ASHP/ buffer tank issue

[Rupe30]

Hi all,

 

Been trying to get to the bottom of why my 16kw lg therma is unable to heat the first floor of my property. We have underfloor heating but only a thin layer of screed (6mm I believe) so the temps need to be fairly high to get any heat into the rooms.

 

I noticed the temperature of the buffer tank displayed on the lg monitor differed considerably from the temp gauge on the maniflow. If I turn the thermostats off and on the lg stat suddenly drops down to the temperature on the manifold. It then starts to heat up to target temp of 55, however before it gets there the same thing happens again, the temp of the buffer gets ahead of the temp on the maniflow, I turn the stats off and on and it drops down to the manifold temp. So it appears the ashp is losing signal to the buffer tank.

 

To add to my confusion earlier today the lg controller was showing buffer tank at 52, but the inlet pipe to the buffer was barely warm. After resetting stats it dropped to 25 degrees on the lg controller. I don't understand where the lg is getting it's temperature reading from and why it thinks the buffer is hotter than it is.

 

It's a 300l buffer. Not sure what other info is useful so let me know!

 

Thanks 

[Marvin]

Hi @Rupe30

 

I'm sure we will be able to help but we're going to need a bit more help from you on this...

 

When you say the first floor, do you mean the ground floor or the floor upstairs?

You speak of a screed ( which makes me think you mean the ground floor as I would call it) could it be 60mm thick? 

 

I am also confused by what you  mean by buffer tank. Usually a 300litre tank would be your hot water tank. My buffer tank is 60litres....

 

Any pictures of the equipment or names of equipment installed would help

 

M

 

 

 

[ReedRichards]

I have an LG Therma V.  Mine has no means of knowing what the buffer tank temperature is.

[Rupe30]

6 hours ago, Marvin said:

Hi @Rupe30

 

I'm sure we will be able to help but we're going to need a bit more help from you on this...

 

When you say the first floor, do you mean the ground floor or the floor upstairs?

You speak of a screed ( which makes me think you mean the ground floor as I would call it) could it be 60mm thick? 

 

I am also confused by what you  mean by buffer tank. Usually a 300litre tank would be your hot water tank. My buffer tank is 60litres....

 

Any pictures of the equipment or names of equipment installed would help

 

M

 

 

 

Thanks @Marvinlet me try and give you some more info.

 

By the 1st floor I do mean upstairs. Downstairs is also underfloor but we have a really deep slab so that runs very efficiently. 

 

By screed upstairs I mean self leveller! Underfloor heating provider (continal) were happy with the 6mm coverage.

 

We're using quantum stats and have 4 zones downstairs and 8 upstairs and a manifold on each floor.

 

We have an internal area of 250m2, using approx 100ltr upstairs and 99ltr downstairs.

 

13.7kw space heating load.

 

5 hours ago, ReedRichards said:

I have an LG Therma V.  Mine has no means of knowing what the buffer tank temperature is.

Ive set the flow temp to max 55 as that's what is on my MCS certificate design temp however looking through my thermodynamic report estimated heat output is based on 50 degree flow. I thought the flow temp symbol on the LG controller was the coil within the buffer tank. Is that not the case?

 

 

 

I turned off the heating downstairs this evening and just ran upstairs, the flow temp on the manifold managed to get to 52, matching the LG controller. It must've run for about an hour before the temp on the manifold started dropping and the LG remained at 52 outlet. 

 

I've had the flow temp set to 55 on the LG controller but temps at the manifolds have been mid 30s for a while now which downstairs can cope with and a 2 bedrooms in the middle of the house upstairs but the bedroom with external walls and our north facing landing cannot (bedroom was 14 degrees this morning when stat is set to 18) It seems the only way I can keep the flow temps high is to turn the stats on and off every 30 mins.

 

Our ASHP is running 24/7. Getting through about 80kwh a day at the moment.

 

Ive attached pics of my LG controller before and after switching the stats off and on. Shows 52, and after reset drops immediately to 43.

 

Any ideas?! 

[ReedRichards]

I don't have UFH, just radiators so I have no direct knowledge of how the controller works with UFH.  In my case the top number in the top section is the temperature of the room where the controller is installed, whereas yours says "Lo".  And the number before that is the same as the outlet flow temperature, as with you.

 

I think the inlet and outlet temperatures are exactly as they say and are measured within the outdoor unit.  My buffer tank has no electrical connections at all so I know it cannot report its temperature.

 

Inlet and outlet temperatures both 43 imply to me that the buffer tank is at 43 C and there is no demand for heat.

