Heat Pump Decisions - BUS or not.

[Pip895]

I have a 15 year old self build that we originally designed to use a GSHP.  When it came to it, we got cold feet and opted for a couple of gas boilers one running at 60 deg and the other at 35 just doing the underfloor heating.  The property is very well insulated and has wet UFH on all floors. We also have an indoor swimming pool - which probably consumes more heat than the rest of the property combined!

I have had quotes for a GSHP but they still look impossibly pricy.  I am now looking at going for an ASHP. 

 

I have two options one ignoring the BUS scheme and the other trying to comply with it and claim the grant.  

 

The first option is a simple system that does the UFH and then dumps excess heat into the pool using an oversized heat exchanger.  There would be very little work with this one, just T into the existing 28mm primary and add the heat exchanger.  Control wise we would run flat out @35 deg during the cheap night rate when the pool pump is running and for a couple of hours in the afternoon. It would then use weather compensation and a 1 deg setback the rest of the time.  Not sure if we would keep the low temperature boiler or not but the High temperature boiler would continue heating the hot water and heating the pool, although hopefully a significant chunk of the workload/gas use would be removed.  

 

I have done quite a detailed Heat loss survey and the house heat loss is under 8kW but we would go for a higher capacity heat pump because the pool would benefit and we can displace more gas that way.  Between a 10 and 16kWh 3 phase unit.

 

Option 2 Where we are trying to comply with BUS, would involve the heat pump or pumps taking over providing the domestic hot water and also doing the space heating in the pool room.  This would require the heat pump to run at a minimum of 55deg, would require the replacement of our existing unvented 330ltr hot water tank and running quite a lot of new pipework.  According to one potential installer we would also need to re plumb our existing solar thermal pipework as our existing copper pipes no longer comply with building regs.  

I can see all this easily eating up the entire BUS grant and as I would probably be removing gas completely it is all much higher risk.  There does seem to be some confusion amongst those administering the BUS scheme as to weather I could keep a gas boiler if it only heated the pool water.  Then there is the threat that they can apparently come and audit the property years after installation and retrospectively remove your eligibility for the grant!  

 

The second option is appealing in that it will bring me much closer to my environmental goal of removing gas from the property altogether and I will end up with a new hot water tank and better pipework paid for mainly by the BUS scheme.  On the other hand, the first option picks most of the low hanging fruit and would make most of the available financial savings.  Probably 80% of the pool water heating and all the underfloor heating would be covered and much of that at 7.5p/kWh electricity over night at a SCOP hopefully of ~ 4.  The DHW and remaining pool heating being concentrated during the very coldest part of the year and at a flow temperature of above 50deg C might only achieve a COP of ~2.

 

I am uncertain how to proceed - every installer I speak to points different ways and so far I have not had either a sensible quote for option 1 or a quote for option 2 that I trust.  Could I even DIY option 1 do you think?   OH is a retired Electronics design engineer so the control side is covered.. 

[JohnMo]

Could you just heat the pool and use it as a thermal store. So heat pump just continually manages pool temp. Then take water direct from the pool and circulate through the UFH? Or take via a plate exchanger to keep them seperate.

 

Could do all with an all seasons pool ASHP. 

 

Use a plate heat exchanger as DHW preheat before cold water is entering an unvented cylinder, and let the ASHP heat that via a 3 way valve a couple of times or even once per day? Or just heat with an immersion as very little work would need to be done.

[eniacs]

I just DIY'd a heat pump install and I wouldn't recommend it for anyone but the hardiest of DIY'ers. I have a similar setup to you, however I have a dedicated pool heat pump. I refitted all my radiators and all new piping so it was very involved. 

 

One thing to note, I have an 8.5kw heat pump and the manufacturers specified 28mm, for their next size up (11kw) they specify 35mm. So the sizes you are proposing may require larger piping than you expect.

 

 

[Pip895]

2 minutes ago, JohnMo said:

Could you just heat the pool and use it as a thermal store. So heat pump just continually manages pool temp. Then take water direct from the pool and circulate through the UFH? Or take via a plate exchanger to keep them seperate.

 

Could do all with an all seasons pool ASHP. 

 

Use a plate heat exchanger as DHW preheat before cold water is entering an unvented cylinder, and let the ASHP heat that via a 3 way valve a couple of times or even once per day? Or just heat with an immersion as very little work would need to be done.

We don't keep the pool warm enough for a thermal store. Heated to 23-24deg only although it has been known to get to 30deg over summer with the solar thermal.  We did consider a dedicated pool heat pump - If we cant get sensible quotes for option 1 and Option 2 doesn't look viable either we could still do it - I really want to do the UFH as well though..

