Gary, we have a 2-story, 4 bedroom house (<1900 sf) with a condensing boiler, hydronic baseboard heat and a mini-split with one head on each floor (2). The mini-split is acceptable for cooling and humidity control. We can use a few fans to help even out the temperatures. We are pretty well insulated and have shade trees on E, S & W, so even without cooling, on a 95F day we will stay below 82F indoors. That's bearable, but the humidity that comes with it is oppressive. We did have to use our mini-split this winter for heat for about 10 days. Our boiler pulled in enough dirt through its combustion air intake to foul the combustion chamber. I have added a filter box to prevent that from reoccurring. So the mini-split and gas log got us thru that January week until the boiler could be put back on-line. But the heat distribution wasn't as good as the hydronic baseboards.

I read that Chiltrix is working on a high temperature ATW heat pump to solve this boiler replacement problem. I have also heard of one other heating solution not available yet: A microwave boiler; an electric heater for hydronic homes. But it will not have cooling. Maybe mini-splits are the answer for cooling, as you say. Or those new window ACs that wrap over the window sill and allow you to see out and open the window. They are supposed to be getting inverters soon, I have read, so they will be a much quieter and more appealing solution, with a SEER2 comparable to mini-splits. I hear they will be reversible too, for heating.

The flare nut connections at each end of the mini-split's refrigeration lines are their Achilles heel. Four opportunities for a leak for each indoor head. After this winter when we used our mini-split as back up while our boiler was repaired, we didn't use it again until a late spring warm spell. But then it didn't respond. After injecting a dye and half a charge of R-410A it worked for a while, then failed again. After checking all the flare joints they finally found the leak in the upstairs head. The dye and UV light showed a tiny crack in one of the flare nuts; I have it here on my desk. That nut cost $900 to replace including another 2.5 lbs of R-410A. In 5 years of operation we have lost 15 lbs of R-410A to leaks. That is the global warming equal of burning gas for 3 full years in our boiler. So that's why I don't think mini-splits that use R-410A or the somewhat better but still high GWP R-32 are not good solutions to decarbonize our homes in order to deal with the climate crisis. They are too likely to leak and make global warming even worse. Central ducted heat pumps avoid the issue with brazed joints, but are an expensive and difficult solution for older homes without ducts.

There is another issue with heat pumps which I never see discussed. Their low-temperature heat makes recovery from night setback long and difficult (I experimented with 120F water this winter and the recovery took 6-8 hours). Mini-split controls omit night setback scheduling because of this. But night setback reduces the number of heating degree days you have to make up for, so was introduced to save energy and money. You have to give that up with low temperature heat pumps, so we not only pay more than gas for the electricity we use in our heat pump, but we have to supply more energy to maintain occupied temperatures all night. The heat pumps that I hope are coming soon CAN recover from night setback with 170F hot water to baseboards, and then reset to a more efficient lower temperature to maintain that occupied setpoint during the occupied hours. They hold the promise of both setback savings and semi-continuous water circulation for uniform comfort temperature with little fluctuation.

Galen, I am thinking about how we can decarbonize on a massive scale. I wanted to reuse as much of the current distribution system as possible to keep the cost down. In the northeast’s relative energy cost structure, heat pumps will not pay for themselves in their lifetime. Indeed, gas customers will have higher heating costs. So retaining the existing distribution system reduces the project cost both now and in 15 years when the mini-split fails and both indoor and outdoor components have to be replaced. The existing hydronic system is good for the life of the house and it gets the heat to every room, which mini-splits are not so good at. As Gary A. points out, we need a drop in replacement for a boiler. The Europeans have that with their propane refrigerant Air-To-Water heat pumps that can make 170F hot water, are less likely to leak their refrigerant because they have factory-sealed refrigerant enclosed in the outdoor unit, and if they should leak their GWP is so low that they won’t create a new climate problem, like R-410A will. And the propane is cheap to replace if it does leak, and the unit can be quickly disconnected and repaired in a shop under good working conditions. If we had these high temperature heat pumps here, they could directly replace the boiler by delivering their to a buffer tank indoors.

That’s sort of good-news/bad news to me. The high-temperature heat pump is a drop-in heating solution, but not easily adapted to cooling. I was trying to manage that with the current US ATW heat pumps that operate in the 105-130F range by adding low-wall fan-coils to the existing piping and using them for heating and sensible cooling cooling only, The fan-coils are required to replace the lost heating capacity due to the lower water temperature. The dehumidification would have to be done by other means. So the low temperature ATW heat pumps available today in the US require a lot of interior additions, but still avoid the expense and disruption of tearing out the existing distribution system and opening the walls to put in a whole new insulated piping system. That’s a really nice solution, but with little-to-no cost savings there is no economic motivation to do it, and I don’t think it would ever happen on a mass scale. Their are cheaper ways to get cooling and dehumidification in these older homes. We only really need it for July and part of August and if you have made some insulation upgrades, the biggest issue is dehumidifying in those months. I have heard that Chiltrix is working on a US high-temperature heat pump, and that would be the perfect boiler replacement for both heat and domestic hot water. That has the potential to make electrification likely at a large scale in the northeast where boilers are so common. As Gary A points out, a simple drop-in solution would be a big deal, making the cost competitive with a boiler replacement.