 

Sorry, I don't know enough about UFH to offer more advice.      

[Rupe30]

10 hours ago, ReedRichards said:

I don't have UFH, just radiators so I have no direct knowledge of how the controller works with UFH.  In my case the top number in the top section is the temperature of the room where the controller is installed, whereas yours says "Lo".  And the number before that is the same as the outlet flow temperature, as with you.

 

I think the inlet and outlet temperatures are exactly as they say and are measured within the outdoor unit.  My buffer tank has no electrical connections at all so I know it cannot report its temperature.

 

Inlet and outlet temperatures both 43 imply to me that the buffer tank is at 43 C and there is no demand for heat.

 

Sorry, I don't know enough about UFH to offer more advice.      

 

 

The "Lo" is just because the controller is in my cold garage, below 12 degrees it just says lo.

 

What I'm confused at is why the LG Controller was showing 53 outlet temp i.e calling for heat, but the outlet temp was actually 43, which it reverted to when I turned the stat on and off.

[Rupe30]

So I've just felt the outlet pipe from the ashp and it's hot, so I think the 53 displaying on the LG controller is correct. The manifold thermostats are both showing 26 degrees. One thing I have noticed is we have a thermostatic mixing valve entering our manifolds which ahould be for high temp heat only. They are open fully, could this be mixing cold in? 

 

 

[JohnMo]

Assume when you say manifold you are talking UFH manifold.  The temp on the manifold will never be the same temp as the buffer.

 

Why, because there is a mixer on manifold, the mixer will alway mix a percentage of the return water with the incoming water, the gauge on the manifold represents a mixed flow temp. There is an issue with most mixers, in that they like to have set temp, 10 degrees lower than the supply temp into it. Because they operate with a fixed minimum mixing all the time.  So if buffer is set to 55 it is unlikely you will get more than 45 on the manifold.

 

Have to say having to heat a large buffer to 55 with a heat pump, doesn't sound the most efficient way to heat a house.  If you are having to run UFH at those high temps sounds like you would have been better with radiators.

[Rupe30]

13 minutes ago, JohnMo said:

Assume when you say manifold you are talking UFH manifold.  The temp on the manifold will never be the same temp as the buffer.

 

Why, because there is a mixer on manifold, the mixer will alway mix a percentage of the return water with the incoming water, the gauge on the manifold represents a mixed flow temp. There is an issue with most mixers, in that they like to have set temp, 10 degrees lower than the supply temp into it. Because they operate with a fixed minimum mixing all the time.  So if buffer is set to 55 it is unlikely you will get more than 45 on the manifold.

 

Have to say having to heat a large buffer to 55 with a heat pump, doesn't sound the most efficient way to heat a house.  If you are having to run UFH at those high temps sounds like you would have been better with radiators.

Ok so first thing to do is remove those mixer valves then, sounds like I'm paying to generate heat only to cool it before it gets to our UFH.

 

Most the house could run cooler temps than 55, it's only a couple of rooms that are losing heat in this cold weather. Do you think the 300ltr buffer is excessive? 

[ProDave]

25 minutes ago, Rupe30 said:

Ok so first thing to do is remove those mixer valves then, sounds like I'm paying to generate heat only to cool it before it gets to our UFH.

 

Most the house could run cooler temps than 55, it's only a couple of rooms that are losing heat in this cold weather. Do you think the 300ltr buffer is excessive? 

Just reduce the "water leaving" temperature on the heat pump.  No point whatsoever having it set to 50 degrees only to be mixed down to a lower temperature by the mixing valves.  But no need to remove the mixing valves.

[Rupe30]

10 minutes ago, ProDave said:

Just reduce the "water leaving" temperature on the heat pump.  No point whatsoever having it set to 50 degrees only to be mixed down to a lower temperature by the mixing valves.  But no need to remove the mixing valves.

How do I get a higher temp to the UFH manifolds though? Will the mixing valves not reduce the water leaving temperature i set as it's mixing with the return water?

[ProDave]

If the manifold water is not hot enough to heat the house, turn the mixing valve up.

 

Once you know what manifold temperature you need, then turn the water leaving temperature on the ASHP down to only slightly above the required manifold temperature.

[ReedRichards]

2 hours ago, Rupe30 said:

Do you think the 300ltr buffer is excessive? 

 

So excessive that 300 l cannot possibly be the correct volume.

[JohnMo]

It does sound big, they are only there to reduce short cycling, when min output from heat source and min load required and small system water volume would lead to a short run times.  They also act as hydraulic sepereration between various pumps.