 

Interesting idea re the plate heat exchanger - getting pipework between the HP & hot water cylinder is a bit of an issue though.

[Pip895]

11 minutes ago, eniacs said:

I just DIY'd a heat pump install and I wouldn't recommend it for anyone but the hardiest of DIY'ers. I have a similar setup to you, however I have a dedicated pool heat pump. I refitted all my radiators and all new piping so it was very involved. 

 

One thing to note, I have an 8.5kw heat pump and the manufacturers specified 28mm, for their next size up (11kw) they specify 35mm. So the sizes you are proposing may require larger piping than you expect.

 

 

As the UFH has been running for 15 years with its existing 28mm pipe and at 35deg, I'm not going to be re doing the primary pipework.  I may need to pick my Heat pump based partly on this sort of requirement though.  What aspects of the job caused most hassle?  We don't have any radiators to replace and are not replacing the primaries..

[JohnMo]

So reverting back to option 1 and 2. Option 2 just looks too complex, so in all probability will be to install and control.

 

Option 1 is basically a hybrid ASHP gas boiler installation. Almost any ASHP is pre equipped to run hybrid mode out the box, some are more difficult than others. The Daikin ASHP especially for hybrid installs is designed to work with other manufacturers gas boilers and you set up to manage which heat source is online based on cost or emmisions. You could also get away with a smaller ASHP as you would design to say +3 degs instead of -3 degs. So ASHP is utilised 95% of the time, gas boiler for other few percent.

[RobLe]

I can see a lot of thermostatic valves for the underfloor stuff - my perhaps obvious comment is that you really want the UFH to run at the same temp as the heatpump flow - if that valve is dropping the temp then it will give a COP reduction.

 

Pool rooms often have high humidity - how do you cope with this?  I suggest that an MVHR would be useful, keeping the RH under control without huge ventillation losses.

Does the pool have enough insulation under it?  Where does the heat go from it, if it is the greatest loss for the house?

 

We have a gshp now.  We used to have solar thermal with a dual coil 200l tank, the top coil was originally for a gas boiler, bott for the thermal.  Now that tank has the coils plumbed in series, the flow going to the top first, lower coil after that.  It increased the surface area of coils - so the tank gets much hotter now with the same flow temp (generally 50C stored water).   It also has the advantage that the gshp is exercised all year round - I'd rather not leave it off for 6 months, only to find a problem when it's actually needed in winter!

nb: the "push" to make the coil change was a particular cold snap, and one of the thermal panels bust, no probs for 10 years then gahhhh.  Freecycled the other panel.

[JohnMo]

Good catch with the mixer valves, the other thought I had was, to route the flow to the UFH first then use the return flow from the UFH to heat the pool. So in series instead of parallel. So your pool demand, acts a huge accumulator. You certainly get the best and steadiest output from the ASHP.

[Pip895]

2 hours ago, JohnMo said:

So reverting back to option 1 and 2. Option 2 just looks too complex, so in all probability will be to install and control.

 

Option 1 is basically a hybrid ASHP gas boiler installation. Almost any ASHP is pre equipped to run hybrid mode out the box, some are more difficult than others. The Daikin ASHP especially for hybrid installs is designed to work with other manufacturers gas boilers and you set up to manage which heat source is online based on cost or emmisions. You could also get away with a smaller ASHP as you would design to say +3 degs instead of -3 degs. So ASHP is utilised 95% of the time, gas boiler for other few percent.

The Heat pump will be oversized for the UFH (in order for it to do the majority of the pool water heating.  This means the gas boiler is unlikely to be used unless we get a very prolonged cold snap say of -10deg, a heat pump break down or a power cut..  

Re the complexity - of the second option - it isn't reallythat much more complex than the former - its just the complexity doesn't involve the heat pump so I didn't draw it..   Having said that one added complexity that does annoy me is that to utilize the "emergency" Retained gas boiler I would need it to buy it a separate heat exchanger because sharing either the Solar heat exchanger or the new low temperature heat exchanger, would result in the gas boiler being connected, even if indirectly, to either the DHW or the heating system.

[Pip895]

1 hour ago, RobLe said:

I can see a lot of thermostatic valves for the underfloor stuff - my perhaps obvious comment is that you really want the UFH to run at the same temp as the heatpump flow - if that valve is dropping the temp then it will give a COP reduction.

