Heat pump operating at reduced capacity

BM2

We live in a large old house in suburban New York and recently had heat pumps installed as part of our AC replacement. They work quite well into the 40s and when it gets below that we switch to an gas-fired steam system.

Earlier this weeks as the temps warmed up I switched back from steam to the heat pumps. We have two systems, one for downstairs and another for upstairs. Each is a 5 ton Bosch BOVA (BOVA60HDN1M20G), connected to Unico air handlers both set to 1000 cfm.

We have been comfortable upstairs as usual, but I noticed the downstairs system was struggling and will run all day without meeting temp. I measured temps at the supply ducts, around 80 degrees downstairs and over 95 degrees upstairs. So something seems definitely wrong with the downstairs system.

I went out to the heat pumps and recorded the codes from each. I'm honestly not sure what to make of them. We had a local HVAC contractor out, they weren't very helpful and said they'd have to call Bosch to see what any of the codes meant.

Here are the codes for both heat pumps:

Code description	Downstairs HP	Upstairs HP
Capacity	H5 (5 ton)	H5 (5 ton)
Mode	Heating	Heating
Compressor freq	26 hz	80 hz
T3 coil temp	43	36
T4 ambient temp	50	50
T5 discharge temp	136	168
Th suction temp	43	30
T3L liquid line temp	79	106
Tf module temp	55	60
Evap pressure	116	101
Condenser pressure	449	391
Evap temp	43	32
Condensing temp	126	117
Target discharge temp superheat	32	32
Compressor discharge superheat	12	57
Openings of EEV	99	252
Fan speed	6	9
Compressor current	7	12
Last fault	PH (low discharge superheat)	P1 (HP protection)

Can these values clue anyone in? I'd really like to keep my family comfortable with the heat pump, especially since it was working so well previously. Thanks for reading my tale of two heat pumps and Happy New Year to you all!

EdTheHeaterMan

Sounds like it may be low on refrigerant.

vhauk

your downstairs is running too slow. Any number of things could be the cause. 

vhauk

Sorry, downstairs compressor is running at a very low speed. Pressures and temps reflect this. The frequency determines the speed of the compressor. The higher the frequency the higher the rpm. 

pecmsg

10-Tons???

How big of a home? I'll assume Down State.

JUGHNE

I don't know anything about Unico air handlers.

But 1000 CFM for 5 ton HP/AC ? (each unit)

How does that work?

Matt_67

With the discharge pressure and liquid line temp it looks like it could be overcharged.  Tough to diagnose in heat pump mode though.

pecmsg

BM2 said:

We live in a large old house in suburban New York and recently had heat pumps installed as part of our AC replacement. They work quite well into the 40s and when it gets below that we switch to an gas-fired steam system.

Earlier this weeks as the temps warmed up I switched back from steam to the heat pumps. We have two systems, one for downstairs and another for upstairs. Each is a 5 ton Bosch BOVA (BOVA60HDN1M20G), connected to Unico air handlers both set to 1000 cfm.

We have been comfortable upstairs as usual, but I noticed the downstairs system was struggling and will run all day without meeting temp. I measured temps at the supply ducts, around 80 degrees downstairs and over 95 degrees upstairs. So something seems definitely wrong with the downstairs system.

I went out to the heat pumps and recorded the codes from each. I'm honestly not sure what to make of them. We had a local HVAC contractor out, they weren't very helpful and said they'd have to call Bosch to see what any of the codes meant.

Here are the codes for both heat pumps:

Code description	Downstairs HP	Upstairs HP
Capacity	H5 (5 ton)	H5 (5 ton)
Mode	Heating	Heating
Compressor freq	26 hz	80 hz
T3 coil temp	43	36
T4 ambient temp	50	50
T5 discharge temp	136	168
Th suction temp	43	30
T3L liquid line temp	79	106
Tf module temp	55	60
Evap pressure	116	101
Condenser pressure	449	391
Evap temp	43	32
Condensing temp	126	117
Target discharge temp superheat	32	32
Compressor discharge superheat	12	57
Openings of EEV	99	252
Fan speed	6	9
Compressor current	7	12
Last fault	PH (low discharge superheat)	P1 (HP protection)

Can these values clue anyone in? I'd really like to keep my family comfortable with the heat pump, especially since it was working so well previously. Thanks for reading my tale of two heat pumps and Happy New Year to you all!

Where's the installing Contractor? BOSCH make a great system but the techs MUST be trained on the proper start up and Commissioning of them.

Aren't the codes on the covers?

BM2

@EdTheHeaterMan seems to think it might be undercharged, and @Matt_67 thinks it could be overcharged?

