Heat Pump Sizing?

michaels

matt_drummer said:

michaels said:

matt_drummer said:

What is 7.5kW? I thought you were looking at a 12.5kW heat pump?

Do you have one or multiple heating circuits?

An automatic bypass valve is fitted to ensure that water can still flow when all trvs are shut, it's a safety device. It used to be done with a gate valve left slightly open.

I have an ABV fitted, Octopus always fit them but mine is closed as I have no trvs, the ABV is not required and is often a source of issues around poor efficiency.

You are still relating a heat pump to the way a gas or oil boiler works where the efficiency never changes by much.

If your 12.5kW heat pump is at its lowest output of say 4kW, when half your radiator output is taken away then one of two things will happen.

The first is, you were running at the lowest flow temperature possible and now you have taken half of the output capacity away. The heat pump won't be able to produce any less heat, the return temperature will rise and the heat pump will stop. It may mean that whilst part of your house will be warm enough, the rest will now get colder and your heat pump will be short cycling.

The second situation is that if the heat pump can still produce its minimum output using only half the radiators then you are running it hotter than you need to with all radiators open. Think about it, if that heat pump can get rid of 4kW with only half the radiator capacity then it would have been able to get rid of 4kW using twice the radiators at much lower flow temperature.

Of course, you won't always be at the lowest output but you'll probably spend more time in the middle and towards the lower end. Radiators whose output are in proportion to the heat loss of the room they are in will allow the lowest flow temperatures and the best efficiency even if they are smaller than they could ideally be in total. 

A heat pump has no idea what it is connected to, it doesn't know how many radiators there are and whether trvs are open or closed.

A heat pump sits in its own environment. All it knows is the outside temperature, the temperature of water coming in and going out and how much of it there is. It will also be told whether to run or not and that may be based on internal temperatures, but may not be.

Those four factors are all that affect the efficiency, and unlike a gas or oil boiler, they make a massive difference.

The skill in designing a heating system that works well with a heat pump is fitting it to the right stuff that enables the water temperatures and flow rates to be at the optimum levels for as much time as possible.

Zones, trvs, buffer tanks, low loss headers, and bypass valves are all best avoided wherever possible.

The big problem I had with my 9kW Daikin was not so much its heat output, I could get it reasonably low (around 3kW) but when it was doing that it used the same amount of electricity as it did producing 4.5kW. This is because there is always a minimum electrical input. This is critical to finding the correct heat pump for your house.

I wrote about this before, one manufacturer may not produce a suitable heat pump for every house.

Did write a long reply to this but lost it so will just go back to the first sentence re sizing.

Octopus calc says 10kwh.  Experience sys that on an ice day the max the gas boiler has ever used is 7.5kwh per hour of gas for heating and this is probably at less than 100% efficiency (95%? as it was in condensing mode with a 53 flow temp).  So my thinking is that if the pipe work can support a sufficient flow rate to allow 7.5k heat transfer with a flow-return delta T of 7C then that will be enough. 

More google suggests that if the pipes were straight the 15mm upstairs section could support the about 8l per min flow (0.9m/s) needed to supply 3.75kw to the upstairs rads and the first downstairs 22mm section could support the 14l per min flow rate (also 0.9m/s) needed transfer 7.5kw.

The rated heat pump output would need to be higher to cover the 20kwh of hot water needed per day and to cover any defrost cycles.

I thought you were going to heat your DHW with an immersion heater?

You are looking at a 12.5kW heat pump when your heat loss is only around 7kW.

I can't see it working very well, it's quite a lot bigger than you need.

Although it could be OK, what does it modulate down to? Do you know?

Do you know what minimum flow rate this Dream heat pump requires?

If your pipework carried enough heat to keep you warm with your gas boiler then a heat pump won't change that, you don't need to deliver any more heat to the house.

The only problem you may have is with the required flow rate of your chosen heat pump.

My Daikin runs quite happily at 6.5 lpm but some others need 15 to 20 lpm

Your large DHW requirement will always be an issue.

I did see one air to air installation where they installed a small gas boiler for their DHW. They had two air to air multi splits with seven or eight indoor units.

Another option, which you won't like, are two air to water heat pumps, one for heating and a smaller one for DHW.

Thanks, some interesting thoughts.  Sorry for not being clear, my thoughts on sizing are that for heating only we need something that can do just over 7kw constant (including any defrost downtime).  The discussion on hot water was to suggest why this number might have been smaller than the Octopus number.

