Need to reduce costs of running Water Source Heat Pump

[MKF]

Hi

Just joined the forum to research ways of improving the efficiency and costs of running our heat pump. It's proved to be much more expensive to run than predicted. The original installer went bust shortly after commissioning it and I can't find anyone who I feel I can trust and has the expertise to investigate the situation  I need to research and understand what needs to be done myself. I'm hoping to find some advice and learn from the experiences of others who I'm sure are going through similar issues.

[ProDave]

Do you have any details?  Make and model of the heat pump?  What water source?  borehole or slinky etc?  And what are you using for heat delivery to the house?  radiators? UFH etc.  

 

Have you had a proper heat loss calculation done on the house?

[MKF]

Thanks for your interest @ProDave.

Briefly,  it's a 1940's stone built mid terrace 3/4 bed house about 120 sq m. including a 15 year old kitchen extension 

The heat pump is a Danfoss DHP Opti Pro 8 SP rated at “heating capacity B0/W35” = 7.51kW

The water source is a 4m borehole, we are close to the river on sand and gravel, the water table is about 1-2 m below ground.  It varies between 5 and 12 C currently 6.5 C,  air temp today was between -2 and +4 C

Radiators mostly connected with 22mm feed from a buffer tank to central manifold then microbore, seems to be well designed. Extn has 15mm pipework with two rads.

The total heat demand was calculated by the installer to be 16,000kWh per year. 

I estimate that the heat pump uses approx 1,600kWh per month when it is cold and an average of 1,000kWh per month in Autumn and Spring.  In summer it's about 200kWh per month.

 

[SteamyTea]

14 minutes ago, MKF said:

The water source is a 4m borehole

Seems very shallow, even allowing for the high water table (read recently that it is even higher at the moment).

What sort of filters are used to get the grit out of the water?

Could they be blocked up?

 

Edited January 8 by SteamyTea

[MKF]

@SteamyTea  - There's certainly plenty of water around at the moment 🙂 Attached a view of the road from the front gate yesterday!  Fortunately gone down today.

 

Even when there's a drought the water table has never dropped too far to affect the water flow from the borehole, I guess its more like an underground river we've tapped into.   Filters are clean there's no issues with flow rate and the heat exchanger, which stops the dirty water reaching the heat pump, is regularly reverse flushed.

[SteamyTea]

Is that Wolvercote?

 

Generally WSHP have a very good CoP.  Do you have any idea what your CoP is currently.

What make of HP is it.

[Kelvin]

Is the reason the borehole is so shallow because of the high water table? I know you can get river source GSHP installs. When I looked at GSHP the minimum depth is typically 15m. I have a redundant borehole so was going to use that but didn’t in the end. 

[Iceverge]

21 hours ago, MKF said:

The total heat demand was calculated by the installer to be 16,000kWh per year. 

I estimate that the heat pump uses approx 1,600kWh per month when it is cold and an average of 1,000kWh per month in Autumn and Spring.  In summer it's about 200kWh per month.

 

 

Digging in these numbers a little. 

 

200kWh sounds like your monthly DHW usage so 2400kWh/annum.  At a COP of 3 that's 7200kWh/year. Quite a bit I'd have said. What temperature do you heat it to and do you have 5 occupants who enjoy a bath daily? 

 

Anyway taking that out of the equation I make the heating load about 10000kWh for the year. If the 16000kWh is correct then the you will only be achieving a COP of 1.6, which isn't great.

 

 

 

Do you have any gas bills pre GSHP to compare or oil usage?

How hot is your flow temp to the rad?

How hot is the house kept?

 

[ProDave]

Which village are you in?  that looks very familliar but I can't quite place it.

 

A previous house was in a low lying river valley and suffered like that, though out house was on a local hump so never got wet.

[RobLe]

Do you have any independent measurements?  I realise that the heatpump may have heat out, electricity used figures, but they may or may not be accurate (accuracy costs money).  If you haven't I would definitely recommend a mid certified electric meter feeding the heatpump (and anything associated, if there is a separate borehole pump etc).  A suitable elec meter is pretty cheap (<£50), I think all heatpumps should have them fitted by default. 

 

You can also get independent heat monitoring equipment - this will cost more, I think £500 or so, and with both meters an accurate COP can be found. 

 

There are many reasons for high electricity useage, and it's quite difficult to untangle them without definitive measurements:

Something else using power

Higher than anticipated heat required

Poor COP:  high temp rads, low input water temp, wrong refrigerant charge, backup immersion on,  internal W2W bypass leaking through

 

Good luck!

[MKF]

1 hour ago, SteamyTea said:

Is that Wolvercote?

 

Generally WSHP have a very good CoP.  Do you have any idea what your CoP is currently.

What make of HP is it.

 

It's Kelmscott, same Thames but further upstream near Lechlade

 

It's a Danfoss/Thermia which doesn't have any way of calculating COP built in.

