I have spent the last few days absorbing all of the excellent information and advice on the various threads here, must admit some of the science is a bit over my head but I think I get the concepts at least. Rather than hijack someone else’s thread I thought I'd start one here and see if I can find some answers to my problems.
Background - 1970's timber framed house, double glazing, loft insulation, suspended timber floor, three bedrooms, about 120sqm total floor area. When we bought the house it had electric storage heating, so we decided from the outset to install ASHP with underfloor heating throughout. The pump is a Vaillant Arotherm Plus 12kw, connected through a 45l buffer to the underfloor system. UFH is retrofit overlay flooring, pipes laid in polystyrene trays at 150mm spacing with 18mm screed board on top. Hot water is from a 300l Vaillant tank. Vaillant system diagram is 30111-1011
The system has been in for about six months now, over the winter it’s been running at a CoP of about 2, and the house has been tolerable but there are times in a cold spell when it’s been unable to get rooms up to 20 deg. With a 12kW pump in a relatively small house I didn’t expect to feel cold!
I had a long and very helpful chat with the Vaillant tech support line and we ran through a bunch of checks which highlighted some potential issues:
- Flow/return temp delta is too low at around 2.5-3deg, Vaillant look for 5deg
- Building circuit flow rate is about 1500, Vaillant say 2000 is ideal (unit spec is 2065l/h)
Am I right in thinking that these two issues are potentially related? Ie if the flow rate is too low, you won’t get as much heat transfer across the buffer to the UFH system.
If that is correct - what could be the cause? The house is a bungalow, so the flow comes out of the heat pump, up the outside wall to a height of about 4 metres and then travels horizontally for about 12 metres to reach the buffer. There is a filter in the circuit with isolator valves (which are open), Vaillant suggested that the filter could need cleaned but it’s been like that since it was installed. I’ve looked at the pipe size and I think it’s correct.
My other thought is that the buffer is too small for a 12kw system, and a larger volume would allow more heat transfer.
Going back to the installer isn't an option as he has retired, and to be honest he was out of his depth anyway. We live in a very remote location so there isn't anyone else around, which means it's down to me to sort this out (with a little help from people who actually know what they're talking about hopefully!) Any thoughts / comments / etc most welcome. Thanks for reading this far!
I have spent the last few days absorbing all of the excellent information and advice on the various threads here, must admit some of the science is a bit over my head but I think I get the concepts at least. Rather than hijack someone else’s thread I thought I'd start one here and see if I can find some answers to my problems.
Background - 1970's timber framed house, double glazing, loft insulation, suspended timber floor, three bedrooms, about 120sqm total floor area. When we bought the house it had electric storage heating, so we decided from the outset to install ASHP with underfloor heating throughout. The pump is a Vaillant Arotherm Plus 12kw, connected through a 45l buffer to the underfloor system. UFH is retrofit overlay flooring, pipes laid in polystyrene trays at 150mm spacing with 18mm screed board on top. Hot water is from a 300l Vaillant tank. Vaillant system diagram is 30111-1011
The system has been in for about six months now, over the winter it’s been running at a CoP of about 2, and the house has been tolerable but there are times in a cold spell when it’s been unable to get rooms up to 20 deg. With a 12kW pump in a relatively small house I didn’t expect to feel cold!
I had a long and very helpful chat with the Vaillant tech support line and we ran through a bunch of checks which highlighted some potential issues:
- Flow/return temp delta is too low at around 2.5-3deg, Vaillant look for 5deg
- Building circuit flow rate is about 1500, Vaillant say 2000 is ideal (unit spec is 2065l/h)
Am I right in thinking that these two issues are potentially related? Ie if the flow rate is too low, you won’t get as much heat transfer across the buffer to the UFH system.
If that is correct - what could be the cause? The house is a bungalow, so the flow comes out of the heat pump, up the outside wall to a height of about 4 metres and then travels horizontally for about 12 metres to reach the buffer. There is a filter in the circuit with isolator valves (which are open), Vaillant suggested that the filter could need cleaned but it’s been like that since it was installed. I’ve looked at the pipe size and I think it’s correct.
My other thought is that the buffer is too small for a 12kw system, and a larger volume would allow more heat transfer.
Going back to the installer isn't an option as he has retired, and to be honest he was out of his depth anyway. We live in a very remote location so there isn't anyone else around, which means it's down to me to sort this out (with a little help from people who actually know what they're talking about hopefully!) Any thoughts / comments / etc most welcome. Thanks for reading this far!
Welcome to the forum.
I sent this post earlier today, but it probably covers your problem to some extent.
It is the heat emitter end where the problem lies. The amount of heat energy that can be emitted by the heat emitters, is dependent upon the physical emitting area, and the temperature difference between the average water temperature inside the heat emitter, and the indoor air temperature. The average water temperature being approximately half way between the temperature of the water entering the heat emitters and the temperature of the water leaving.
If, to meet the present heat demand, the average water temperature at the heat emitters needs to be 35C, and there is a 5C DeltaT across the heat emitters, then the water entering the heat emitters will need to be approximately 37.5C.
If there is mixing occurring within a buffer tank or LLH, there could possibly be a 5C temperature loss between the LWT from the heat pump, and the water going to the heat emitters. The heat pump would therefore need a LWT of 42.5C to meet the heat demand rather than the 37.5C required by the heat emitters.
The heat pump therefore uses more electrical energy to absorb a smaller quantity of heat energy from the outside air, so is obviously operating in a less efficient manner.
To try to balance the temperatures across a buffer tank or LLH, the water flow rate going in from the heat pump needs to be the same, or slightly higher than the water flow rate coming out and going to the heat emitters.