 

Pool rooms often have high humidity - how do you cope with this?  I suggest that an MVHR would be useful, keeping the RH under control without huge ventillation losses.

Does the pool have enough insulation under it?  Where does the heat go from it, if it is the greatest loss for the house?

 

We have a gshp now.  We used to have solar thermal with a dual coil 200l tank, the top coil was originally for a gas boiler, bott for the thermal.  Now that tank has the coils plumbed in series, the flow going to the top first, lower coil after that.  It increased the surface area of coils - so the tank gets much hotter now with the same flow temp (generally 50C stored water).   It also has the advantage that the gshp is exercised all year round - I'd rather not leave it off for 6 months, only to find a problem when it's actually needed in winter!

nb: the "push" to make the coil change was a particular cold snap, and one of the thermal panels bust, no probs for 10 years then gahhhh.  Freecycled the other panel.

There are individual room stats that control the valves on each of the 4 UFH manifolds.  These are pre-existing and although I wouldn't design it this way now if I was starting from scratch - I am reluctant to get rid of something that is working.  The temperature of the water flowing through each zone is around 35deg but it is entirely possible that the only part of the house demanding heat is say the family bathroom (probably because someone has left the window open AGAIN! )  In a standard setup this would be very inefficient with the Heat pump cycling badly.  In my case though with the pool accepting all the extra heat it should be possible to minimise cycling even when like now there is very little demand for heat from the house.

The pool room is humidity controlled by a Heatstar unit that also heats the air via a ducted air system.  This is connected to the high temp boiler and wouldn't change in scenario 1.  We also have MVHR in the main house.   

Interesting idea regarding putting the coils of your HW tank in series but we want to retain our solar thermal system - it works well and although we never heat our pool above 24deg using gas the pool is usually above 27 deg throughout the summer.

[Pip895]

1 hour ago, JohnMo said:

Good catch with the mixer valves, the other thought I had was, to route the flow to the UFH first then use the return flow from the UFH to heat the pool. So in series instead of parallel. So your pool demand, acts a huge accumulator. You certainly get the best and steadiest output from the ASHP.

I did consider the Idea of putting the pool heat exchanger in series with the UFH but as the pool water heating season is more than twice as long as the house heating season, for much of the year the only thing the heat pump will be doing is heating the pool. 

The complication I have is that the pool filter pump is on only 8hrs a day what do I do the rest of the time?  I am split between getting a relay to turn on a smaller pool circulation pump if the house demands heat or just using fairly aggressive weather compensation.

[eniacs]

19 hours ago, Pip895 said:

As the UFH has been running for 15 years with its existing 28mm pipe and at 35deg, I'm not going to be re doing the primary pipework.  I may need to pick my Heat pump based partly on this sort of requirement though.  What aspects of the job caused most hassle?  We don't have any radiators to replace and are not replacing the primaries..

Yes i would pick around 8.5kw as if the manufacturers are correct - which presumably they are, then 35 deg water cannot flow 11kw in a 28mm pipe.

 

Repiping everything and new rads were a significant hassle. However the primary pipework and internal parts of the system (pump/electronics etc) were larger and more work than i first thought.

 

In option 1, why not just replace the system with 1 large heat pump. Virtually all heat pumps do 35deg or 55deg. The only change it looks like is you would need an hour or 2 somwhere to heat the hot water

[Pip895]

1 hour ago, eniacs said:

Yes i would pick around 8.5kw as if the manufacturers are correct - which presumably they are, then 35 deg water cannot flow 11kw in a 28mm pipe.

 

Repiping everything and new rads were a significant hassle. However the primary pipework and internal parts of the system (pump/electronics etc) were larger and more work than i first thought.

 

In option 1, why not just replace the system with 1 large heat pump. Virtually all heat pumps do 35deg or 55deg. The only change it looks like is you would need an hour or 2 somewhere to heat the hot water

The problem with picking an 8.5kW pump would be that I'm not going to be making enough of a dent in the pool heating that way, particularly when you remember that the pool is only heating during the 6-8hrs I have the pool pump on.  Having the connections from the Pump in >40mm isn't going to be a problem and I can use the same size going to the swimming pool heat exchanger as its only a few meters away.  The House on 28mm is never going to draw more than 8kW so the 28mm should be fine.

 

Re doing the hot water - what you are describing is my Option 2.  People seem to be discounting it because its too complicated but its exactly as you have described - the heat pump splits its time between the 35deg and the 55deg flow (which in option 1 is still done with a gas boiler).    

