I don't own a heat pump, unless you count a fridge freezer. But it does appear as though the defrost operation of ASHPs isn't often spoken about. It would great to learn how installers / vendors / customers cope with cold water being pumped into the heating system. Is it even a problem? I keep thinking about "average flow temperature", if there's a large volume in the system the average flow temperature should be fairly stable. If the volume is low the flow temperature would fluctuate a bit. It seems as though a heat pump system is more like an old solid fuel system which requires constant operation and large water volumes than a modern "combi boiler" which tend to be on/off with small water volumes.
I don't own a heat pump, unless you count a fridge freezer. But it does appear as though the defrost operation of ASHPs isn't often spoken about. It would great to learn how installers / vendors / customers cope with cold water being pumped into the heating system.
I suspect the experience differs a lot from heat pump to heat pump and house to house. When my 7kW Vaillant (no buffer tank or volumiser) defrosts the water in the system goes down to about 25C, so not cold enough to chill the house, just to stop heating it. The pause isn't long enough to notice, but of course it has the potential to reduce the net output.
The full cycle seems to take about 15-20 mins in total, about 5 for the actual defrost operation (ending is an impressive cloud of vapour being ejected from the front) and about 10-15 to recover the flow temperature fully to the starting point. During the recovery phase the heat pump seems initially to ramp up the compressor to maximum (then to cut it back) and the flow temperature overshoots by a few degrees. This behaviour is logical in a way, but does make it relatively noisy (the heat pump is otherwise whisper quiet). I have so far witnessed defrosts occurring as much as once per hour if there is freezing fog, to once every several hours on days like today when its been cold but dry.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Hi. Just a quick question during my quest of identifying the reasons and possible remedies for the mixing i observe and the low- 1 degree Dt. Is there a certain way to connect the pipes to a buffer tank? In all the diagrams i have seen the flow moves in a straight and parallel direction for the top and bottom connections of the tank. On my system it seems that flows crosses the tanks and moves in the opposite direction. Can this be a reason for the issues i face? Please see the attached diagram
Hello,
Buffer tanks have several functions which determines whether or not they are necessary in an installation.
1. Hydraulic separation for systems which have >1 circ pump on the heating side and one primary pump in/connected to the heat pump primary flow and return since pumps *should* not be connected in series
2. Providing sufficient volume for a defrost cycle in units where zones may shut down or there is simply not enough volume for defrost even when all the zones are open
3. Reducing cycling in systems where the heating demand of the house may be satisfied when the system has not run for long periods or where there are many zones (so the zones can pull off the buffer tank and not trigger the ASHP to run every time a microzone calls for heat).
Mixing across a buffer tank is frequently down to differing flow rates across the primary and secondary side, so this is worth investigating. How many pumps do you have on the heating side and are any of the pumps adjustable (eg they have a button on the top saying flow speed 1, 2, 3 etc).
Heat pump consultant and designer at Cirrus Energy. Thinking about installing a heat pump? Or already have one but it’s not performing as it should? Book a one-to-one session with Rob to discuss things here.
@rob_cirrus_energy there are two pumps. One in the primary circuit (from ashp to buffer) and one to the ufh manifolds (from buffer). Primary pump has an operating flow of 2 cubic meters/h and the secondary pump a nominal flow of 33l/min. I can only adjust the flow of the primary pump from the control panel but not of the secondary. There are ball valves however from the buffer to the secondary pump and i have closed them a bit. The primary pump is set to automatically adjust the flow based on the Dt. I can see that with the current adjustments i am losing about 3 degrees from the 6 i was losing before. Not much of a problem but when there is demand for only a single zone, the Dt drops so the ashp starts cycling. I don't know if this is caused by mixing in the buffer or not
Primary pump has an operating flow of 2 cubic meters/h and the secondary pump a nominal flow of 33l/min.
At 2 cubic meter/h that is around 33l/min, so the two pumps look like they are much the same flow.
But when you stop a zone then flow will be effected, as the CH does not require as much water etc. So the HP will start to use the buffer.
If its possible and without compromising your comfort, can you lower the WC by 1° both ends, or at least the top end. This may help keep the zone that shuts off to work longer and keep that flow going. After making the change leave it on that setting for 24hrs, or longer, and recheck.
If you see the zone runs longer and/or the temp different dropping, then think about dropping the WC another degree at the top.
After 2 adjustments I would leave it upto a week before making any further adjustments as that will cover a range of outside temp etc.
@jamespa most HP recycle water around the system when its below say 4° but not supplying heat. To stop the water freezing.
How does that effect the house ?
Probably not a lot unless you have UFH, although there is a risk of condensation on the pipes
When ASHPs are used for cooling one problem that occurs is that the cold water collects and stays at the bottom of the radiators and so little or no cooling takes place. Thats why fancoils (or UFH) are usually recommended if the cooling function is specified.