When the tech was here he checked the liquid and suction pressures using a gauge and thought it "seemed fine". I know that's not the same as measuring the charge, but if there was a leak wouldn't that result in a pressure loss? How else could it lose charge other than a leak?

And if it's overcharged, that means it was commissioned incorrectly? Why would I only observe reduced capacity now, over a year later?

I am curious--if you came to my house, what steps would you take to diagnose this? Yep, it's a big old house in Westchester county.

pecmsg

Unless this Big Old house is 7 - 8K Sq Ft with bad windows and no insulation that equipment is seriously oversized.
My house on eastern isl of long the 2nd floor is 100 years of no insulation and horse hair plaster with balloon construction. I installed a 2-Ton Fujitsu heat pump. Balance point is 20°f. Then I switch over to the Hot Water back up.
Each of your units can run as low as 3-Tons! so there's 6-Tons minimum.
Go to the BOSCH web site and find a contractor that's been trained on these units.

EdTheHeaterMan

BM2 said:

@EdTheHeaterMan seems to think it might be undercharged, and @Matt_67 thinks it could be overcharged?

When the tech was here he checked the liquid and suction pressures using a gauge and thought it "seemed fine". I know that's not the same as measuring the charge, but if there was a leak wouldn't that result in a pressure loss? How else could it lose charge other than a leak?

And if it's overcharged, that means it was commissioned incorrectly? Why would I only observe reduced capacity now, over a year later?

I am curious--if you came to my house, what steps would you take to diagnose this? Yep, it's a big old house in Westchester county.

That is why I thought under charged. Over charged would hav been a problem all along. Under charged = a slow leak or a recent leak. I agree with @pecmsg, get a Qualified Bosch trained tech there ASAP.

Matt_67

Yep you’re going to need a tech. 47 degrees of subcooling is too high, it’s a matter of why it’s that high. I didn’t realize it was over a year old. A restriction in the liquid line could also result in that type of reading, that could be caused by a plugged filter drier or a couple of other items. Or a faulty sensor, though I doubt that based on the other readings. Ultimately you’ll need a tech with both pressure gauges and temperature clamps to get to the bottom of it.

ethicalpaul

You're in good hands with the troubleshooting, but a couple things you said really have me scratching my head.

Any new heat pump should be pretty good down to like 30 degrees in my head anyway. Does Bosch say how low they can reliably and efficiently run? For comparison, my mom's Carrier is over 10 years old and I have it going down to 25 and it heats her house fine.

Second, we don't have to "switch it over" manually. She has a really quite basic appearing programmable thermostat that lets us set the temperature at which the oil furnace is used instead of the heat pump. Surely your installer gave you a thermostat that can do this, or they are doing you a disservice IMO.

vhauk

Something is telling your downstairs compressor to run at a low rpm. Resulting in a low capacity. AC electrical motors are now being driven by inverters to control speed, giving the unit a higher efficiency rating. The higher the frequency/hertz, the higher the compressor rpm. 

ratio

Discharge head is up, the controller is trying to manage that. Discharge superheat is low. Check the charge.

JUGHNE

Old school guy here, still sitting in the back of the room with my hand up.....

How does a 1000 CFM Unico AH do 5 tons??

ratio

I figured that, since it worked before, that was a typo.

JUGHNE

Perhaps, but we don't know the tonnage of the originals.

The total of 10 tons sounds above and beyond for most houses.

Matt_67

I think he has the Unico high velocity air handlers, the 5 ton is rated for 1,000 CFM for “normal” conditions. They can be switched to 1,250 for “dry” climates. The Bosch heat pumps work well on them because the compressor speed varies based on suction pressure and they won’t ice up. I suspect the OPs is sized to give more heating capacity.

BM2

Thanks everyone for the suggestions. @Matt_67 is right that 1000 CFM is within the nominal range for 5 tons on Unico. We bump it up to 1150 CFM for cooling. And he is also right that some excess capacity was added for heating, with the Bosch heat pump able to ramp up and down as needed.

Another thing @Matt_67 mentioned is the filter drier. I just looked again and the downstairs HP does not have one (I looked at the lines outside at the HP and inside near the air handler). The upstairs HP does. If I recall, I think the installer told me this was because the downstairs air handler was located in basement below the height of the HP and it didn't need one. Is that right?

Matt_67

I think Unico wants a sight glass at the air handler if its above the condenser. But there should be a filter drier somewhere on the system regardless of whether there is a sight glass or not.