 

41 minutes ago, ProDave said:

Which village are you in?  that looks very familliar but I can't quite place it.

 

A previous house was in a low lying river valley and suffered like that, though out house was on a local hump so never got wet.

 

Fortunately yes, we're in exactly the same situation, the house was built on a hump in 1947 after a big flood

 

1 hour ago, Iceverge said:

 

200kWh sounds like your monthly DHW usage so 2400kWh/annum.  At a COP of 3 that's 7200kWh/year. Quite a bit I'd have said. What temperature do you heat it to and do you have 5 occupants who enjoy a bath daily? 

 

 

That's interesting, 200kWh might be an overestimate, there's only two, sometimes three of us.  

 

1 hour ago, Iceverge said:

Do you have any gas bills pre GSHP to compare or oil usage?

How hot is your flow temp to the rad?

How hot is the house kept?

 

I'll dig out our old oil bills, that's a good idea.

 

Currently rads are about 35-45 C with an outside temp of -1 C

 

House is generally about 20 C

 

1 hour ago, Iceverge said:

 

Anyway taking that out of the equation I make the heating load about 10000kWh for the year. If the 16000kWh is correct then the you will only be achieving a COP of 1.6, which isn't great.

 

You're right, GSHP's are supposed to be much more efficient than that.  The borehole has a 750W pump which may well contribute a fair bit to the 200kWh.

 

1 hour ago, RobLe said:

 I would definitely recommend a mid certified electric meter feeding the heatpump (and anything associated, if there is a separate borehole pump etc).  A suitable elec meter is pretty cheap (<£50), I think all heatpumps should have them fitted by default. 

 

You can also get independent heat monitoring equipment - this will cost more, I think £500 or so, and with both meters an accurate COP can be found. 

 

That's a really sensible thing to do.

 

2 hours ago, Kelvin said:

Is the reason the borehole is so shallow because of the high water table? I know you can get river source GSHP installs. When I looked at GSHP the minimum depth is typically 15m. I have a redundant borehole so was going to use that but didn’t in the end. 

 

Yes that's right, the river is normally about 300 metres away.  The house originally had a shallow well for drinking water .

 

 

[ProDave]

8 minutes ago, MKF said:

The borehole has a 750W pump which may well contribute a fair bit to the 200kWh.

Ouch.  750W of "wasted" (not contributing to the heating) power all the time the heating is on.  Many on here say run a heat pump at a low temperature 24/7.  That would be 18kWh "wasted"  so I would not suggest that.

 

This is a hidden "cost" when people say a GSHP is more eficcient than an ASHP.

 

So a start to solving this high usage might be to increase the radiator temperature and run the heating for shorter periods?  Also forget zoning, all on or all off.  Zoning may keep the system running just to get a last bit of heat into the cold room.  So better run it all on or all off and balance the system to even out temperatures.

[SteamyTea]

21 minutes ago, MKF said:

Kelmscott, same Thames but further upstream near Lechlade

Right, I have been there, but not since I was at school near Witney.

22 minutes ago, MKF said:

The borehole has a 750W pump which may well contribute a fair bit to the 200kWh.

0.75 kW would be 540 kWh over 30 days.

10 minutes ago, ProDave said:

So a start to solving this high usage might be to increase the radiator temperature and run the heating for shorter periods?

Yes, can't see any other option initially.

[Adrian Walker]

On 08/01/2024 at 20:09, MKF said:

 

 

 

The water source is a 4m borehole, we are close to the river on sand and gravel, the water table is about 1-2 m below ground.  It varies between 5 and 12 C currently 6.5 C,  air temp today was between -2 and +4 C

 

 

That's quite a wide temperature range and I would suspect that this is not a "true" aquifer which should be 10-15C the whole year round.

 

[RobLe]

1 hour ago, ProDave said:

Ouch.  750W of "wasted" (not contributing to the heating) power all the time the heating is on.  Many on here say run a heat pump at a low temperature 24/7.  That would be 18kWh "wasted"  so I would not suggest that.

 

This is a hidden "cost" when people say a GSHP is more eficcient than an ASHP.

That is one big pump - perhaps it is turned down?  We have a 2.5kW(heat) gshp; the glycol pump is 40W and is included in the elec meter figures.  A common rule of thumb is for the pump to be below 10% of the compressor power.  A similar comment could be applied to an ashp; sometimes only the active compressor power is considered, not the external fan nor controls nor other pumps. 

[SteamyTea]

Having slept on this, I feel that the use of a WSHP with such a shallow water supply is always going to cause problems.

 

So, what to do.

 

Fill in our favorite heat loss spreadsheet.

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=18333

 

That should give you an idea of what is actually needed size wise.

 

Then look at what improvements can be done i.e. insulate and air tightness improvements.

 

Then look at junking the WSHP (sell it on eBay) and fit a properly sized modulation ASHP.  A bit of shopping about and you could pick up a decent one for not much more than 2 months winter heating bill.

