Pip895

I will need to calculate it but even with defrosts I think the heat pump will win over gas - once we go down below about -1 we will be on gas though. How do you control it all? Can you do it within the Heat pump programmer? I was intending getting my OH to program a Raspberry Pi or something.

I am looking to do this with our system. I intend to run flat out for 6 hrs overnight at 7.5p/kWh (Intelligent Octopus Go) then if the house really needs more during the day It will probably be via a gas boiler at the moment, with gas at 5p/kWh a heat pump running on electricity at 30p/kWh just cant compete!

I want to upgrade my current 15 year old two gas boiler system to a heat pump. I have been trying to decide weather to go all the way and apply for the BUS grant or to simplify things and concentrate on the under floor heating and heating our swimming pool water which both require lower temperatures and are ideally suited to an ASHP. My previous discussion on the subject is here:- Having now had another couple of installers in (both of whom seemed to know what they were talking about- not just sales men) I really have to make a decision on which way to jump. The first installer quoted for a full BUS compliant system a 12kW Vaillant doing DHW and house heating - it also involved putting in radiators into our swimming pool room which is currently heated by a blown air system. This solution would hopefully be BUS compliant and should end up with a cost of around 10k to us. One boiler would be retained but would only be heating the pool water. The second installer came down on the other side of the argument and thought all the BUS grant and more, would be used up by replacing the tank and re running its pipework to little advantage (we have solar thermal providing much of our DHW) and he suggested a 16KW Midea unit - approximate cost to us similar, although I am yet to get a formal quote. This installer is also quite flexible and happy to install any system we want within reason. I like the Vaillant unit but am not keen on the amount of disruption or on radiator(s) in the pool room so we are currently looking at the option without the BUS grant or involving changing the DHW tank. Because we aren't using the BUS grant we have the option to create a true bivalent system, whereby the gas boiler can take over when conditions dictate eg. when temperatures are sub zero and during the day when we have run out of battery power to run the heat pump. I have come across the NRG Awareness manifold that looks like it could greatly simplify the installation - has anyone got experience of these? Does this look like a reasonable plan?

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?

I am facing a similar dilemma - you may have seen my other thread. I think the pro's & cons for me (may well be similar for you) are as follows. Self install Pro:- Fast - can start as soon as I want so any savings will start this winter. Kit seems relatively cheap (compared to the prices I have been quoted for a professional install) & I think the simpler install would be done in a day if a Heat pump team did it - will take us much longer though. Low risk - I can keep my existing system as a backup to use in the very coldest conditions, if the heat pump breaks down or if there is a power cut. (We have a battery backup so could run the pumps etc but obviously the Heat pump itself would drain the battery too quickly.) The % of gas displaced would be done at a very good COP as the pump would work 100% at 35deg or below - The % gas (in our case) saved could be in the region of 70 to 80% if we size the pump correctly. No need to replace our existing Hot water tank or otherwise mess with a working system. Wont have to pay for expensive yearly maintenance to maintain warranty (as we wont get it anyway..). R32 would be ideal at the low temperatures and Larger R290 units aren't yet available in all product ranges. Disadvantage No grant to help with the costs. Warranty on the HP may be reduced or non existent after first year. No reduction in costs of DHW DHW tank is 15 years old and may need replacing in the next 10 years anyway at our expense. Less far on the road to completely removing gas from the property (not relevant to you). House will have a less standard set up with no professional backup.

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).

Isn't it just people using the hot water rather than the tank loosing heat?

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.

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.

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.

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..

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.

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..