 

So your heat of pump should turn down to about 5kW.  So have run some run-times calcs, that show the following.

 

With a minimum CH heat demand load of 1kW.

 

System volume 300 plus 100, 400L in the ch system, run time 28.mins, off time 111 mins.

 

Reducing the total system to 145 incl a buffer, would give a run time of 10.1 mins and off time 40.4 mins. Which isn't short cycling for an ASHP.

 

Increasing you min heating load to 2kW gives a 13.5 min run time.

 

So for run times and eliminating short cycling a system volume of 145 L is fine, you may already have close to this without a buffer.

[Rupe30]

4 hours ago, ProDave said:

If the manifold water is not hot enough to heat the house, turn the mixing valve up.

 

Once you know what manifold temperature you need, then turn the water leaving temperature on the ASHP down to only slightly above the required manifold temperature.

 

Mixing valve is on max. The issue is I can't get enough heat to the manifolds. 

4 hours ago, ReedRichards said:

 

So excessive that 300 l cannot possibly be the correct volume.

I'm sure it is, we've got a horizontal 210litre water tank (which can't even fill a hot bath but I won't get started on that!) Which I would say is smaller than our buffer tank.

 

3 hours ago, JohnMo said:

It does sound big, they are only there to reduce short cycling, when min output from heat source and min load required and small system water volume would lead to a short run times.  They also act as hydraulic sepereration between various pumps.

 

So your heat of pump should turn down to about 5kW.  So have run some run-times calcs, that show the following.

 

With a minimum CH heat demand load of 1kW.

 

System volume 300 plus 100, 400L in the ch system, run time 28.mins, off time 111 mins.

 

Reducing the total system to 145 incl a buffer, would give a run time of 10.1 mins and off time 40.4 mins. Which isn't short cycling for an ASHP.

 

Increasing you min heating load to 2kW gives a 13.5 min run time.

 

So for run times and eliminating short cycling a system volume of 145 L is fine, you may already have close to this without a buffer.

Our system volume is 230litres over the two floors in the underfloor system so perhaps a small buffer necessary?

 

I reset the heatpump today and with a 6/9 

 zones on upstairs and 1/5 downstairs couldn't get a flow temp above 40 degrees, the manifolds were getting the correct temperature though (just below 40) so I haven't needed to turn the stats on and off at all. I turned off the zones downstairs and flow increased to 47degrees. Is it possible the buffer tank is too much for the ashp to power? I find it hard to believe a 16kw heat pump could be underpowered for our property. Looking at our smart meter the ashp has been running at 4kw for most of today. 

 

Thanks for everyone's input. 

 

 

[JohnMo]

How is the buffer heated - is a pressurised one heated direct from the heat pump, or are you heating via a coil in the buffer.  If a coil what size coil is it?

[ReedRichards]

30 minutes ago, Rupe30 said:

I find it hard to believe a 16kw heat pump could be underpowered for our property. Looking at our smart meter the ashp has been running at 4kw for most of today. 

 

 

If your 16 kW heat pump was underpowered for your property it would have been running at close to 16 kW for most of the day.

[Nickfromwales]

20 hours ago, JohnMo said:

So for run times and eliminating short cycling a system volume of 145 L is fine, you may already have close to this without a buffer.

Not on a system that has actuators and rooms stats. That can run down to just one active loop, so would need the equivalent capacity in the buffer as 'run-off' when the heat demand is down to the minimum / house near to set temp.

23 hours ago, Rupe30 said:

Ok so first thing to do is remove those mixer valves then, sounds like I'm paying to generate heat only to cool it before it gets to our UFH.

Nope, do NOT do that!!

The mixing valves take cool water from the loops, where the water in the loops gets recirculated until cool enough to be reheated by being blended with the incoming ( available ) heated water. These are there to protect the flooring / occupants against damage / injury.

If you open these manifold mixing valves to their max setting, and the 1st floor heating is not getting the house up to temp, the issue is with the design and spec / execution of install, and nothing to do with the system not working properly ( functioning at its max potential ). You can create a furnace of heat, but if the emitters cannot convey that heat to the air space in the rooms then it's a paperweight.

 

The 6mm screed is a concern, but essentially not much difference to that vs the Wunda overlay system with a tile bonded atop so the question needs to be, what is below it?

 

What is the floor covering upstairs?

Have you opened the mixing valves to max setting ( what does the valve say that is? 1-6 etc or ^oC? ) and did action then show on the manifold thermometer?

[JohnMo]

12 minutes ago, Nickfromwales said:

Not on a system that has actuators and rooms stats.