I am not sure on the flow rate, I thought what you suggest with regards to th egas boiler working ok but heat geek for example suggest that what matters is the flow rate multiplied by the temperature difference between the flow and return.  Basically if the gas boiler has a bigger difference between flow and return temps then the water can flow more slowly for the same heat transfer: the same amount of heat input raises 14l per minute by 7 degrees as raises 7l per minute by 14 degrees.

matt_drummer

michaels said:

matt_drummer said:

michaels said:

matt_drummer said:

What is 7.5kW? I thought you were looking at a 12.5kW heat pump?

Do you have one or multiple heating circuits?

An automatic bypass valve is fitted to ensure that water can still flow when all trvs are shut, it's a safety device. It used to be done with a gate valve left slightly open.

I have an ABV fitted, Octopus always fit them but mine is closed as I have no trvs, the ABV is not required and is often a source of issues around poor efficiency.

You are still relating a heat pump to the way a gas or oil boiler works where the efficiency never changes by much.

If your 12.5kW heat pump is at its lowest output of say 4kW, when half your radiator output is taken away then one of two things will happen.

The first is, you were running at the lowest flow temperature possible and now you have taken half of the output capacity away. The heat pump won't be able to produce any less heat, the return temperature will rise and the heat pump will stop. It may mean that whilst part of your house will be warm enough, the rest will now get colder and your heat pump will be short cycling.

The second situation is that if the heat pump can still produce its minimum output using only half the radiators then you are running it hotter than you need to with all radiators open. Think about it, if that heat pump can get rid of 4kW with only half the radiator capacity then it would have been able to get rid of 4kW using twice the radiators at much lower flow temperature.

Of course, you won't always be at the lowest output but you'll probably spend more time in the middle and towards the lower end. Radiators whose output are in proportion to the heat loss of the room they are in will allow the lowest flow temperatures and the best efficiency even if they are smaller than they could ideally be in total. 

A heat pump has no idea what it is connected to, it doesn't know how many radiators there are and whether trvs are open or closed.

A heat pump sits in its own environment. All it knows is the outside temperature, the temperature of water coming in and going out and how much of it there is. It will also be told whether to run or not and that may be based on internal temperatures, but may not be.

Those four factors are all that affect the efficiency, and unlike a gas or oil boiler, they make a massive difference.

The skill in designing a heating system that works well with a heat pump is fitting it to the right stuff that enables the water temperatures and flow rates to be at the optimum levels for as much time as possible.

Zones, trvs, buffer tanks, low loss headers, and bypass valves are all best avoided wherever possible.

The big problem I had with my 9kW Daikin was not so much its heat output, I could get it reasonably low (around 3kW) but when it was doing that it used the same amount of electricity as it did producing 4.5kW. This is because there is always a minimum electrical input. This is critical to finding the correct heat pump for your house.

I wrote about this before, one manufacturer may not produce a suitable heat pump for every house.

Did write a long reply to this but lost it so will just go back to the first sentence re sizing.

Octopus calc says 10kwh.  Experience sys that on an ice day the max the gas boiler has ever used is 7.5kwh per hour of gas for heating and this is probably at less than 100% efficiency (95%? as it was in condensing mode with a 53 flow temp).  So my thinking is that if the pipe work can support a sufficient flow rate to allow 7.5k heat transfer with a flow-return delta T of 7C then that will be enough. 

More google suggests that if the pipes were straight the 15mm upstairs section could support the about 8l per min flow (0.9m/s) needed to supply 3.75kw to the upstairs rads and the first downstairs 22mm section could support the 14l per min flow rate (also 0.9m/s) needed transfer 7.5kw.

The rated heat pump output would need to be higher to cover the 20kwh of hot water needed per day and to cover any defrost cycles.

I thought you were going to heat your DHW with an immersion heater?

You are looking at a 12.5kW heat pump when your heat loss is only around 7kW.

I can't see it working very well, it's quite a lot bigger than you need.

Although it could be OK, what does it modulate down to? Do you know?

Do you know what minimum flow rate this Dream heat pump requires?

If your pipework carried enough heat to keep you warm with your gas boiler then a heat pump won't change that, you don't need to deliver any more heat to the house.

The only problem you may have is with the required flow rate of your chosen heat pump.

My Daikin runs quite happily at 6.5 lpm but some others need 15 to 20 lpm

Your large DHW requirement will always be an issue.

I did see one air to air installation where they installed a small gas boiler for their DHW. They had two air to air multi splits with seven or eight indoor units.

Another option, which you won't like, are two air to water heat pumps, one for heating and a smaller one for DHW.