Curt, You are correct. ALL the dehumidifier energy does get rejected to the space. I was thinking that the half used to condense the water would end up there, but on reflection, I think you are right. I first thought of the dehumidifier as a solution to separating the latent cooling from the sensible because of a few articles by Joe Lstiburek in their Building America project, in which a low cooling load house had trouble with high humidity and they solved it with a dehumidifier in the hall closed and a louver door, because a whole-house dehumidifier was rather expensive. Maybe a small window AC unit running on low speed would be a better solution, eh? But even so, the 1389 Btu/hr isn’t a huge load.

We have millions of existing homes in the northeast with boilers and baseboard or radiators, and I have been trying to think of a way to utilize their existing hydronic system but convert to a heat pump. The two problems I was struggling with is with the low water temperature of the current ATW heat pumps, the high-temperature heating terminals only have about 30% of their capacity, so would have to be augmented. The second issue is that the old system is not designed to handle chilled water and deal with the condensate. But cooling is an important incentive to conversion, because there will be no cost reduction for gas customers and not enough savings to pay for the installation over it’s lifetime for oil customers. So the only solution I could think of was to add low-wall fan-coils to the existing piping to regain heating capacity and use them for sensible-only cooling, but add something else for the latent cooling. My first thought was a free-standing dehumidifier on the first floor, but then I thought with most of these houses having accessible basement ceilings we could add one or two fan-coils on the first floor piped for 45F chilled water to do the dehumidfying for the whole house.

I have done this on commercial buildings with radiant floors or ceilings, but maybe it’s too complicated for residential. It would probably need a special controller that will always keep the cooling water supply to the old hydronic system above the current air dew point in the house. All those new fan coils will get expensive, but unlike a multi-split air Air-to-Air system, they would not have to be replaced every 15 years when the heat pump fails. They would last the life of the house. Thanks for correcting my thinking on where the dehumidifier energy goes.

John Siegenthaler is an early proponent of ATW heat pumps, but his designs involve new insulated piping home runs to each room’s fan-coil. That works for new construction and single story homes, but in 2 and 3 story homes the damage and disruption caused by the new piping is expensive and may require the occupants to move out for a while. I was looking for a cheaper and less disruptive solution.

Don’t guess at loop field dimensions, tube lengths, diameter, etc. Closed loop water source heat pump installers use software and data from site-specific conditions to size and configure loop fields.

1) The entire 407 Watts of electric energy are added to the room as sensible heat (407 * 3.413) = 1389 Btuh. That comes from heat of compression, motor windings, fan energy etc. Every single electrical energy Btu is rejected into the room since the dehumidifier and its air flow is entirely within the room.

2) But wait, there’s more: For the sake of example let me guess that yours is a nominal 30 pint per day model performing at specification. That’s 1.25 pints removed per hour (condensed from air and drained or stored between tank emptying chores).

In round numbers, the heat transfer needed to evaporate a pint of water is about 1000 Btu. To condense a pint of water from vapor to liquid requires extracting 1000 Btu from the air / water mixture. At 1.25 pints per hour, that’s 1250 Btuh extracted from the room’s air to condense the vapor to water. That heat is extracted by the low pressure (evaporator or “cold side” coil) of the dehumidifier. Where does it go? It is added to the heat energy rejected by the dehumidifier via the high pressure (condenser or “hot side” coil) which explains why the air leaving the dehumidifier is considerably warmer than room temperature.

So the net effect of operating your dehumidifier is the removal of 1250 Btuh of latent heat from the room’s air and the addition of 2639 Btuh sensible heat (1389 from the electricity plus 1250 Btuh converted from latent to sensible via condensing the water vapor).

For simplicity, I’ve omitted the additional heat transfer required to first cool the room’s air down to dewpoint before condensing occurs, but that washes out when the very temporarily cooled air enters the dehumidifier’s high side coil.

I wonder if the 50-50 rule you cite comes from the approximate equality of energy used relative to moisture removed. (1389 Btuh consumed vs 1250 Btuh transferred).

A dehumidifier’s ability to make a cold wet space both warmer and drier is useful when operated in clammy chilly basements in New England, the upper Midwest, and Oh Canada. Pretty much anywhere else the waste heat is a nuisance to be endured or additional load to be rejected by air conditioning systems.

The only way for a dehumidifier to remove heat from a room is to direct its high side condensing coil heat transfer somewhere other than to the air in the room. Window / portable air conditioners do that, as do heat pump water heaters.

This all explains why heat pump water heaters work so well in warm climates – they leverage their modest electricity use (similar to a small dehumidifier) so that all the electrical energy PLUS the sensible heat from the surrounding room air PLUS the latent heat from water condensed from surrounding room air are ALL transferred to the water tank to produce domestic hot water.

I hope this explanation is correct, understandable and helpful…but if not, oh well, the price was right!