 

 

 

[RobLe]

A WSHP with a shallow supply isn't fundamentally a problem.  It sounds like the temperature of supplied water is higher than we get with our horizontal pipes 900mm below ground - I think they are around 3C now, which is ok as they have glycol in them.   For comparison, our flow rate is 12lpm, so I anticipate you'd need around 36lpm due to the 7.6kW(heat) rather than our 2.5kW(heat) system.  The power needed to lift water by 4m at 36lpm I think is 4m * (36/60) * 10 = 24W, ie. not significant.  Perhaps the pump is a problem; maybe it isn't running at that power though.

 

If we had a 4m borehole that could supply that water, I would have used it instead of hiring a trencher - the borehole is cheaper, simpler, and better.  The only issue with it might be contamination, but I understand you backflush to prevent this.

 

[SteamyTea]

53 minutes ago, RobLe said:

think they are around 3C now, which is ok as they have glycol in them

Your slinkies have antifreeze in them, the water in the ground doesn't.

 

@MKF is there any way to check that the water in the borehole heat exchanger isn't freezing?

[saveasteading]

Always learning on BH.

 

Despite my interest i had not realised that directly pumped water was taken to the heat pump. I thought it was a closed loop through the borehole, absorbing heat.

Where does the 'wasted' pump energy go? If it's in the house it isn't lost, just inefficient.

I'm assuming that the spent water is discarded some distance away. Any idea the (typical, theoretical, actual) temperature after the energy has been taken out?

[RobLe]

22 minutes ago, SteamyTea said:

Your slinkies have antifreeze in them, the water in the ground doesn't.

 

@MKF is there any way to check that the water in the borehole heat exchanger isn't freezing?

You're right, can't argue with that - more trenching is always better.  It's always a compromise though - effort versus efficiency - and it's made it through 2 winters so I think it's ok.  It's not like "ground freezing" is a on/off thing - it would be gradual as there is so much latent heat released during that phase transition, I'm not worried.  Allegedly shallow horiz systems get better over time, as the earth compacts.

 

@saveasteading The "wasted" pump energy will raise the water temperature of the flow water marginally - if indeed an oversized pump is used.  It would be pretty much waste - it's definitely not all going into the house.  I am to be convinced this is the case; it's an odd install decision.

[MKF]

Lots to think about, I really appreciate everyone's input.  Obviously a lot to learn!

 

On 09/01/2024 at 19:29, ProDave said:

So a start to solving this high usage might be to increase the radiator temperature and run the heating for shorter periods?  Also forget zoning, all on or all off.  Zoning may keep the system running just to get a last bit of heat into the cold room.  So better run it all on or all off and balance the system to even out temperatures.

 

I'm going to try that, good suggestion.

 

13 hours ago, SteamyTea said:

I feel that the use of a WSHP with such a shallow water supply is always going to cause problems.

 

Why do you think that?  In theory extracting heat directly from lake or river water, which this effectively is, is supposed to be much more efficient...

 

13 hours ago, SteamyTea said:

 

Fill in our favorite heat loss spreadsheet.

 

great, thanks for that, looks like an excellent idea.

 

 

 

6 hours ago, RobLe said:

The power needed to lift water by 4m at 36lpm I think is 4m * (36/60) * 10 = 24W, ie. not significant.  Perhaps the pump is a problem; maybe it isn't running at that power though.

 

Yes, you may well be right there, I need to work out what the optimal flow rate should be, the (dirty) ground water doesn't go directly to the heat pump, its pumped through a brazed plate heat exchanger then back into the water table some distance away from the extraction. At that point it's 2 - 3 C cooler.  There's a short 'brine' (glycol) closed loop between that and the heat pump.

 

5 hours ago, SteamyTea said:

 

@MKF is there any way to check that the water in the borehole heat exchanger isn't freezing?

 

I don't think that's a problem, currently the ground water return is 6.5C 

 

This might help, there are temp sensors on each port of the heat exchanger plus an outside air temp one (darker blue).  I think it shows the glycol circuit (black and pale blue) extracts up to 9 C  which seems very effective, any thoughts? :-

 

[RobLe]

@MKF The graph’s useful, shows the right sort of behaviour from a on/off type of gshp.  While the water loop has about a 2C delta, the glycol loop shown has a 7C delta - indicates the glycol loop could do with a higher flow rate.  For comparison, we have a DT of about 2C input to output of our glycol circuit.  Roughly every 4C drop costs a 10% COP hit.  
Given the glycol out of the heatpump is often below zero, is there ice on the pipes?
 

[MKF]

 

 

18 hours ago, RobLe said:

the glycol loop could do with a higher flow rate

 

That's a really useful observation, thanks very much @RobLe,  there's no ice, pipes are well insulated. Now I just have to work out how to increase the flow rate..