EBEBRATT-Ed

It looks like the refrigerant system may be restricted to me. But with variable speed compressors it's hard to tell

BM2

So it sounds like the consensus is that I need to get a tech in to do some or all of the following:

• Check (weigh) the charge
  
• Purge or otherwise evacuate the lines for any obstructions
  
• Install a filter drier since there isn't one
  
• Recommission according to Bosch installation instructions Chap 14 "System Charge Adjustment"
  

Were there other steps I'm missing? Would this most likely resolve my problem?

Regarding the tech, the original installer was BOSCH Certified and I was not too happy with their work, I learn today they have omitted the filter drier (which is clearly listed in the installation instructions), and the installation is past the 1 year warranty anyway.

The tech who came last week from a different company was also BOSCH Certified, poked around my house for a couple of hours, charged me dearly for it, said he had no idea what was wrong and would have to call BOSCH.

If it were you would you call back either contractor?

I would be grateful if anyone had recommendations for a solid Westchester County, NY tech, ideally someone part of the HeatingHelp community. Please DM me!

mattmia2

A tech wouldn't do any of those things. They would troubleshoot and figure out what the problem is then fix that. It looks like you have all the information there to figure out if the charge is correct or of there are other sealed system problems without opening the system. I'm not sure if there is a lot of point in adding the filter dryer now unless you need to open the system anyhow to fix whatever the problem is.

pecmsg

Remember
70+% of all edged Techs are actually salesman!

Have you called BASCH and explained the issues you have. I know most times they won’t talk directly to the end consumer but you don’t know until you try. 

EdTheHeaterMan

JUGHNE said:

Old school guy here, still sitting in the back of the room with my hand up.....
How does a 1000 CFM Unico AH do 5 tons??

High velocity air moving thru smaller ducts = lower CFM with a larger ∆T. It is all in the literature.
It was invented by some rocket scientist over 50 years ago. Or he might have beed in the aviation industry. Interesting story. You should read the book. That's what us Old School Boys do.

RocketScience

I see this old thread wondered if you fixed it? I work with these Bosch units a lot.

First Bosch requires you to put the unit in Force mode to really tell what is going on.

Your unit is overcharged. The restriction is the eev is closed down on purpose. Once the charge is correct the eev will open back up.

Have your technician weigh the charge out the name plate has the exact amount needed for 15 feet of line set for each 10 additional feet = .6 oz.

Once you know the charge is correct run both system in force mode. They shoukd be close to each other in numbers.

if you still have a problem you would look at the airflow side of the equation . To be certain we woukd need static reading and delta T across coil. High super heat could mean return air infiltration. Put the fan speed on medium speed for the test.

for the old school guy when you have 1,000 cfm at constant volume the unit will cool to the equivalent of a 5 ton forced air 2,000 cfm traditional unit. Traditional equipment only runs for 20 minutes over cools or heats the space and satisfies. The Bosch has 30 turn downs so 5 tons can go as low as 1/4 of 1 ton.

i like the guys thoughts on running it down to 25 degrees in winter. Then bring gas on as auxiliary or w3 call. If the unit does not keep up. This will save a ton in natural gas expenses.

HVACNUT

@RocketScience, I missed this thread in January. I know the 5 ton Bosch inverter can handle 1000 cfm, but this system is mismatched with a high velocity Unico air handler. Have you ever seen that?

RickyJGDE

Hi @BM2 did you ever get a resolution on this? The information you provided is pretty detailed, but of course just one snap shot in time. Others have already commented on a lot of this but I'll repeat:

Having 10 total tons doesn't seem a little oversized it seems a lot oversized, especially if you are switching over to a fossil fuel system at 40 degrees F. Such an oversized heat pump system ought to be able to supply all your heating all winter and then some. Which one is more economical depends on the fuel vs electricity cost and the heat pump efficiency.

I assume you have a downstairs thermostat and upstairs thermostat? How far away from setpoint is the downstairs thermostat? Its not surprising for a heat pump of that size on a mild day (50F) to be running at a low speed, but as another commenter pointed out, the sub-cooling is high, suggesting maybe that the air handler is running at too high a speed (1000cfm) with respect to the heat pump compressor's low speed (26Hz).

My best guess is that this is a controls issue with your heat pumps, thermostats and air handlers, and any other controllers not coordinating properly for your application.

Its really confusing why one heat pump is operating close to max speed while the other is operating close to min speed. One explanation could be that they are configured in stages where the second heat pump is only called on to assist the first if it cannot heat the whole house. Just a guess but without knowing more, the control theory of operation vs what is actually happening would be the place I would dig in.

This is an interesting problem so let us know how it works out.