Obviously cold water circulating in the house will cause some cooling of the house, but if the occupant has been away long enough to trigger this function its probably pretty cool anyway.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@uk_pete_2000 thank you for your response. As i am a newb in all of this, can you be a bit more specific on what should i change? The attached screen shows the wc curve i see on the panel. Not my panel but a screenshot found online. My limits are 25-40c. Thanks.
@jamespa I was more concerned about the HP in the winter.
I only run my HP for about 6-8 hrs a day and not overnight. So when the temp outside gets to 4° the HP starts just its own pump and circulation takes place to stop any pipes from freezing.
I have UFH downstairs and rads upstairs
If that was running all night then without my buffer it would have to send the water around the house and I suspect will cool it down.
I only run my HP for about 6-8 hrs a day and not overnight. So when the temp outside gets to 4° the HP starts just its own pump and circulation takes place to stop any pipes from freezing.
FWIW my boiler did the same. Sometimes I heard it coming on at 3am in the morning. It heated as well during these forced 'on' periods.
But yes, if your heat pump circulates cold water the house and in particularly the UFH will cool if there is no buffer. In any event all the water in the buffer will cool and you will have to reheat it in the morning. I wouldn't use this as an argument for a buffer, there are better ways to solve the problem.
If its cold enough to freeze I personally wouldn't run a heat pump for only 6-8 hours per day, its very likely indeed to cost more than running it 24*7 at a lower flow temperature.
This post was modified 1 month ago 4 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@uk_pete_2000 thank you for your response. As i am a newb in all of this, can you be a bit more specific on what should i change? The attached screen shows the wc curve i see on the panel. Not my panel but a screenshot found online. My limits are 25-40c. Thanks.
No probs. Looking at screen shot the system is running 55° at -2c to 54° at +16c
This means this system is set up more like just using a set flow rate, so I would take both ends down to 54 & 53 then 53 & 52.
A normal WC has a much greater range over the two settings, say 36° at -3c to 25° at +15c. So with this we may just take down the higher one to 35 then 34°
As each HP has a different way of changing the WC curve, I am not sure of the actual instructions. Instructions are normally included with the HP, if you can not find them, let us know which HP you have and which controller you are using and I'll take a gander for the instructions
@jamespa I'm very much in the anti buffer side and looking to revamp my system over summer to see about removing/replacing it in 2026. So I'm going to add valves/bypass to the buffer and run various scenarios, to get the best option.
The reheating of the buffer is okay as I have got the difference between the HP and CH sides down to just below 2°. But to me that means I have to run the HP those 2° higher than I need too.
As for running the HP 24Hrs, I see it as a waste of energy and a false idea.
So with that statement I better explain. For this I am going to make some assumptions which may or may not be correct, but it shows how I work it.
You probably run yours 24hrs, with a "setback" overnight. So let's say 21° day/evening 18° overnight, but does it really work that way. If the HP has done its job, then when you use the "setback" you must be turning your HP off because to room is 21° and you have now set it to 18°. Then when you set it back to 21° the HP will turn itself back on and start heating etc.
If the HP does not turn itself off, then your house has never reached 21° and more likely to be at 17°
You can only run "setback" correctly if it changes the flow temp. Which I believe some HP may be able to do.
The only otherway of making 24hrs running work is to have your flow temp set exactly at the amount of heat to cover the house loss at all times of the day. WC goes someway towards this, but overtime the house will either overheat or be cold and the WC will need tweeking or the HP has turned itself off for a time.
Now to the energy used. In 24hrs running the HP should work more efficiently, but it will use x amount of power. If you know how long your house take to drop 1° and also how long it take to heat 1° you can work out what you can save in kwh. You can then use that to set up your timers to get the correct heat at the correct times
Now the trick, if the house takes say 6hrs to cool, but only 4hrs to heat, you have saved some energy, but you have to take in the extra energy you have used to start your HP. If the house takes the same or longer to heat up, then fine run 24hrs
With our house it takes 12hrs to lost 1° but only takes about 6-8hrs to heat it up. So our schedule is heat from 08.00hrs to 16.00hrs then off for the rest of the time. All I do is change the length of the running time to match the conditions outside. At present the house is gaining about 0.1 to 0.2° each day, but next week it might lose that as it gets colder.
At present we can run the house on either WC or flow temp, but it only needs to run at 25-27° flow, to get the house to 20°. Next year I'm hoping we can get it down to 23° and still maintain the same degree of comfort.
Each person's house is different, ours is very efficient, but by getting to know your house as much as your HP, you can start to save energy
Edit - Regarding overall cost, the last couple of years we used 1300kwh to heat the house each year. Hoping to reduce the to below 1000kwh per year
This post was modified 1 month ago by Uk_pete_2000
Thinking about installing a heat pump but unsure where to start? Already have one but it’s not performing as expected? Or are you locked in a frustrating dispute with an installer or manufacturer? We’re here to help.