I should have qualified my statement, but the point was the 300L buffer is huge.  And depending if direct or indirect heating could be part part of the issue.

[Marvin]

28 minutes ago, JohnMo said:

I should have qualified my statement, but the point was the 300L buffer is huge.  

Plus one. Heat sump? Hot water tank? Buffer tank? I suppose a design could incorporate it as any of theses, but I cannot see the benefit unless you have spare PV energy used to for water heating, or used as a hot water tank because you have many people in the property.

 

Perhaps someone will educate me....

 

 

Edited December 18, 2022 by Marvin
clarification

[SteamyTea]

1 hour ago, Marvin said:

Perhaps someone will educate me

About why you need a buffer tank?

[Marvin]

13 minutes ago, SteamyTea said:

About why you need a buffer tank?

You've done that already.

[Marvin]

1 hour ago, Marvin said:

Plus one. Heat sump? Hot water tank? Buffer tank? I suppose a design could incorporate it as any of theses, but I cannot see the benefit of one so big unless you have spare PV energy used to for water heating, or used as a hot water tank because you have many people in the property.

 

Perhaps someone will educate me....

 

 

 

[Nickfromwales]

18 hours ago, JohnMo said:

I should have qualified my statement, but the point was the 300L buffer is huge.  And depending if direct or indirect heating could be part part of the issue.

Big, not huge imo, especially if it were in a 'low temp' dwelling. A 100L buffer gets sucked bone dry ( of heat ) incredibly quickly, as I'm finding on my current project where we've been firing up the het pump in this cold snap. I'm designing one system atm which will utilise either a 200L or 250L buffer + 9kW HP, but this is to cope with 1) a passive setting with very low heat output, which is also married to a detached annex likely to be heated in the future by FCU's that 2) will consume most of what the system has to offer when all is at 'full chat'. It'll be getting 2 lots of foam spray ( 100-110mm ) to keep standing losses at an absolute minimum. A bigger buffer should give a faster response / recovery time here vs being reliant on the HP alone.

 

If the OP has a buffer which is known to be sitting at >40^oC, and that is not arriving at / being delivered to the manifolds, then the issue may simply be that it is not being ( getting ) circulated between the two. If the heated water is there, then there it should be getting delivered into the floors. Controls / sensor issues seem to be at play here.

[Rupe30]

On 18/12/2022 at 13:07, Nickfromwales said:

Not on a system that has actuators and rooms stats. That can run down to just one active loop, so would need the equivalent capacity in the buffer as 'run-off' when the heat demand is down to the minimum / house near to set temp.

Nope, do NOT do that!!

The mixing valves take cool water from the loops, where the water in the loops gets recirculated until cool enough to be reheated by being blended with the incoming ( available ) heated water. These are there to protect the flooring / occupants against damage / injury.

If you open these manifold mixing valves to their max setting, and the 1st floor heating is not getting the house up to temp, the issue is with the design and spec / execution of install, and nothing to do with the system not working properly ( functioning at its max potential ). You can create a furnace of heat, but if the emitters cannot convey that heat to the air space in the rooms then it's a paperweight.

 

The 6mm screed is a concern, but essentially not much difference to that vs the Wunda overlay system with a tile bonded atop so the question needs to be, what is below it?

 

What is the floor covering upstairs?

Have you opened the mixing valves to max setting ( what does the valve say that is? 1-6 etc or ^oC? ) and did action then show on the manifold thermometer?

 

We're using the ThermoDEK -18 system upstairs (https://www.continal.co.uk/system/thermodek) which is lay over chipboard flooring. There's acoustic insulation in the ceiling space below.

 

I spoke with the UFH providers who had designed the system, they informed me the mixing valves should be set to 4 with my ASHP flow temp set to 50 degrees. I've also set the correct flow rates on the manifolds, everything was just open to the max (3-4ltr) when most our flow rates needed to be between 1-2ltr. I'm also running AI mode settings 29 50 as mentioned in the Therma V thread (I had tried that before when house was struggling to keep temp but it didn't help). And finally I've re-done the lagging outside as that was useless.  Since then its been running well. I know the weather has warmed up a little, lately its been down to 3 degrees at night, 8-9 in the day and our ASHP has consumed an average of 23kwh each day over the last 3 days. The prior 4 days to that it averaged 95kwh per day although it was down to -5 over night and barely above 0 during the day. So will see when we get another cold snap if the house is able to keep temperature with these latest adjustments. 

 

I'll try and get in the loft tomorrow to take a picture of the water tank setup and get your thoughts. 

 

Thanks again!