[Originaltwist]

On 20/11/2023 at 12:13, Pip895 said:

As the UFH has been running for 15 years with its existing 28mm pipe and at 35deg, I'm not going to be re doing the primary pipework.  I may need to pick my Heat pump based partly on this sort of requirement though.  What aspects of the job caused most hassle?  We don't have any radiators to replace and are not replacing the primaries..

Can 28mm pipe convey 11kW at 35'? It's actually all down to flow and Dt. Having a play with my models... first - Flow.  22 litres per minute is 0.69 metres per second (26mm ID) which is reasonable for this example I trust (like 2 showers running). A pipe is just a very long tank so turning to the 'power to heat a tank' model you can see that for that flow to make 11kW a Dt of about 7 is required. So 35' to the pool and 28' back.

So yes, it looks possible if that Dt can be reached and a good pool heat exchanger might help with that.

[eniacs]

Twist your maths checks out. Im just parroting what ecodan say for their heat pumps. It may be they are limited by their heat exchangers? Or is it their experience determining that flow rates like that arent possible? I dont know.

 

What do other manufacturers say for the primary piping sizes i wonder?

 

I did think you could fit a larger unit, and it would just modulate to a lower power as soon as it saw the flow rate it was getting.

[sharpener]

1 hour ago, eniacs said:

What do other manufacturers say for the primary piping sizes i wonder?

 

I am planning on a 12kW Vaillant Arotherm Plus. The quoted nominal flow rate is 2065 l/hr and the minimum is 995. The former is 34 l/min which according to this flow calculator is 1.0 m/s in a 28 mm Cu pipe which is about the max to avoid noise and erosion (opinions differ a bit). Vaillant do not make any specific recommendation about pipe sizes.

 

1 kW is 860 litre-degrees per hour (3600/specific heat of water). So to check the heat transfer capacity for 2065 l/hr at 5 deg delta T you divide 5 x 2065 by 860 which comes out at 12.005 kW.

 

So yes it is possible, an 11kW unit with 28mm primaries will require a flow rate of only 11/12 as much for 5 deg i.e. 0.917 m/s which is fine and gives you a margin for using glycol.

 

 

Edited November 21, 2023 by sharpener

[JohnMo]

1 hour ago, eniacs said:

What do other manufacturers say for the primary piping sizes i wonde

Basically what ever they say as a throw away statement about pipe sizes, will only be true for a certain set of circumstances. 

 

You/designer still have design the piping system to manage total pressure drop, so you do not exceed the allowable system back pressure, and to ensure the circulation pump can meet the minimum flow rate requirement.

 

Talking about exact pipe sizes, without a full design, is rubbish in, rubbish out and pretty meaningless.

[Pip895]

We have nearly 1.7km of UFH pipe installed in the house.  The total design flow rate is 44 l/min - calculated by adding all the individual room/area flows together.  We also have 15 room stats controlling the flow.  The Pool Heat exchanger will be sized to be able to transfer the entire output of the heat pump at a dT (between pool & Primary) of 10deg - the pool heating season is far longer than the house heating season, so there will be months where the only demand on the HP will be from the pool. The issue I can see might be that particularly in cold weather when the house is demanding heat, the return temperature in the primary might drop too far for the heat pump to cope - well over the 5 or 7 degrees the heat pump is designed to run at.  How does the heat pump react to this?  Is the flow temperature allowed to drop? - does the same thing happen in a heat pump when it is trying to heat DHW?  With very large primaries running from the HP to the pool heat exchanger I am concerned that the pool is going to essentially steal the heat from the UFH - which is why I put a pressure valve in the heat exchanger circuit, but might the two things (pressure valve & large primaries) just counteract each other?

 

I can't quite get my head around any reason why a heat pump providing heat to a circuit at a given flow temperature (35deg) needs to pump any faster than a system boiler working at the same temperature!  What I can see is that a heat pump in normal operation might see a drop in return temperature as a good reason to drop the primary temperature and increase flow driving up the COP.   Whilst my pool pump is running & electricity prices are low I will want the HP to be running more or less flat out while there is demand for heat.  I think holding the temperature at 35deg and varying the flow would be a good way of achieving this.  During periods when the pool pump is off and there is no where to dump the excess heat I would want temperature compensation to reduce the cycling that would tend to result from an oversized heat pump - If I find this to be a big problem I might look at adding a small circulation pump in the swimming pool circuit.  Are these types of HP controls get at able in most heat pumps?  Would some makes be particularly good or worth avoiding?

 

 

 

 

Edited November 21, 2023 by Pip895