Thanks, some interesting thoughts.  Sorry for not being clear, my thoughts on sizing are that for heating only we need something that can do just over 7kw constant (including any defrost downtime).  The discussion on hot water was to suggest why this number might have been smaller than the Octopus number.

I am not sure on the flow rate, I thought what you suggest with regards to th egas boiler working ok but heat geek for example suggest that what matters is the flow rate multiplied by the temperature difference between the flow and return.  Basically if the gas boiler has a bigger difference between flow and return temps then the water can flow more slowly for the same heat transfer: the same amount of heat input raises 14l per minute by 7 degrees as raises 7l per minute by 14 degrees.

Yes, you are correct on the relationship between dT (between flow and return) and flow rate.

We discussed this weeks ago if you recall, maybe another thread, I don't remember.

It is why 30c flow and 25c return at 10lpm produces as much heat as 75c flow, 70c return at 10lpm

Perhaps it was a discussion about high temperature heat pumps.

I run at the lowest flow rate possible with the biggest dT I can get. Because my radiators are so big I don't vary my flow temperature much. As it gets colder my dT increases as the house loses more heat, my heat output increases automatically so I don't use weather curves or anything like that.

I have a very limited range of flow temperatures I can run at in any case, above about 37c my heat pump can't produce any more heat, so that is the upper limit.

I have the heating on now but turned the flow temperature down a few hours ago as it got colder, that is just how mine works best.

It's good that you now understand the relationship between dT and flow rate and that the actual flow temperature is irrelevant to the heat produced.

The flow temperature required to get the dT needed to supply the heat the house needs is determined by the radiator's ability to deliver that hear demand.

But you also need to consider the minimum flow rate required by your choice of heat pump.

Many heat pumps won't run at 7 lpm, they have a minimum flow rate. My 9kW Daikin wouldn't go lower than 10lpm and some others won't go below 15 or even 20 lpm.

Many people like to run at a higher flow rate and a smaller dT, like 2 or 3c. They do this because for any given flow temperature the mean flow temperature is higher and that means you can run a lower flow temperature but improve radiator output.

I run at something like 31c flow and 25c return. That means my mean flow temperature is 28c and that is what determines the output of my radiators.

But I could run at 30c flow and 28c return giving a mean flow temperature of 29c. My radiator output would be higher at the same indoor temperature even though my flow temperature is 1c lower. It would be more efficient in theory.

But to get the same heat at 30/28 as I do at 31/25 I would have to circulate my water 3 times faster (20lpm instead of 6.5lpm) which I don't want to do. Some heat pumps are designed to work at lower dT's and higher flow rates.

Daikins are good because you have a wide range of adjustment and their control is based on maintaining a set dT. Other heat pumps are controlled differently and have less adjustment. Daikin heat pumps vary their flow rates to maintain the dT you have requested. They reach their set flow temperature very quickly and maintain it. Vaillants work completely differently as do many others where the flow temperature gradually increases but the flow rate is constant. Daikin heat pumps are very aggressive at the start of a heating cycle, they get to flow temperature very quickly at maximum flow rate and then after about 20 minutes start to drop the flow rate to bring the dT into line. It can be difficult to manage, the whole process takes 30 minutes and you get a lot of heat in that time. The return temperature rises very quickly, enough to shut the heat pump down, and it is the reason why I avoid cycling as much as I can, they need to just run as low as necessary for as long as possible. It is also another reason why I have massive radiators, I need them to deal with the first 20 minutes of a Daikin heating cycle.

So, you need to know what the minimum flow rate is of the heat pump you choose, the bigger they are, the higher the flow rate they generally require.

Some don't even have pwm controlled circulation pumps, they just run at a constant speed.

I think some, if not all Samsungs are like this as standard.

This is just another thing you need to look at when choosing a heat pump.

greenbee

I've been following this discussion with interest as I'm trying to work out how best to prepare for a heat pump. Currently the house is heating by oil, with electric towel rails. The house is around 3000 square feet, and has been extended twice - so four heating zones - upstairs, downstairs radiators, utility room wet UFH, and extension UFH. As the extension is open plan to the kitchen (which has a radiator) the zoning seems pointless.

The oil tank is only 1000l which means I'm constantly waiting for it to hit the 500l mark so I can refill it, which comes round remarkably quickly during cold weather!

Several radiators have recently been replaced, but looking back at the paperwork they've been sized for 45C flow, so some may need replacing again. However, the sitting room downstairs now has 3 type 22 radiators instead of 1 type 21 and warm enough on the lowest setting on the TRVs, and the new pipework is 22mm. The bedroom has been upgraded from type 21 to 22, and the ensuite from type 11 to 22, and all pipework is 15mm.

