Heat Pump Sizing?

rjmachin

wrf12345 said:

There does not seem to be any incentive to fit the simplest possible system, rather endless complication to ramp up the cost, the grant seen as a nice bonus for the installer rather than saving the consumer real money. Octopus the best of a bad bunch but not yet installing everywhere in England, and even there it is more a tick-box installation than a carefully crafted system to max out efficiency - but, hey, they make money out of selling electricity.

I had my heat pump installed by Octopus earlier this year, not been through a winter yet, but happy with that outcome so far.

I only paid £1360 for the entire system, which took 5 days to complete, with an electrician, two installers and the team leader / supervisor on site for most of the week.

michaels

So now I am trying to understand about pipe sizing, flow rates and flow and return deltaT (as opposed to water temp vs room temp deltaT)

Seems like the max suggest circuit temp drop is 7C.  I also need to put out about 7.5kw from which I can work out a flow rate.

Seems like the upstairs leg of my heating circuit which is in 15mm copper would see a pressure drop of about 0.27bar and would be able to put out the required 3.5kw at a flow rate of 7l/min with a dt of 7C.  This then comes after the downstairs section on 22mm pipe which in turn supports the required flow rate of 14l/min to output the 7.5kw required for the whole house and this section would have a pressure drop of 0.15bar

Total pressure drop is 0.42 bar or just under 4m of water header pressure.  Standard circulation pumps seem to do about 6m head.

MY question is can you do the maths like this, look at the 15mm bit as a 'sub circuit' of the overall 22m circuit with about half the flow not going through this section but gong through the downstairs rads?  Obviously if those rads TRVs closed then all the flow would try and go through the small section which would not be possible but I think you can get a pressure bypass valve to address this?  Presumably as some or all the rads TRV close the returning water DT is smaller and the heat pump modulates down but this is the behaviour you would want.

matt_drummer

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.

Reed_Richards

A couple of caveats to the comments by @matt_drummer.

A heat pump (or a boiler) will cycle if the minimum amount of heat it can provide exceeds the current requirement.  These cycles could be of short duration but a well-controlled heat pump (or boiler) will ensure that they are not.

A heat pump might also know the set room temperature and the desired room temperature but this mode of control, "Load Compensation" is not very common.  And it's more about responsiveness than economy.  Indeed there may be other conflicts between control and economy; eliminating TRVs and zones might be good for economy but it eliminates some easy control options.

matt_drummer

Reed_Richards said:

A couple of caveats to the comments by @matt_drummer.

A heat pump (or a boiler) will cycle if the minimum amount of heat it can provide exceeds the current requirement.  These cycles could be of short duration but a well-controlled heat pump (or boiler) will ensure that they are not.

A heat pump might also know the set room temperature and the desired room temperature but this mode of control, "Load Compensation" is not very common.  And it's more about responsiveness than economy.  Indeed there may be other conflicts between control and economy; eliminating TRVs and zones might be good for economy but it eliminates some easy control options.

All true, and I did say the same.

Heat pumps stop for four reasons, they are told to, they need to defrost, there is a fault or the returning water temperature is too hot.

@Reed_Richards uses a third party controller to limit short cycling by setting a minimum time between cycles but you don't usually get that with the heat pump's own controller. They just want to get heating again as soon as possible.

Setting a limit on the time between cycles is fine if your house is already warm enough but in my example it wouldn't be.

If the heat pump has stopped because the upstairs radiators are shut by trvs but the downstairs still need heat then delaying the heat pump starting again using a third party controller is just going to make the downstairs even colder.

A well controlled heat pump only works with a well designed rest of the system.

My Daikin has load compensation if I choose to use it but it only works if you aren't already at the minimum output of the heat pump. To load compensate it has to be able to reduce as well as increase output.

As always, economy is a balance between cost and comfort.

But, by their nature, trvs are only necessary if one area of the home reaches the design temperature before other areas, they just compensate for deficiencies in the system design.

Reed_Richards

Or you can use TRVs to vary the desired room temperature at different times.  For example, you might want to sleep in a cool room but have it warm when you wake up whilst another room might want to be warm all the time.  A programmable TRV could achieve the variation in temperature you want, provided you coordinate with the heat pump schedule.  Getting rid of all direct thermostats and zones works best or most easily if you want your house to remain at the same temperature 24/7.

michaels

What about if you want the whole house to be cooler at night and warmer by day, you are doing well if your circulation and all your rads are perfectly sized to achieve this as it also means that none are oversized and everything is perfectly balanced.  and how about if a room is prone to solar gain or due to exposure to the wind, higher heat loss at some times than others, how can this be handled with out TRVs?

michaels

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.

matt_drummer

michaels said:

What about if you want the whole house to be cooler at night and warmer by day, you are doing well if your circulation and all your rads are perfectly sized to achieve this as it also means that none are oversized and everything is perfectly balanced.  and how about if a room is prone to solar gain or due to exposure to the wind, higher heat loss at some times than others, how can this be handled with out TRVs?

That is what I do if you are asking me. 

My radiators suit the heat losses of the rooms they are in.

It's not always perfect because sometimes it is sunny, sometimes windy, we just live with it.

I aim for efficiency and most of the time the house is comfortable everywhere. The sun coming out is natural and I have never found that trvs would have enough affect to counter the solar gain, it is what it is.

You and anybody else can fit what you/they want and run your heating to suit you. I am not the law!

The topic was about heat pump sizing, choice and efficiency.

All these things are related. 

So, I started at the most efficient design and anybody can compromise efficiency for comfort, it just depends what you want.

It's much easier to add some extra comfort to an efficient system than it is to make an inefficient system efficient.

matt_drummer

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.