speed pump I've got pricing on still outstrips a pump/bypass combo by more than $100 to $150. Now that would see payback... as long as the pump doesn't fail! and i don't expect it to, but our industry is littered with things that did what we didn't expect it to

If the daytime storage allows you to, say, run an average of a COP higher... you could justify a fairly good size tank. I don't know that it would, but I could see how it could, daytime to nighttime temperature swing is easily 20+ degrees even in "non wild" areas oftentimes... more, in desert climates. That has a big affect on heat pump performance. If that takes you from 2 to 3 COP, for example, that's a 33% energy usage decrease... surely significant.

course i'm guessing on the COP affect!

Well as soon as I manage to get a COP curve out of a manufacturer I'll post it but the American companies are loath to publish them. The Germans do, real engineers there, and I think theirs should be close to ours, but one can't be sure. I'll have real data on the installation I'm doing now by the end of the year so I can tell you at least how I did.

On the other hand you need to figure standby loses, plus some areas have time of day metering where the cost of the ac is less than half the daytime rate after about 10 pm. In those cases you would like a tank too, but you'd run it the other way around, at night.

Since this whole game of spending each incremental dollar where it gets the largest savings I've spent a fair amount of time trying to understand where that is. The answer I've come up with is infiltration. Almost any modern house is so well insulated as far as the walls that there isn't a lot to be gained doing any more. Windows on the other hand are terrible, but the cost to make them better is prohibitive. What's cheap is to keep a hold of that nice warm (or cool) air and the humidity (or lack thereof). It's not much work to get a house to .3 ACH and frankly not a lot more work to get it to .1 or even .05 ACH, but the amount of energy saved is enormous. Funny thing is practically no one does it. There are a lot of reason, fear mostly, since anyone schooled in the old way of building thinks a house that doesn't "breath" is going to have mold (actually its the reverse) but old ways die hard.

This building I engineered is amazingly tight. Not only does it stay warm, but there are no drafts a benefit I really hadn't focused on. I was up in a wind storm where we were seeing 40 mph gusts and not only was it calm inside, it was quiet. I was really amazed. This wasn't something I had thought about, but now I'm a believer.

Best of all it is so easy to do. Simple Caulk and Seal is something any sheetrocker can master in a few minutes. That plus caulking all the penetrations, around the outlets and a variable permeable membrane instead of a standard vapor barrier and you're done.

I highly recommend anyone interested in doing a better structure that they read the books from Scientific Building Corporation, take one of the courses the Department of Energy runs (free) and give it a shot. It changes everything

hope you provide for ventilation I"m sure you did, but it cannot be overlooked so I mention it for future readers.

I have but actually as I've noted it might not need it. It's a big house, over 5500 sf. Even with very low leakage rates its a lot of air (5500 sf x 10' ceilings=55,000cf, x.05ACH=2750cfh or 45 cfm.

With only 4 occupants, often less, you don't need much leakage/cubic foot of volume to get plenty of air.

But there are ERVs. They double as bath fans and with a cute trick also provide the make up air for the range hood and open hearth fireplace.

I am a fan of make up air in general.

I find it imparative in tight new buildings.

O2 is imparative for life.

Leakage doesn't guarantee air quality where you need it. Smaller, closed rooms we breath in all night long come to mind. That requires mechanical ventilation.

Again, a good point except in the home I'm talking about it is a one story open floor plan. For purposes of air circulation its pretty close to one large room. In addition the main blowers have ECM motors and a fall back speed of 300 cfm in addition full speed that I have the ability to program to come periodically to stir things up.

The reason for this is because the preferred topology for ERV/HRV is to keep them completely independent of the main HVAC system. Instead the fresh air comes in via dedicated duct that is integrated into one of the return air vents. In this way the fresh air (if needed) goes directly into the room right in front of the return air vent where it creates a plumb. Then when the main HVAC system comes on this air is right where you want it to spread it around the house.

The nice thing about this setup is that it works well whether the HRV's are running 24/7 or not at all. In the case of the former the air is always coming in but the energy consumption is minimized because only the HRV fan is running (which is a lot better then buying a bigger HRV and then running it only 1/3rd of the time which is crazy). The HVAC system can be used as much or as little as necessary to disperse the air depending on the layout of the home.

Likewise if the HRVs aren't needed at all, they stay off (except when needed as exhaust fans, to supply make up air or when the house has a lot of guests or pollution for some reason).

It is very nice to be able to deal with dispersion and fresh air separately. System that introduce the fresh air directly into the return air duct can't do this. Worse, because they present the HRV with wild swings in pressure they unbalance them. I can't see why anyone would want to do such a thing when it is just as easy to duct the fresh air directly into a room at or near a return. You save money with a smaller HRV, the back pressure is constant, and you can control the whole system much simpler.

But as I have mentioned before old habits often die hard. Either that or I'm missing something.

Bottom line. The home has HRVs but I believe that they will not generally be needed for fresh air. I'll bet the energy recovery from them will be trivial. They're just a convenient way to supply balanced air with free make up tossed in.

I don't understand how shared duct work, with HRV tie in on return and supply, unbalances anything or changes the size of the HRV you need? Usually HVAC ductwork is twice the size you need for IAQ, or larger. That should indicate no need for more powerful HRV fan..?

Also, you get no fresh air distribution if the HVAC is not running.. except one dump point in the middle of the house. As I pointed out, this does not get you fresh air where you need it. You'll get fresh air from that point to your exhaust points which, I assume, are not in bedrooms. Even if you have sufficient bedroom undercuts on your doors, you won't get airflow to them unless you are inducing that flow some other way.. right?

In other words, I would say if you have ventilation, you at least want supplies in bedrooms. If the HVAC ductwork allows that, great. If not, dedicated ductwork should be used.