I should have paid more attention to what the plumbers were doing, but there was a lot going on at the time and I hadn't really done enough research at that point. I'm aware that I could put a small heat pump in to run the UFH alongside the oil boiler and reduce my oil consumption that way, but obviously wouldn't then qualify for a grant. I also have ongoing renovation work to do, and clearly need to make sure that replacement radiators are sized for lower temperatures.

I do also have solar panels - 6.16kw, with a 5kw myenergi Libbi inverter and 10kw battery. All panels currently on the south-facing garage roof, and I'm currently eyeing up the south/east/west facing bits of the main roof - even the north side of the garage gets a lot of sun during summer!

How do I find someone who will design an ASHP setup with efficiency in mind? It seems to be very hit and miss. 

Reed_Richards

greenbee said:

How do I find someone who will design an ASHP setup with efficiency in mind? It seems to be very hit and miss. 

Well the main thing to maximise efficiency is to design for as low a flow temperature as you can achieve without making your radiators unacceptably large or numerous.  Some of the larger installers seem to have a preconceived idea of what flow temperature to use so that won't necessarily give you the best efficiency.

michaels

Reed_Richards said:

greenbee said:

How do I find someone who will design an ASHP setup with efficiency in mind? It seems to be very hit and miss. 

Well the main thing to maximise efficiency is to design for as low a flow temperature as you can achieve without making your radiators unacceptably large or numerous.  Some of the larger installers seem to have a preconceived idea of what flow temperature to use so that won't necessarily give you the best efficiency.

I think there is a trade of between design efficiency and installation cost, for example Octopus seem to look for the minimum install cost option regardless almost of running costs.  Working out where on that trade off you want to be is pretty hard as it depends on factors like expected future energy prices over the next 20 years.....

silverwhistle

Gawd knows how I'd go about sizing a heat pump for my single occupancy 3 bed terrace on the mild south coast. I'm tempted to go for A2A with a split: lounge and dining room, the latter also helping the kitchen which doesn't have (or really need) heating.

Upstairs I'm not bothered about: there's an electric towel rail I can control remotely and MHRV, but otherwise I'm either active or under the covers and the fabric of the house retains heat fairly well (DG/CW etc.) and decent solar gain.

I've 4kWp of solar and an EV and my annual electricity consumption is <3000kwh and gas  <2190 (last year) but the obfuscating issue is that for heating when it's cold I largely use the wood burner in the lounge which then indirectly heats the whole house, and the gas largely comes on when relative from warmer climes arrives (or I'm under the weather). That could be dealt with by direct acting space heating, although I've never had any comments about chillyness!..

I'm tempted to undersize for efficiency and top up with the wood stove, plus the odd burst of direct acting space heating when Agile is cheap or when the heat pump may not be coping. I'd certainly like to get rid of the gas standing charge which is more or less the same as my actual gas usage (on Tracker).

Any thoughts about this approach, including the sizing aspect? I'm aware it wouldn't be eligible for any grant.

Spies

For me its the ability to get rid of the Gas meter as well as the standing charge, A2A would mean I'd still have to have a gas meter for the combi boiler in order to have hot water.

Just ask Octopus for a quote, nothing to lose.

greenbee

Spies said:

Just ask Octopus for a quote, nothing to lose.

Apart from the endless emails and phonecalls asking whether you've made a decision on major expenditure within 5 minutes of filling in the details!

There's a lot to think about - I'm aware that I could have done better with my solar if I'd had the patience to track down someone who would install what I asked for based on the advice and insight from this forum. But I ended up going with the least worst option. And to be fair, they were quick, efficient, and tidy - the paperwork all arrived the next day and my export was up and running in about 3 working days. But I'm trying to be more considered about the next phase. 

silverwhistle

Spies said:

For me its the ability to get rid of the Gas meter as well as the standing charge, A2A would mean I'd still have to have a gas meter for the combi boiler in order to have hot water.

I've got an immersion and any spare solar gets diverted to it, so for me no need. Even in winter I get some input and would then top up when electricty is cheap.

matt_drummer

silverwhistle said:

Spies said:

For me its the ability to get rid of the Gas meter as well as the standing charge, A2A would mean I'd still have to have a gas meter for the combi boiler in order to have hot water.

I've got an immersion and any spare solar gets diverted to it, so for me no need. Even in winter I get some input and would then top up when electricty is cheap.

A heat pump is far more efficient than your immersion heater.

I heat my water at around 300% efficiency on average so it costs me 5p per kWh and I export my solar at 15p per kWh

The heat pump will always be cheaper.