I don't understand why you need bypass valves if you ensure that there are no zone valves or TRV's that can close and fully shut off the water flow. Your present diverter valve and water pump are adequate at present flow rates, which could be lower if the system is operating more efficiently.
If you were to modify the system as I suggested, if problems occur it would be a simple job to reverse the process.
Initially you could try lowering the secondary pump speed and/or increasing the primary pump speed, to try to balance the flow rates. If you measure the temperature at the inlet flow pipework and the exit flow pipework, this should give some indication of when the flow rates are reasonably balanced. The objective being to obtain the minimum temperature difference.
I would suggest that you measure the temperature of the pipes around your buffer tank, the flow in and out, and the return in and out. If you find a temperature drop across the buffer tank then try the suggestions detailed above.
The system has been in for about six months now, over the winter it’s been running at a CoP of about 2
Keeping this simple - COP is [directly] a function of only two parameters COP = f(ambient outside temp, flow temperature), without moving location you can only influence the latter parameter, a good design will focus on keeping that temperature as low as possible. Given yours is in place, you need to understand all the factors that drive flow temperature up (other than it being colder outside) and deal with them.
As Derek pointed out, flow temperature in your case will be determined by two design decisions #1 - your emitters (which I think is UFH) and #2 - your buffer tank configuration and pump settings
For #2 - follow Dereks suggestion to make sure you have almost no drop in temp between input to buffer tank and output as this will increase you COP.
For #1 -try to identify the optimal flow temperature and deltaT for your UFH pipes and set this in your heat pump. The below questions may help there.
do you you have just polystyrene or polystyrene aluminum spreader plates?
do you know the insulation factor of the 18mm overlay board?
how much insulation is under the UFH pipes?
what is the diameter of your UFH pipes?
how many ufh circuits do you have?
how long is each ufh circuit?
what is the heat load in each room at the design temperature?
what is the design outside temperature?
With the above data, you should be able to get to a target flow temperature and dT
Finally., enable weather compensation in your heat pump.
I would suggest that you measure the temperature of the pipes around your buffer tank, the flow in and out, and the return in and out. If you find a temperature drop across the buffer tank then try the suggestions detailed above.
Thanks Derek, that's a good suggestion. I've ordered another Shelly temperature monitoring module which will do the job, should be here in a few days, will report back then.
Are you measuring this at the heat pump side? - Dereks suggestions may help here.
Thanks for the suggestions. The trays are polystyrene with thin aluminium foil. As to the rest, I don't think any kind of heat loss calculations were done, and I certainly never saw anything with that sort of information that you list, which makes this all a bit challenging. I've experimented with the heat curve by trial and error. Currently running well at 0.6, but it's been quite mild for the last couple of weeks and I'm not sure that will be enough in a cold spell.
Weather compensation is off as that's what Vaillant specify for the design of system, don't really understand the logic of that.
Flow/return temperatures come from the heat pump itself, viewable on the console. I'm going to put sensors on each of the in/out pipes on the buffer as Derek suggested and see what I can find.
@squonk flow rate is a function of pump "head" (how hard it pushes to the layman) and how much resistance to flow there is in total coming from the water circuit: the heating device itself, valves, pipework, radiators, , and any other plumbing fittings like filters, air separators. The type of liquid matters too as well - glycol is "thicker" so slows it down. The maths to get to "total resistance" can be quite complex, but at high level it's simple thing : To get more flow rate you can either get a "stronger" pump (but thats not usually a good idea to jump straight to unless its obviously undersized) or - better - look at the total resistance to flow and try to reduce it. It can be surprising how much resistance to high flow rates there can be from "standard plumbers merchant" valves that were designed for the much lower flow rates of a boiler, and they are not that difficult to source alternatives for and replace. For example, you mentioned a "filter". It will matter what type of filter that is.
suggest you try to draw up a diagram of your circuit, following the example of this thread (click for link) and where the pipework goes through a valve, filter or basically anything that is not a pipe , bend or tee, get a part number / model number / close up photo for that fitting and post it here. we'll then be able to help you identify any devices putting an obvious drag on your flow rate.
I would suggest that you measure the temperature of the pipes around your buffer tank, the flow in and out, and the return in and out. If you find a temperature drop across the buffer tank then try the suggestions detailed above.
Just to follow on from this, finally got hold of the sensors (Shelly) and set them up on the buffer tank.
This was the result:
Any thoughts?
(Note - the sensors are labelled according to the diagram showing how to connect the buffer up. I am 100% certain they are correct)
I would suggest that you measure the temperature of the pipes around your buffer tank, the flow in and out, and the return in and out. If you find a temperature drop across the buffer tank then try the suggestions detailed above.
Just to follow on from this, finally got hold of the sensors (Shelly) and set them up on the buffer tank.
This was the result:
Any thoughts?
(Note - the sensors are labelled according to the diagram showing how to connect the buffer up. I am 100% certain they are correct)
Could you please provide a photo of how the sensor are installed around the buffer tank. Identify each sensor and the pipework.
There would appear to be something wrong with how the sensors are installed or how the sensors or the pipework is being identified.
@derek-m That was my first thought, but I’m 100% sure it’s correct, which leads me to think the flow & return into the buffer could be the wrong way round.
@derek-m That was my first thought, but I’m 100% sure it’s correct, which leads me to think the flow & return into the buffer could be the wrong way round.
The only way to be certain would be to physically trace the pipework from your heat pump to your buffer tank, and then from your buffer tank to heat emitters. Also ensure that any water pumps that may be installed are pumping in the correct direction.
